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Antenna Design Basics + Amplification


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#1 alanh

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Posted 15 December 2004 - 11:34 PM

This post has been removed by the author and saved by him.

Edited by alanh, 25 March 2013 - 11:56 AM.


#2 jaybonzi

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Posted 17 December 2004 - 07:53 AM

Dipole
This is the most fundamental antenna. This is your typical rabbit’s ears antenna with the ears flat. Its total length is related to the frequency to be received.
Length (mm) = 15 000/frequency (MHz). As examples for channel 0, (48.5 MHz) the antenna should be 3 metres long. Channel 6 (177.5 MHz) 842 mm, channel 69 (816.5 MHz) 183 mm. If the above formula is used it will be a maximum for that frequency.
The dipole has a figure of 8 directivity pattern, with the maximum sensitivity at right angles and in the same plane as the metal rods. End on it is not very sensitive. The front to back ratio is 0 dB or equally sensitive. If you look at the dipole end on it is omnidirectional.

Gain measured in dB
This is the logarithmic ratio of the signal power of the antenna under test to the signal power from a dipole at the test frequency. Usually the bigger the better.

For a fixed signal strength, to get a fixed output voltage you need to have more gain as the frequency (real channel number) rises. There is also more loss down cables and in the air. The transmitters usually use increased radiated power as the channel number rises, everything else being equal.

Quality of the signal
Carrier to noise ratio(c/n) usually measured in dB. I an indication of how strong the signal is compared to the noise. The noise may come from interference or the noise generated in an amplifier. This ratio can be improved by using a more directional antenna (One containing more elements)

The importance of this reading is that if the carrier and the noise are nearly equal, you have fallen over the digitial cliff and you will see either no signal message or pixellation and sound plops.

Noise figure is the carrier to noise ratio at the output (dB) - carrier to oise ratio at the input of an amplifier. This figure for modern masthead amplifiers can be as low as 1 dB. This is how much worse the amplifier is making the carrier to noise ratio.

Bit Error Rate If the carrier to noise ratio is low, then there will be a large bit error rate. This can also be caused by reflected signals particularly those nearing the main signal's strength. This is where antenna design is important as is using an antenna which is directional enough.

Antenna Types
The most common types in TV reception are
Yagi, Log Periodic and Phased Array

Yagi
This antenna has a dipole which is connected to the down lead via a balun. The dipole is commonly folded.
A reflector is placed on the side away from the transmitter to prevent signals from the back being received and to reflect any signals that got past the dipole.
Directors are placed parallel with the dipole to focus the electromagnetic waves on the dipole. The greater the number of directors the more sharply focused the antenna becomes and the stronger the signal. Remember the magnifying glass lighting the paper!

The entire antenna is connected to earth except the dipole. A ferrite transformer is used to keep the dipole above earth (this is a balun), since the receiver end of the cable is likely to be earthed.

Log Periodic
This antenna consists of cross connected dipoles. None of the antenna is earthed. It works by having a longer dipole behind acting as a short to signal from behind at that frequency. The dipole in front can have no function at all; act as a second dipole or as a director. This depends on the number of dipoles near that frequency. Since there are a large number of dipoles a linear transformer is used to obtain the standard 75 ohm output to match the cable. Since the antenna must not be connected to earth either the whole antenna is not earthed or insulators are used on each dipole.

This antenna usually has a very good front to back ratio, and medium gain depending on the frequency range required and the number of dipoles.

Phased Array
Assuming that all the other antennas are mounted horizontally, this antenna has an even number of dipoles mounted one above the other. Typically each of these dipoles is accompanied by a reflector. The vertical spacing is related to the frequency, as is the length of the dipoles. All of these antennas are earthed except the dipole.

Directivity in the horizontal direction is not great because there is only a dipole and a reflector. In the vertical direction the directivity is increased because of the stacking of the dipoles.

Polarisation
For maximum received signal strength the receiving antenna should be parallel to the one on the transmitter tower. In Australia we use both horizontal and vertical transmitting antennas. This means we can put two transmitters on the same channel much closer together that if we only used one polarisation. In some areas of Australia band 3 transmitters are vertically polarised and band 4 & 5 horizontal on the same transmitter tower. eg. Canberra and Wide Bay.

Comparison of Characteristics
Dipole: On its own is usually only used in indoor antennas and when bent into a near circle for boats. This is where we do not want directivity! These antennas are not very sensitive and some have in build amplifiers. The will pick up reflected signals and this gives ghosted images in analog and if it is bad enough will cause pixellation and sound plops. ie the Bit Error Rate (BER) will be high.

Yagi:They have a sensitivity which is related to the number of directors, but element for element more gain in a restricted range of frequencies, the range widens as the frequency to be received increases.

It is less likely to receive lower frequencies from power line interference, car ignitions and high powered radio transmitters. This is because only the dipole is connected to the down lead.

Log Periodic:This antenna has less sensitivity but can maintain it over a wide range of frequencies. The gain is related to the range of frequencies and the number of dipoles.

Phased Arrays:Since this antenna has better vertical directivity it is typically used for long distance and paths blocked by terrain and buildings. (Imagine looking at the sunset over the sea. What you see is a thin bright line on the horizon.) Reflected signals are usually not a problem in weak signals as they will be weaker.

Combined antennas: Your average bands 1-3, 4-5 antenna (Channels 2-12 + channels 28-69) consists of a Log periodic for bands 1-3 and a Yagi for bands 4 – 5.

Log periodics drop rapidly in sensitivity outside their design range which is why the older style combined antenna will not receive channels 11 & 12.

For Australian Digital TV no channel under channel 6 will be used so any sensitivity there makes you susceptible to interference causing pixellation and sound plops.

Band 4 European Antennas
These are designed to receive channels 21-35. In Australia channels 21-27 are not used for TV but for communications including UHF CB radio. This 56 MHz is not only not required but can cause pixellation and sound plops if the interference is strong enough.

Vertically Polarised Signals
If reflected signals are a problem, phased arrays are better than Yagis and Log Periodics. If you want long distance or diffuse signals then multi element Yagis are the way to go.

Advertising
An element is a reflector, dipole or a director. All are mounted at right angles to the boom. I see advertised an SF91 antenna, if you look at a picture how many elements do you get? 22!


AlanH

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Then whats BAND 5 :blink:

#3 alanh

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Posted 17 December 2004 - 08:04 PM

TV Bands
TV band 1 channels 0-2. 45-70 MHz
TV Band 2 Channels 3-5 85-108 MHz, yes this includes the FM radio band
TV channel 5a was only used in Australia and is being phased out
TV band 3 channels 6-12, 174-230 MHz, includes VHF trial radio DAB
TV band 4 channels 28-35 526-582 MHz
TV band 4+ channels 28-50, 526-687 MHz
TV band 5 channels 36-69 582-820 MHz

Bands 1,2 and channel 5A will not be used for digital TV anywhere in Australia.


AlanH

#4 Chicken Man

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Posted 18 December 2004 - 04:36 PM

AlanH,

That was a good intoduction to antennas,informative but not too technical.

Cheers, C.M

#5 alanh

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Posted 19 December 2004 - 03:53 PM

All,
The reason I check which RF channels are to be used in a particular installation is to get the antenna which matches the channels.

The reasons for this are;
that the antenna size for size will give more signal on those channels.

to reduce the possibility of interference.

This reduces the chances of pixellation and sound plops.

Interference: For example no station in Australia will use a channel below channel 6 because the Australian Broadcasting Authority recognises that interference from power mains and car ignitions increases as the channel number is reduced. There is also 2 way radio communications between channel 5a and channel 6 High powered FM radio transmitters in channels 3-5

Channels 21-27 are used for communications including UHF CB

Just above channel 69 there is paging transmitters and mobile phones.

The only advantage of wideband antennas (eg Channel 6-69 and to a lesser extent channel 21-69) is that you do not have to change the type when you move. They are good for mobile applications.

AlanH

#6 Gizmomelb

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Posted 01 February 2005 - 07:00 PM

Hi AlanH,

thanks for the very informative read - though I have one regarding antennas for laptops:

what should be the minimum element length for the 'most useful' coverage, for a monopole antenna DVB-T?

and I guess the follow-up to that would be - can you recommend any brands/models for laptop users?

Thank you very much for any info on this.

Regards,
Gizmomelb

#7 alanh

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Posted 01 February 2005 - 07:50 PM

Gizmomelbourne,
On the end of each Get the best reception
Which transmitter and which antenna
http://www.dtvforum....showtopic=12503
The required length depends on the channel to be received. Fortunately in Australia the lowest channel number is channel 6 745 mm, down to 215 mm for band 5. So it will have to be telescopic. It also needs to be either horizontal or vertical depending on the transmitter.

It needs to be remembered that the internal electronics of the computer radiates signals in the TV bands causing interference particularly when signals are weak. So it would be better to plug in an antenna.

AlanH

#8 Mr Antenna

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Posted 26 May 2005 - 10:51 PM

Channels
TV Bands
TV band 1 channels 0-2. 45-70 MHz
TV Band 2 Channels 3-5 85-108 MHz, yes, this includes the FM radio band
TV channel 5a was only used in Australia and is being phased out
TV band 3 channels 6-12, 174-230 MHz, includes VHF trial radio DAB
TV band 4 channels 28-35 526-582 MHz
TV band 4+ channels 28-50, 526-687 MHz
TV band 5 channels 36-69 582-820 MHz

Bands 1,2 and channel 5A will not be used for digital TV anywhere in Australia.

Real Channel Number and Logical Channel Numbers
The receiving section of TV receivers work on the channel numbers indicated in TV Bands. If you wish to add an additional transmitter to repeat the program in an adjacent area you will have to use another channel, otherwise there will be interference between the signals. To use the same channel the transmitters must be at least 400 km apart. (Digital SFN excepted. SFN is a Single Frequency Network which has repeaters on the same channel in the same coverage area. The signals frome each transmitter must be made identical a point halfway between the transmitters. Does not work in analog.)

Since the metropolitan networks use their main transmitter's channel number as the network name (eg, 7,9,10) for digital TV they transmit a logical channel number This means that the receiver may be using channel 69 but the program name as displayed on the front of the STB may be 7 for example.
In installation the STB usually will show the real channel number, as well as the LCN

Logical Channel Numbers LCN
The network is identified by the LH digit displayed on the front of the STB. It is also used by the remote control for channel selection.
1 = TEN Network
2 = ABC
3 = SBS
4 = Miscellaneous (including Datacasting)
5 = TEN affiliates (eg. Southern Cross)
6 = Seven affiliates (eg. Prime, GWN)
7 = Seven Network
8 = NINE affiliates (eg. WIN, NBN, Impaja)

All digital transmissions are capable of being split to carry multiple programs. (eg ABC and ABC2)

Polarisation
For maximum received signal strength the receiving antenna should be parallel to the one on the transmitter tower. In Australia we use both horizontal and vertical transmitting antennas. This means we can put two transmitters on the same channel much closer together that if we only used one polarisation. In some areas of Australia band 3 transmitters are vertically polarised and band 4 & 5 horizontal on the same transmitter tower. eg. Canberra and Wide Bay.

Dipole the Most Basic Antenna.
Dipole
This is the most fundamental antenna. This is your typical rabbit’s ears antenna with the metal rods folded out into a straight line. Its total length is related to the frequency to be received.
Length (mm) = 15 000/frequency (MHz). As examples for channel 0, (48.5 MHz) the antenna should be 3 metres long. Channel 6 (177.5 MHz) 842 mm, channel 69 (816.5 MHz) 183 mm. If the above formula is used it will be a maximum for that frequency.
The dipole has a figure of 8 directivity pattern, with the maximum sensitivity at right angles and in the same plane as the metal rods. End on it is not very sensitive. The front to back ratio is 0 dB or equally sensitive. If you look at the dipole end on it is omnidirectional.

The effect of the frequency or real channel number
It should be remembered that the gain is relative to a dipole at that frequency.

So if the field strength is 1 Volt/metre then a channel 6 (177.5 MHz) antenna has a dipole length of 0.842 m so the voltage from the dipole of this length tested at 177.5 MHz was 0.842 V.

Repeat this for channel 69 (816.5 MHz) the dipole length is 0.183 m. So its dipole output is 0.183 V with a 816.5 MHz signal.

So just by changing channels from 69 to 6 there is an of signal strength of  13 dB

So to get an antenna for channel 69 to give as much signal as one designed for channel 6 you need to add enough elements to increase the sensitivity of the antenna by 13 dB. Hence you pay for around the same amount of metal for the same voltage. It is just cut differently!

As a reminder there is usually an increase in power from the transmitter for the same coverage area of between 4 & 8 dB

Gain measured in dB
This is the logarithmic ratio of the signal power from the antenna under test to the signal power from a dipole at the test frequency. Usually the bigger the better. This is usually achieved by focussing the signal on the dipole.

Antenna Types
The most common types in TV reception are
Yagi, Log Periodic and Phased Array

Yagi
This antenna has a dipole which is connected to the down lead via a balun. The dipole is commonly folded.
A reflector is placed on the side away from the transmitter to prevent signals from the back being received and to reflect any signals that got past the dipole.
Directors are placed parallel with the dipole to focus the electromagnetic waves on the dipole. The greater the number of directors the more sharply focused the antenna becomes and the stronger the signal. Remember using the sun through a magnifying glass lighting the paper!

The entire antenna is connected to earth except the dipole. A ferrite transformer is used to keep the dipole above earth (this is a balun), since the receiver end of the cable is likely to be earthed.

Log Periodic
This antenna consists of cross connected dipoles. None of the antenna is earthed. It works by having a longer dipole behind acting as a short to signal from behind at that frequency. The dipole in front can have no function at all; act as a second dipole or as a director. This depends on the number of dipoles near that frequency. Since there are a large number of dipoles a linear transformer is used to obtain the standard 75 ohm output to match the cable. Since the antenna must not be connected to earth either the whole antenna is not earthed or insulators are used on each dipole.

This antenna usually has a very good front to back ratio, and medium gain depending on the frequency range required and the number of dipoles.

Phased Array
Assuming that all the other antennas are mounted horizontally, this antenna has an even number of dipoles mounted one above the other. Typically each of these dipoles is accompanied by a reflector. The vertical spacing is related to the frequency, as is the length of the dipoles. All of these antennas are earthed except the dipole.

Directivity in the horizontal direction is not great because there is only a dipole and a reflector. In the vertical direction the directivity is increased because of the stacking of the dipoles.

Quality of the signal

Carrier to noise ratio(c/n) usually measured in dB. I an indication of how strong the signal is compared to the noise. The noise may come from interference or the noise generated in an amplifier. This ratio can be improved by using a more directional antenna (One containing more elements)

The importance of this reading is that if the carrier and the noise are nearly equal, you have fallen over the digitial cliff and you will see either no signal message or pixellation and sound plops.

Noise figure is the carrier to noise ratio at the output (dB) - carrier to oise ratio at the input of an amplifier. This figure for modern masthead amplifiers can be as low as 1 dB. This is how much worse the amplifier is making the carrier to noise ratio.

Bit Error Rate If the carrier to noise ratio is low, then there will be a large bit error rate. This can also be caused by reflected signals particularly those nearing the main signal's strength. This is where antenna design is important as is using an antenna which is directional enough.

Comparison of Characteristics
Dipole: On its own is usually only used in indoor antennas and when bent into a near circle for boats. This is where we do not want directivity! These antennas are not very sensitive and some have in build amplifiers. The will pick up reflected signals and this gives ghosted images in analog and if it is bad enough will cause pixellation and sound plops. ie the Bit Error Rate (BER) will be high.

Yagi:They have a sensitivity which is related to the number of directors, but element for element more gain in a restricted range of frequencies, the range widens as the frequency to be received increases.

It is less likely to receive lower frequencies from power line interference, car ignitions and high powered radio transmitters. This is because only the dipole is connected to the down lead.

Log Periodic:This antenna has less sensitivity but can maintain it over a wide range of frequencies. The gain is related to the range of frequencies and the number of dipoles.

Phased Arrays:Since this antenna has better vertical directivity it is typically used for long distance and paths blocked by terrain and buildings. (Imagine looking at the sunset over the sea. What you see is a thin bright line on the horizon.) Reflected signals are usually not a problem in weak signals as they will be weaker.

Combined antennas: Your average bands 1-3, 4-5 antenna (Channels 2-12 + channels 28-69) consists of a Log periodic for bands 1-3 and a Yagi for bands 4 – 5.

Log periodics drop rapidly in sensitivity outside their design range which is why the older style combined antenna will not receive channels 11 & 12.

For Australian Digital TV no channel under channel 6 will be used so any sensitivity there makes you susceptible to interference causing pixellation and sound plops.

Band 4 European Antennas
These are designed to receive channels 21-35. In Australia channels 21-27 are not used for TV but for communications including UHF CB radio. This 56 MHz is not only not required but can cause pixellation and sound plops if the interference is strong enough.

Vertically Polarised Signals
If reflected signals are a problem, phased arrays are better than Yagis and Log Periodics. If you want long distance or diffuse signals then multi element Yagis are the way to go.

Advertising
An element is a reflector, dipole or a director. All are mounted at right angles to the boom. I see advertised an SF91 antenna, if you look at a picture how many elements do you get? 22!

Impulse Interference
This causes the picture to break up and plops in the sound.

It is usually caused by switching of electrical devices in the house, dirty powerline insulators (particularly after a long dry period then drizzle), petrol engines. This interference is picked up by the antenna, and the cable to the receiver. Spikes on the mains cable are an unlikely cause. So spike suppressor powerboards are usually ineffective.

Stopping this problem
Install an antenna according to your area. See
Get the best reception. Which transmitter and which antenna http://www.dtvforum....showtopic=12503
Use Quad shielded Coax cable and keep it away from electrical wiring. If it has to cross it keep it at right angles. Use F connectors.

The best way in areas where you still use analog channels 0-5A, is to keep digital and analog wiring separate.[/b]

Areas which use Band 3 (Real channels 6-12)
Many of you have bought expensive antennas which are designed to receive channels 0-5A. If you are not using it for analog, then feed the antenna into a GME Kingray TF160 High pass filter (http://www.gme.net.au/matv/index.php). You will have to purchase a cover as well.

Another option is to use a LT3014 diplexer from Jaycar (http://www.jaycar.com.au). Leave the band 1 input disconnected.

If masthead amplifiers are being used see near the end of this post.

If band 3 is not used in your area, then get the correct UHF antenna and forget the paragraph above!


Separation of antennas on the mounting pole.
The theory is at least 1/2 wavelength separation. This would be  around the length of the element which has the cable connected. If it is a log periodic, that will be the longest dipole.

All separations in the list below are in mm. For the lowest mounted antenna it is the roof particularly if it is metal.

Lowest edge of Band 3, Channel 6: 862
Lowest edge of Band 4, Channel 28:  285
Lowest egge of Band 5, Channel 35: 258

Channel 0: 3 333 mm
Channel 1: 2 679
Channel 2: 2 381
Channel 3: 1 765

FM radio: 1 714

If vertical polarisation is used I would double these figures.


Amplification
Masthead amplification is the best so that any signals picked up on the downlead is not amplified causing interference (pixellation and sound plops). They are necessary where there is low signal strength at the antenna and where long downlead is used. Splitter amplifiers are required where more than 4 receivers/STB/Videos are used.

Amplifiers can make the situation worse. Where strong signals are amplified with weak ones the weak signal will have more interference than with no amplifier. This also occurs with a corroded antenna.

To overcome the amplifier problem, the solution is to use only antennas designed for the range of channels required and to use an input filter to reject unwanted signals. Lastly the amount of amplifier gain may need to be reduced to prevent overloading.

See Get the best reception. Which transmitter and which antenna
http://www.dtvforum....showtopic=12503
for the recommended antennas and amplifiers for your area.

Difficult installations.
I have recommended a single masthead amplifier in the above posts. Where the signals are very weak, and more than one band is used you can get more gain by using two amplifiers at twice the price!

For example 1
band 3 and 4(+)
Band 3
Kingray MHV44HLG  The gains are B3 44 dB B4-5 10 dB Feed the B3 antenna into the V input
Band 4 & 4+
Kingray MHU44G The gains are B1-3 -1 dB, B4-5 44 dB
Feed the B4 or 4+ antenna into the U input. The next choice is to feed the output of the B3 amplifier into the V input of this amplifier. You get more gain the other way around, but 54 dB is a huge gain.

Example 2
band 3 and 5
Kingray MHV44HLG  The gains are B3 44 dB B4-5 10 dB Feed the B3 antenna into the V input
Band 5
Kingray MHU44B5G B1-3 -1 dB, B5 44 dB
Feed the B5 antenna into the U input. The next choice is to feed the output of the B3 amplifier into the V input of this amplifier. You get more gain the other way around, but 54 dB is a huge gain.

In both cases the power is supplied from a PSK08 on a receiver end of the downlead. There is a link to allow the power to be fed through the B4 or B5 amplifier to the B3 amplifier. Check the instructions. Also remove the link connecting the V & U inputs together.

Faultfinding

Bit Error Rates of more frequent than 1 e-4 cannot guarantee pixellation and chirp free reception.

This BER indicates the amount of errors found in the signal from the transmitter. The signal contains extra data for error detection and correction. This measurement in receivers is often called quality.

The problem is if the error rate is too high what is causing it?

The usual causes are;
1. Insufficient signal or excessive noise.
2. Delayed signals from either reflections or another SFN transmitter.
3. Additional signals from either impulse interference or intermodulation occuring in overloaded Masthead amplifiers or corrosion in the antenna.

How do you separate these factors.

1. The carrier to noise ratio (measured in dB) if this ratio is low then you need more signal from the transmitter. This can be achieved by one or more of the following methods.

Increase or vary the elevation of the antenna, change the antenna location.

Ensure the antenna design channel range matches that of the channels being used.

Minimise the loss from the antenna to the receiver through the use of the shortest low loss cable.

Increase the directivity of the antenna to focus more signal onto its dipole.

For longer distances you need to increase the capture area by reducing the vertical acceptance angle and and keeping a horizontal acceptance angle. (Think of a sunset as a thin horizontal line on the horizon).

Use a masthead amplifier to overcome cable losses.

2. A more focussed antenna (one with more elements is required). The amount of focussing is its "gain" in dB. A high front to back ratio may be required if you are on a line between the two SFN transmitters.

3. To reduce the level of impulse interference which will give sporadic high BER and low C/N readings, the following will help.

Use quad shielded RG6 cable and F connectors.

Ensure the antenna is only designed to receive the channels in use. Therefore in Australia no antennas designed to receive channels 0-5A

If a masthead amplifier is used it must have a filter on its input to pass only the channels of interest, otherwise the impulse interference will get worse.

In severe cases a 160 MHz high pass filter and even a braid breaker can be used at the receiver to minimise the interference picked up on the cable. This is also useful where AM, FM, two way radios, CB and amateur radio transmitters give trouble.

Where some channels are much stronger than others or all signals are strong, the gain of any masthead amplifer must be reduced because you will get interfering signals in the channels you really want.

So if you get a high BER and a low C/N you need to get more signal if C/N is high and the BER is poor then look at points 2 & 3. Point 3 is likely to cause rapid variations in BER and C/N

Note. The signal strength reading on STBs and cards is a measure of the Automatic Gain Control signal. This signal does not have a linear relationship  between the strength and the reading. Also it is not calibrated. Measuring meters are calibrated!


Questions on this post should be in this strand.

AlanH

<{POST_SNAPBACK}>



#9 Mr Antenna

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Posted 26 May 2005 - 10:58 PM

Hi
Just wondering if you have anymore detailed information on master head
amps in response to digital reception on band 5. Interested to find out
more information on better brands of amps and the total amount of ampliflication
allowed before pictures play up. Also doubling amps up is that sucessful
please let my know thanks
Matty

#10 alanh

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Posted 08 June 2005 - 09:37 PM

Matty,
Sorry for the delay because I must have missed your post.
For Band 5 I recommend Band 5
Kingray MHU44B5G with its Power supply PSK08
Kingray
It has two inputs a B1-3 (VHF input which has a very slight loss of -1 dB, the Band 5 input has up to 44 dB gain. This means it increases the voltage by a maximum of 159 times. So the input voltage must be small otherwise the output will overload. The advantage of this amplifier is that it only amplifies band 5 because it has in input filter.

If you need band 4 & 5 then use the MHU44G.

AlanH

#11 Fingers

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Posted 22 June 2005 - 05:52 PM

Hey Alan

any chance of you dropping up to Gympie (under the Sunshine Coast section in Qld) to put up me new antenna......?

hehe...

I can't even work out which way to point mine...... :blink: :P :P


Fingers.

#12 alanh

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Posted 22 June 2005 - 09:09 PM

Fingers,
I do not live anywhere near you. However, if you give me the name of the suburb in which you live I will give you the direction to point your antenna.

AlanH

#13 ewoud

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Posted 22 June 2005 - 09:38 PM

:ph34r: Dear mr AlanH
I live in Perth (Connolly) and have read your extensive information about antenna,s.
Being 65 + it goes a bit over my head.
Could you recommend an antenna man in the Perth area who knows all those things you write about and could give me advice and possible install the best antenna for my area ?

Regards
Ewoud

#14 CARLTON

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Posted 30 June 2005 - 04:04 PM

[size=7]Channels
TV Bands
TV band 1 channels 0-2. 45-70 MHz
TV Band 2 Channels 3-5 85-108 MHz, yes, this includes the FM radio band
TV channel 5a was only used in Australia and is being phased out
TV band 3 channels 6-12, 174-230 MHz, includes VHF trial radio DAB
TV band 4 channels 28-35 526-582 MHz
TV band 4+ channels 28-50, 526-687 MHz
TV band 5 channels 36-69 582-820 MHz

Bands 1,2 and channel 5A will not be used for digital TV anywhere in Australia.

Real Channel Number and Logical Channel Numbers
The receiving section of TV receivers work on the channel numbers indicated in TV Bands. If you wish to add an additional transmitter to repeat the program in an adjacent area you will have to use another channel, otherwise there will be interference between the signals. To use the same channel the transmitters must be at least 400 km apart. (Digital SFN excepted. SFN is a Single Frequency Network which has repeaters on the same channel in the same coverage area. The signals frome each transmitter must be made identical a point halfway between the transmitters. Does not work in analog.)

Since the metropolitan networks use their main transmitter's channel number as the network name (eg, 7,9,10) for digital TV they transmit a logical channel number This means that the receiver may be using channel 69 but the program name as displayed on the front of the STB may be 7 for example.
In installation the STB usually will show the real channel number, as well as the LCN

Logical Channel Numbers LCN
The network is identified by the LH digit displayed on the front of the STB. It is also used by the remote control for channel selection.
1 = TEN Network
2 = ABC
3 = SBS
4 = Miscellaneous (including Datacasting)
5 = TEN affiliates (eg. Southern Cross)
6 = Seven affiliates (eg. Prime, GWN)
7 = Seven Network
8 = NINE affiliates (eg. WIN, NBN, Impaja)

All digital transmissions are capable of being split to carry multiple programs. (eg ABC and ABC2)

Polarisation
For maximum received signal strength the receiving antenna should be parallel to the one on the transmitter tower. In Australia we use both horizontal and vertical transmitting antennas. This means we can put two transmitters on the same channel much closer together that if we only used one polarisation. In some areas of Australia band 3 transmitters are vertically polarised and band 4 & 5 horizontal on the same transmitter tower. eg. Canberra and Wide Bay.

Dipole the Most Basic Antenna.
Dipole
This is the most fundamental antenna. This is your typical rabbit’s ears antenna with the metal rods folded out into a straight line. Its total length is related to the frequency to be received.
Length (mm) = 15 000/frequency (MHz). As examples for channel 0, (48.5 MHz) the antenna should be 3 metres long. Channel 6 (177.5 MHz) 842 mm, channel 69 (816.5 MHz) 183 mm. If the above formula is used it will be a maximum for that frequency.
The dipole has a figure of 8 directivity pattern, with the maximum sensitivity at right angles and in the same plane as the metal rods. End on it is not very sensitive. The front to back ratio is 0 dB or equally sensitive. If you look at the dipole end on it is omnidirectional.

The effect of the frequency or real channel number
It should be remembered that the gain is relative to a dipole at that frequency.

So if the field strength is 1 Volt/metre then a channel 6 (177.5 MHz) antenna has a dipole length of 0.842 m so the voltage from the dipole of this length tested at 177.5 MHz was 0.842 V.

Repeat this for channel 69 (816.5 MHz) the dipole length is 0.183 m. So its dipole output is 0.183 V with a 816.5 MHz signal.

So just by changing channels from 69 to 6 there is an of signal strength of  13 dB

So to get an antenna for channel 69 to give as much signal as one designed for channel 6 you need to add enough elements to increase the sensitivity of the antenna by 13 dB. Hence you pay for around the same amount of metal for the same voltage. It is just cut differently!

As a reminder there is usually an increase in power from the transmitter for the same coverage area of between 4 & 8 dB

Gain measured in dB
This is the logarithmic ratio of the signal power from the antenna under test to the signal power from a dipole at the test frequency. Usually the bigger the better. This is usually achieved by focussing the signal on the dipole.

Antenna Types
The most common types in TV reception are
Yagi, Log Periodic and Phased Array

Yagi
This antenna has a dipole which is connected to the down lead via a balun. The dipole is commonly folded.
A reflector is placed on the side away from the transmitter to prevent signals from the back being received and to reflect any signals that got past the dipole.
Directors are placed parallel with the dipole to focus the electromagnetic waves on the dipole. The greater the number of directors the more sharply focused the antenna becomes and the stronger the signal. Remember using the sun through a magnifying glass lighting the paper!

The entire antenna is connected to earth except the dipole. A ferrite transformer is used to keep the dipole above earth (this is a balun), since the receiver end of the cable is likely to be earthed.

Log Periodic
This antenna consists of cross connected dipoles. None of the antenna is earthed. It works by having a longer dipole behind acting as a short to signal from behind at that frequency. The dipole in front can have no function at all; act as a second dipole or as a director. This depends on the number of dipoles near that frequency. Since there are a large number of dipoles a linear transformer is used to obtain the standard 75 ohm output to match the cable. Since the antenna must not be connected to earth either the whole antenna is not earthed or insulators are used on each dipole.

This antenna usually has a very good front to back ratio, and medium gain depending on the frequency range required and the number of dipoles.

Phased Array
Assuming that all the other antennas are mounted horizontally, this antenna has an even number of dipoles mounted one above the other. Typically each of these dipoles is accompanied by a reflector. The vertical spacing is related to the frequency, as is the length of the dipoles. All of these antennas are earthed except the dipole.

Directivity in the horizontal direction is not great because there is only a dipole and a reflector. In the vertical direction the directivity is increased because of the stacking of the dipoles.

Quality of the signal

Carrier to noise ratio(c/n) usually measured in dB. I an indication of how strong the signal is compared to the noise. The noise may come from interference or the noise generated in an amplifier. This ratio can be improved by using a more directional antenna (One containing more elements)

The importance of this reading is that if the carrier and the noise are nearly equal, you have fallen over the digitial cliff and you will see either no signal message or pixellation and sound plops.

Noise figure is the carrier to noise ratio at the output (dB) - carrier to oise ratio at the input of an amplifier. This figure for modern masthead amplifiers can be as low as 1 dB. This is how much worse the amplifier is making the carrier to noise ratio.

Bit Error Rate If the carrier to noise ratio is low, then there will be a large bit error rate. This can also be caused by reflected signals particularly those nearing the main signal's strength. This is where antenna design is important as is using an antenna which is directional enough.

Comparison of Characteristics
Dipole: On its own is usually only used in indoor antennas and when bent into a near circle for boats. This is where we do not want directivity! These antennas are not very sensitive and some have in build amplifiers. The will pick up reflected signals and this gives ghosted images in analog and if it is bad enough will cause pixellation and sound plops. ie the Bit Error Rate (BER) will be high.

Yagi:They have a sensitivity which is related to the number of directors, but element for element more gain in a restricted range of frequencies, the range widens as the frequency to be received increases.

It is less likely to receive lower frequencies from power line interference, car ignitions and high powered radio transmitters. This is because only the dipole is connected to the down lead.

Log Periodic:This antenna has less sensitivity but can maintain it over a wide range of frequencies. The gain is related to the range of frequencies and the number of dipoles.

Phased Arrays:Since this antenna has better vertical directivity it is typically used for long distance and paths blocked by terrain and buildings. (Imagine looking at the sunset over the sea. What you see is a thin bright line on the horizon.) Reflected signals are usually not a problem in weak signals as they will be weaker.

Combined antennas: Your average bands 1-3, 4-5 antenna (Channels 2-12 + channels 28-69) consists of a Log periodic for bands 1-3 and a Yagi for bands 4 – 5.

Log periodics drop rapidly in sensitivity outside their design range which is why the older style combined antenna will not receive channels 11 & 12.

For Australian Digital TV no channel under channel 6 will be used so any sensitivity there makes you susceptible to interference causing pixellation and sound plops.

Band 4 European Antennas
These are designed to receive channels 21-35. In Australia channels 21-27 are not used for TV but for communications including UHF CB radio. This 56 MHz is not only not required but can cause pixellation and sound plops if the interference is strong enough.

Vertically Polarised Signals
If reflected signals are a problem, phased arrays are better than Yagis and Log Periodics. If you want long distance or diffuse signals then multi element Yagis are the way to go.

Advertising
An element is a reflector, dipole or a director. All are mounted at right angles to the boom. I see advertised an SF91 antenna, if you look at a picture how many elements do you get? 22!

Impulse Interference
This causes the picture to break up and plops in the sound.

It is usually caused by switching of electrical devices in the house, dirty powerline insulators (particularly after a long dry period then drizzle), petrol engines. This interference is picked up by the antenna, and the cable to the receiver. Spikes on the mains cable are an unlikely cause. So spike suppressor powerboards are usually ineffective.

Stopping this problem
Install an antenna according to your area. Geographic Viewers' Forums Then see
Get the best reception. Which transmitter and which antenna
Use Quad shielded Coax cable and keep it away from electrical wiring. If it has to cross it keep it at right angles. Use F connectors.

The best way in areas where you still use analog channels 0-5A, is to keep digital and analog wiring separate.

Areas which use Band 3 (Real channels 6-12)
Many of you have bought expensive antennas which are designed to receive channels 0-5A. If you are not using it for analog, then feed the antenna into a GME Kingray TF160 High Pass Filter. You will have to purchase a cover as well.

If masthead amplifiers are being used see near the end of this post.

If band 3 is not used in your area, then get the correct UHF antenna and forget the paragraph above!

Separation of antennas on the mounting pole.
The theory is at least 1/2 wavelength separation. This would be  around the length of the element which has the cable connected. If it is a log periodic, that will be the longest dipole.

All separations in the list below are in mm. For the lowest mounted antenna it is the roof particularly if it is metal.

Lowest edge of Band 3, Channel 6: 862
Lowest edge of Band 4, Channel 28:  285
Lowest egge of Band 5, Channel 35: 258

Channel 0: 3 333 mm
Channel 1: 2 679
Channel 2: 2 381
Channel 3: 1 765

FM radio: 1 714

If vertical polarisation is used I would double these figures.


Amplification
Masthead amplification is the best so that any signals picked up on the downlead is not amplified causing interference (pixellation and sound plops). They are necessary where there is low signal strength at the antenna and where long downlead is used. Splitter amplifiers are required where more than 4 receivers/STB/Videos are used.

Amplifiers can make the situation worse. Where strong signals are amplified with weak ones the weak signal will have more interference than with no amplifier. This also occurs with a corroded antenna.

To overcome the amplifier problem, the solution is to use only antennas designed for the range of channels required and to use an input filter to reject unwanted signals. Lastly the amount of amplifier gain may need to be reduced to prevent overloading.

Geographic Viewers' Forums Then see Get the best reception. Which transmitter and which antenna
for the recommended antennas and amplifiers for your area.

Difficult installations.
I have recommended a single masthead amplifier in the above posts. Where the signals are very weak, and more than one band is used you can get more gain by using two amplifiers at twice the price!

For example 1
band 3 and 4(+)
Band 3
Kingray MHV44HLG  The gains are B3 44 dB B4-5 10 dB Feed the B3 antenna into the V input
Band 4 & 4+
Kingray MHU44G The gains are B1-3 -1 dB, B4-5 44 dB
Feed the B4 or 4+ antenna into the U input. The next choice is to feed the output of the B3 amplifier into the V input of this amplifier. You get more gain the other way around, but 54 dB is a huge gain.

Example 2
band 3 and 5
Kingray MHV44HLG  The gains are B3 44 dB B4-5 10 dB Feed the B3 antenna into the V input
Band 5
Kingray MHU44B5G B1-3 -1 dB, B5 44 dB
Feed the B5 antenna into the U input. The next choice is to feed the output of the B3 amplifier into the V input of this amplifier. You get more gain the other way around, but 54 dB is a huge gain.

In both cases the power is supplied from a PSK08 on a receiver end of the downlead. There is a link to allow the power to be fed through the B4 or B5 amplifier to the B3 amplifier. Check the instructions. Also remove the link connecting the V & U inputs together.

Faultfinding

Bit Error Rates of more frequent than 1 e-4 cannot guarantee pixellation and chirp free reception.

This BER indicates the amount of errors found in the signal from the transmitter. The signal contains extra data for error detection and correction. This measurement in receivers is often called quality.

The problem is if the error rate is too high what is causing it?

The usual causes are;
1. Insufficient signal or excessive noise.
2. Delayed signals from either reflections or another SFN transmitter.
3. Additional signals from either impulse interference or intermodulation occuring in overloaded Masthead amplifiers or corrosion in the antenna.

How do you separate these factors.

1. The carrier to noise ratio (measured in dB) if this ratio is low then you need more signal from the transmitter. This can be achieved by one or more of the following methods.

Increase or vary the elevation of the antenna, change the antenna location.

Ensure the antenna design channel range matches that of the channels being used.

Minimise the loss from the antenna to the receiver through the use of the shortest low loss cable.

Increase the directivity of the antenna to focus more signal onto its dipole.

For longer distances you need to increase the capture area by reducing the vertical acceptance angle and and keeping a horizontal acceptance angle. (Think of a sunset as a thin horizontal line on the horizon).

Use a masthead amplifier to overcome cable losses.

2. A more focussed antenna (one with more elements is required). The amount of focussing is its "gain" in dB. A high front to back ratio may be required if you are on a line between the two SFN transmitters.

3. To reduce the level of impulse interference which will give sporadic high BER and low C/N readings, the following will help.

Use quad shielded RG6 cable and F connectors.

Ensure the antenna is only designed to receive the channels in use. Therefore in Australia no antennas designed to receive channels 0-5A

If a masthead amplifier is used it must have a filter on its input to pass only the channels of interest, otherwise the impulse interference will get worse.

In severe cases a 160 MHz high pass filter and even a braid breaker can be used at the receiver to minimise the interference picked up on the cable. This is also useful where AM, FM, two way radios, CB and amateur radio transmitters give trouble.

Where some channels are much stronger than others or all signals are strong, the gain of any masthead amplifer must be reduced because you will get interfering signals in the channels you really want.

So if you get a high BER and a low C/N you need to get more signal if C/N is high and the BER is poor then look at points 2 & 3. Point 3 is likely to cause rapid variations in BER and C/N

Note. The signal strength reading on STBs and cards is a measure of the Automatic Gain Control signal. This signal does not have a linear relationship  between the strength and the reading. Also it is not calibrated. Measuring meters are calibrated!


Is there a digital antenna as opposed to an analog antenna?

Analog
Vision uses vestigial sideband and a pair of FM modulated carriers for Sound. Colouring signals are double sideband suppressed carrier.

So as the signal strength goes down you get more noise appearing as dots and the sound is generally unaffected until it drops out of limiting.

So for vision there is a gradual reduction in quality as compared to signal strength.

Secondly any reflected signals will appear to the left of the original signal so antenna directivity is used to remove these ghosts.

Impulse noise appears as rows of dots with the sound generally unaffected.


Digital
DVB-T uses 1705 of carriers spread over the 7 MHz bandwidth. This is the same bandwidth used for analog. It is like using parallel data, one carrier per bit.

Digital uses channels adjacent to analog ones, this includes high power transmitters. You cannot use adjacent analog channels because the interference is horrific. Digital receivers can reject the analog signals, but the analog TVs cannot. So digital is transmitted at a quarter of the power of the analog transmitters to prevent interference to analog signals.

Since the digital signal consists of 2 states, on or off, the signal can be limited like in FM. So you can just look for transitions at the half amplitude point. This means that as the signal level reduces there is no effect of the program quality until the demodulator comes out of limiting. The error correction will help to remove some errors.

If impulse noise is added to a digital signal errors occur, the will cause pixellation (picture breaks up in to blocks and either a gap in sound or a squark).

The effect of the above on antenna design
Since the amplitude of impulse noise from sparking in switches, power lines etc increases as the frequency decreases the Australian Broadcasting Authority has decreed that there will be no digital transmitters below 174 MHz (Channel 6) So your antenna should not be designed like many are to receive from 45 to 144 MHz (channel 0-5A) They pick up interference well and that metal work could be used to increase gain in the required channels above channel 5A.

Without signals below 174 MHz, masthead amplifiers can be set to a higher gain particularly if they have a bandpass filter in their inputs. Then it will only amplify the wanted signal and not overload on unwanted signals. This cause intermodulation distortion and interfere with the wanted signals.

As in good telecommunications practice you should use an antenna which is designed to cover the frequency range in use. This varies all over the country so see Get the best reception. Which transmitter and which antenna in the geographic forums at the bottom of the home page of the main forum.

Digital ready antennas are typically advertised in areas where digital is using channel 11 & 12 which is all capital cities (except Canberra and Darwin). These channels have been added since digital. So what has happened is that a lot of digital ready antennas are designed from channels 1-12,+ 28-35. These antennas are not ideal. Some add an F connector for a more reliable connection with less reflections.

Antenna gain is a issue because of the lower transmitted power and the pick up of impulse noise not only in the antenna but also in the downlead. Remember the effect of some impulse noise on analog TV is usually some intermittent or continuous rows of dots. So as long as the signal to noise ratio is adequate impulse noise will not be seen.

They are usually quite small on the screen. There is no effect on the sound. In digital the picture breaks up into much larger squares and the sound either is muted or produces a loud screech. You miss an important part of the plot!

Directivity may or may not be an issue. Since many carriers are used this makes each pulse longer. So short term reflected signals can be ignored by the receiver. Decoding will be perfect.
The ABA is now specifying Single Frequency Networks (SFN) in some areas of Sydney, Central Coast NSW, Newcastle, Melbourne, Gold Coast and Sunshine Coast.

Lastly the ABA has to make duplicates of all analog transmitters in digital and not use channels 0-5A. This can mean different antennas are required. Sometimes the polarisation changes as well. There are also a number of high powered sites using vertical polarisation for VHF and horizontal polarisation for UHF.

If you take the Darling Downs as an example all stations are in band 4+ (channel 28-50) with Southern Cross on channel 0. This antenna should be removed when converting to digital. In Toowoomba, all stations are in band 5 so no antenna changes should be required provided the installation is in good condition. The ABA has tried to allocate digital channels in the same band as analog ones so that a minimum number of households have to buy new antennas.

So yes there can be differences between digital and analog antennas particularly at VHF.

Questions on this post should be in this strand.

AlanH

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#15 CARLTON

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Posted 30 June 2005 - 04:14 PM

thanks for the insight on helping to receive digital tv more successfully !!!! i was just wondering i have a rover tda 6 digital and analogue field strength meter and it seems to be quite good but not totally sure on the ber readings and what they actually mean. i am aware that its a level of the amount of info received into stb and the errors have to be kept to a minimum . i have been informed that the best measure for ber is <10-8 is this true? and with other readings how do you translate? would appreciate some help thankyou!!!!!

#16 bellotv

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Posted 30 June 2005 - 07:41 PM

Carlton
Meters generally give two BER measurements,The first is the channel BER which is the BER of the signal as arrives at the input.This is sometimes referred to as B BER on some brands of meter as it is the BER BEFORE viterbi correction.

The second BER measurement is the post viterbi BER or PV BER sometimes referred to as A BER or AFTER VITERBI BER.

People are always looking for the magic pass/fail good /bad type of figures.

STB,s will just show a perfect picture with Channel BER as low as 1e-02 and a PV BER just under 1e-04 but these are absolute minimums.

A Channel BER of 1e-04 or higher would be a realistic value for reliable service.This would generally relate to PV BER of 1e-06 - 1e-08

#17 `felix`

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Posted 01 July 2005 - 12:06 AM

:ph34r: Dear mr AlanH
I live in Perth (Connolly) and have read your extensive information about antenna,s.
Being 65 + it goes a bit over my head.
Could you recommend an antenna man in the Perth area who knows all those things you write about and could give me advice  and possible install the best antenna for my area ?

Regards
Ewoud

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Did you get your antenna system sorted out? I also am in Perth - i had installed my antenna system myself and have had excellent result. PM if you would like some more information.

#18 CARLTON

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Posted 01 July 2005 - 05:21 PM

Carlton
Meters generally give two BER measurements,The first is the channel BER which is the BER of the signal as arrives at the input.This is sometimes referred to as B BER on some brands of meter as it is the BER BEFORE viterbi correction.

The second BER measurement is the post viterbi BER or PV BER sometimes referred to as A BER or AFTER VITERBI BER.

People are always looking for the magic pass/fail  good /bad type of figures.

STB,s will just show a perfect picture with Channel BER as low as 1e-02 and a PV BER just under 1e-04 but these are absolute minimums.

A Channel BER of 1e-04 or higher would be a realistic value for reliable service.This would generally relate to PV BER of 1e-06 - 1e-08

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#19 CARLTON

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Posted 01 July 2005 - 05:28 PM

thanks for your help bellotv appreciate it !!! is there anywhere where i can get more info on understanding the formulas for ber some of the readings are a bit strange to me like "1x10-4" etc

#20 alanh

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Posted 03 July 2005 - 03:47 PM

Ewoud,
I have been away without contact with the forum. I am now reading the 27 pages of new posts!
Have you read WA Geographic Viewers' Forum. WA subforum Select your geographic subforum and then open Get the best reception. Which transmitter and which antenna

Connelly is also subject to salt spray not only on antennas but also on the power lines which can cause the picture to break into blocks and the sound to chirp.

Depending on your location (can you see the Darling Range in the Bickley area) or not. If you can use one of the first recommendations for the Main transmitter, if not I would use the weak signal recommendations.

Any further posts should be in the Perth Viewers' forum.

AlanH

#21 apsilon

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Posted 12 August 2005 - 12:40 PM

I'm looking at trying one of the TF160 high pass filters. Given it's a "trade" filter it probably won't come with much in the way of instructions. I'm sure I can figure it out but one thing I want to check is should it be installed before or after the masthead amplifier?

I'd prefer to put it after to save getting up on/in the roof until I see if it actually works if that possible.

#22 bellotv

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Posted 12 August 2005 - 06:51 PM

HP filter needs to be installed before masthead to stop lower frequencies (and hopefully the impulse noise) from even being amplified and more likely in the case of impulse noise from overloading the amp.

Fitting it after the masthead is like shutting the gate after the horse has bolted.

#23 apsilon

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Posted 12 August 2005 - 06:58 PM

HP filter needs to be installed before masthead to stop lower frequencies (and hopefully the impulse noise) from even being amplified and more likely in the case of impulse noise from overloading the amp.

Fitting it after the masthead is like shutting the gate after the horse has bolted.

<{POST_SNAPBACK}>


But will it work (at all) after the amp? Just as a temporary test to save me the time of climbing up into the roof just to test. It's only the outlet that has the STB on it that I'm concerned with. If it works, even only as a reduction rather than complete cure I can then look at installing it permanently.

#24 mtv

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Posted 12 August 2005 - 07:04 PM

It will work to an extent, however if you have already amplified the offending signals/interference it may be totally useless.

#25 7seven

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Posted 01 September 2005 - 10:17 AM

is there an amp avail to cover the entire uhf range as mt tamborine on the gold coast hinterland trans mits from uhf 36 - sbs to uhf 68 nbn digital.

thanx


7seven