Jump to content


Photo

Home Made Yagi And Lpda Antennas For Brisbane Tv


  • Please log in to reply
7 replies to this topic

#1 JoeyJojo

JoeyJojo

    AV Forum Member

  • New Member
  • 2 posts

Posted 25 December 2014 - 10:08 PM

Greetings,
 
I thought I might share some info on a couple of TV antennas I recently made (one LPDA and one Yagi), for use at 2 locations near Brisbane.

If you need a new antenna, maybe this will inspire you to have a go at making your own (even if you just copy the exact dimensions I used). It was simple, cost was minimal, and it took less than a day to make 2 antennas.

Both designs had no problem picking up all channels in a fringe reception area. This is a location which is about 30km from the towers, with no line of sight (mountains), rated as "variable coverage" by MySwitch website, which only got some channels with an older style "analog" antenna (designed for the lower frequencies analog tv was broadcast on, with very long elements), and only when it was strategically positioned sitting on my deck at the top of my stairs (yes, you had to step over it to enter/exit the house...).

Long story short, I needed 2 new antennas, to replace poor condition old style "analog" ones. Being a tight arse, and since I had a couple of old antennas to butcher for parts, I decided to have a go at making my own.

Here's a few things I've learned, during my "Bachelor of Wikipedia in Antenna Design":
- In Brisbane, TV is broadcast in band 3, channels 6-12, VHF, 174MHz - 230MHz.
- There are no channels broadcast in UHF frequencies, so I only needed a VHF antenna, and not VHF/UHF combined.
- This made the design and construction easier.
- The 2 main types of antenna suitable for this purpose are LPDA and Yagi.
- Generally speaking, LPDA has a wider bandwidth, and a more even gain across the range.
- Generally speaking, Yagi has a narrow bandwidth, a higher gain, and higher front to back ratio.
- A design based on emperical data (that has not been optimised through testing or simulation) will work well, unless you have problems with reception at your location, in which case, you're probably better off getting a pro to select and install an antenna for you.

I guessed I would need a high gain antenna for my fringe location, so I decided that a Yagi would be best. However, I couldn't find any good info on Yagi bandwidth, except that the general concensus was "a few percent" is the usual. Since I need around 28%, I was concerned that it may not have enough bandwidth to adequately pick up the upper and lower frequencies (and I didn't want to add the complexity of making the elements split into the "X" shape). For this reason, I decided to make both an LPDA, and a Yagi.

 

Since both worked fine at the location I tested them (and the tv I tested them with didn't have a signal strength function), I can't really tell you which one worked better.

 

 

LPDA
====

Specs:
- Min. Frequency    174MHz
- Max. Frequency    230MHz
- Boom length    1481mm
- Elements    8
- Gain    approx 8dB

Cost:
- Less than $20

Materials:
- An old "analog" antenna -> aluminium tube, post clamp, and balun.
- 3m length of 19x19mm square hollow aluminium extrusion (1.2mm wall thickness), about $20 at hardware.
- 4mm aluminium pop rivets
- An old plastic chopping board (approx 10mm thick).

Tools:
- Hacksaw
- Tape measure
- Marker
- Drill
- Rivet gun

Description:
- I decided to make the "twin boom" design, which means all of the elements are alternately fixed to the upper and lower booms, and the booms are the electrical conductors (rather than a single boom, insulated elements, and criss-crossing conductors to electrically connect the elements).
- This means that all elements can be directly fixed/riveted to the booms, and do not have to be insulated.
- Elements are fitte through holes drilled through sides of booms, and riveted from top side.
- Then the booms are mounted together with an insulating material (in this case, the chopping board).
- One thing I would do differently, is to use a plastic that has good UV stability. Although the LLDPE chopping board doesn't have terrible UV stability, it's not going to last as long as the rest of the construction (if I get 5 years before it falls apart, I'll be very happy).
- I used the balun from the old antenna, along with the post clamp.
- Note: the post clamp needs to be electrically insulated from the upper and lower booms - see photos for more detail.
- Each element length shown below is for a single element (there are 2 of each element).
- The 3m length of extrusion was cut in half, which limited the total boom length to 1500mm.

Dimensions:
Element 1 (rear)    
- Length    431mm
- Position    10mm
Element 2
- Length    386mm
- Position    297mm
Element 3
- Length    346mm
- Position    555mm
Element 4
- Length    310mm
 -Position    785mm
Element 5
- Length    277mm
- Position    992mm
Element 6
- Length    249mm
- Position    1177mm
Element 7
- Length    223mm
- Position    1343mm
Element 8 (front)
- Length    199mm
- Position    1491mm


YAGI
====

Specs:
- Centre Frequency    202MHz
- Boom length    2226mm
- Gain    10dB
- Elements    8

Cost:
- About $30

Materials:
- An old "analog" antenna -> aluminium tube, post clamp, and balun.
- 3m length of 25x25mm square hollow aluminium extrusion (1.2mm wall thickness), about $25 at hardware.
- 3m length of D10mm round tube aluminium extrusion (1.2mm wall thickness), about $5 at hardware.
- 4mm aluminium pop rivets
- An old plastic chopping board (approx 10mm thick).

Tools:
- Hacksaw
- Tape measure
- Marker
- Drill
- Rivet gun
- Simple tube bender (home-made with plywood, using a drill press and angle grinder)

Description:
- YAGI is (mostly) more simple to make than LPDA -> single boom, one set of elements.
- Folded dipole adds slight complication, but I made mine with a home-made timber bend jig, and it turned out pretty good.
- The bend diameter for each end of the dipole is about 50mm.
- The jig bend die (round "pulley" in middle), was made by cutting a bit of ply into a rough circle, drilling through the middle, using a bolt as a mandrel to fit in drill press, and using an angle grinder to shape it round, and grind a groove around the edge for the tube to fit into (this holds the tube cross-section as it is bent around the die).
- The "clamp" and "slide" were similarly shapoed with a groove for the tube, with an angle grinder.
- Make sure you fill the tube with sand before you bend it, to ensure it maintains the round section. And empty the sand after bending...
- I decided to make the YAGI with the same number of elements as the LPDA (8 elements), which meant it was a bit longer, but that worked out ok, because the 3m extrusion didn't have to be cut in half for 2 booms like in the LPDA, and could be used for a longer single boom.
- The insulating clamp block holding the centre of the dipole was made by clamping 2 bits of chopping board together with a sheet of thick cardboard in between them, and drilling through the join. After drilling, the cardboard is discarded, and this allows the two halves of plastic to be adequately clamped around the aluminium tube of the dipole. See photo of assembly for more detail.
- I used the balun from the old antenna, along with the post clamp.

Dimensions:
Reflector (back)    
- Length    730mm
- Position    50mm
Folded Dipole
- Length    707mm (this is tip-tip length, not "unfolded" tube length. Unfolded tube length was approx 1465mm)
- Position    347mm
Director 1
- Length    645mm
- Position    458mm
Director 2
- Length    637mm
- Position    725mm
Director 3
- Length    630mm
- Position    1044mm
Director 4
- Length    623mm
- Position    1415mm
Director 5
- Length    617mm
- Position    1831mm
Director 6 (front)
- Length    611mm
- Position    2276mm

 

 

A picture of both antennas. Yagi is about 800mm longer than LPDA, although both have same number of elements. Bottom of the image looks skewed due to perspective...
bhlg7t.jpg
 

For some reason, I can't post more images, so here's some links:

 

LPDA:

http://i61.tinypic.com/sy3yab.jpg

 

LPDA, showing end of boom, and element fixing method:
http://i60.tinypic.com/35mmfbr.jpg
 

LPDA, showing how the upper and lower booms were fixed. The rivets I had were too short to go all the way through the chopping board, so they had to be counterbored:
http://i57.tinypic.com/2uerxqg.jpg
 

LPDA, showing post clamp fitted through plastic plates (electrically insulated from upper and lower boom)
http://i60.tinypic.com/2ug02m9.jpg
 

LPDA, showing old balun fitter to tip of booms:

http://i62.tinypic.com/2udz11z.jpg

 

LPDA installed:

http://i62.tinypic.com/2j2izyr.jpg

 

YAGI:
http://i62.tinypic.com/63zz21.jpg

 

YAGI, showing bottom of folded dipole. Free ends were bolted to balun.
http://i59.tinypic.com/fjkq3a.jpg

 

YAGI, showing top of folded dipole, and plastic clamp. Bottom of clamp was firsat riveted to the boom, then the top clamp was screwed through into the boom:
http://i61.tinypic.com/fwiquc.jpg

 

YAGI, showing post clamp (doesn't need to be insulated):

 

http://i61.tinypic.com/4vs10l.jpg


Edited by JoeyJojo, 25 December 2014 - 10:09 PM.


#2 alanh

alanh

    AV Forum Member

  • Senior Member
  • 12,680 posts

Posted 26 December 2014 - 01:28 AM

Joey,

Good on you. A TV is no good for comparing the sensitivity of antennas. 

You need the spectrum analyser that good antenna installers use. The gain should be compared with a dipole whose length is equal to 1/2 wavelength of the channel under test. The dipole needs to be at right angles to a line between it an the transmitter with no blockages in the path. The polarisation must be the same. The antenna under test should be similar to that and then rotated through 360 degrees in horizontal and vertical directions to get the directivity.

 

Alanh



#3 James T Kirk

James T Kirk

    AV Forum Member

  • Member
  • PipPipPipPip
  • 765 posts

Posted 26 December 2014 - 08:37 AM

JoeyJojo

 

There are all sorts of people in this world, some are creative, have natural talent, are inquisitive and have the potential to turn a need into reality.

Some other people are quick with advice but in reality could not produce a single original thought.

You are one of the former, absolutely very well done.

 

James



#4 Digital Penetration

Digital Penetration

    AV Forum Member

  • Member
  • PipPipPip
  • 477 posts

Posted 26 December 2014 - 10:21 AM

Well done! If you ever do it again, might want to get a simple plumber's pipe cutter for the elements. The sort used to cut copper tube, they work well with Aluminium, too. You twirl them around the tube, gradually tightening the cutter roller blade. Much nicer finish, stops that "rough-cut hacksaw edge" you get otherwise.

 

I made an old yagi into a 3 element VHF antenna when our restack happened a couple of months ago, getting rid of the UHF channels. But I'm only 7km from the transmitter, so a low gain antenna is fine for me. Used the QuickYagi program to calculate the element lengths and spacings required. Now the J-pole boom bolted to the house fascia doesn't sway in the wind anymore since the antenna is now much smaller.



#5 Malich

Malich

    AV Forum Member

  • Senior Member
  • 2,797 posts

Posted 26 December 2014 - 05:48 PM

Yeah, well done from me too! It's always nice to see someone thinking about stuff, experimenting, and actually trying things out - rather than pontificating from misremembered theory and naysaying based on misunderstood press releases.
 
(Don't worry too much about anything Alan says; he never has really understood the relationship between claims and evidence. Or, indeed, even what "claims" and "evidence" are - since he often conflates the two, and frequently gets them mixed up with "press release"...)
 

- There are no channels broadcast in UHF frequencies, so I only needed a VHF antenna, and not VHF/UHF combined.

 
Wellll.... :unsure: :huh:
 

- A design based on emperical data (that has not been optimised through testing or simulation) will work well, unless you have problems with reception at your location, in which case, you're probably better off getting a pro to select and install an antenna for you.

 
Very good point - in their standard form at least both Yagis and LPs are hard to cock up and, with average care, will come pretty close to their expected (if not theoretical) performance.
 
Of course part of the trick there is to know what will affect performance how, so you know what you can and can't get away with ;)!
 

- An old plastic chopping board (approx 10mm thick).
...
- One thing I would do differently, is to use a plastic that has good UV stability. Although the LLDPE chopping board doesn't have terrible UV stability, it's not going to last as long as the rest of the construction (if I get 5 years before it falls apart, I'll be very happy).

 
You sure it's LLDPE and not HDPE? Most of the ones I've seen and cut up are HDPE (though it may depend on whether you buy the "3 for $10" ones from Kmart, or the "3 for $5" ones from Waynee's or the Reject Shop ;)). Either way, the stuff is an antenna-maker's (& general hobbyists') dream - hard enough for light structural uses but easily worked (even if it is filthy stuff to machine or cut!), excellent and stable dielectric properties, good-to-excellent excellent resistance to weathering / chemicals / UV, etc. Hardest part is finding decent thicknesses these days; the manufacturers seem to shave a mm or 2 off each year...
 

- Make sure you fill the tube with sand before you bend it, to ensure it maintains the round section. And empty the sand after bending...


Surprising how many people have never learned this 'trick', and how many places where it should be mentioned don't mention it. And yes, I too have forgotten to empty the sand out before crimping the ends for termination... :(
 

- The bend diameter for each end of the dipole is about 50mm.


Just to note for anyone else: this isn't critical provided it's a fraction (say, <5%) of the wavelength (i.e. < ~10% of the driven element length). Usually not a problem at VHF, but can get tricky at high frequencies.
 

- Length    707mm (this is tip-tip length, not "unfolded" tube length. Unfolded tube length was approx 1465mm)

 
Good point. Far too many texts skip over that little fact...
 
But yeah, nice job and good pics. It's true that the canonical Yagi design has a fairly sharp & narrow bandwidth (as you say, typically quoted as 'a few percent') vs a LP (an octave or 2), and an 8-element yagi has a theoretical gain of ~13dB - but, realistically, once you chuck in all the real-world factors that conspire to reduce gain and broaden the bandwidth, you tend to find that actual gain is somewhere around 11dB (you quoted 10dB) and 6dB bandwidth is ~±10% (e.g. 180MHz-220Mhz)

 

So, long story short, of your expected/predicted +10dB gain it's probably still got +4dB gain or better at RF channels 8 & 11, and even likely some worthwhile gain at RF channels 7 & 12...



#6 JoeyJojo

JoeyJojo

    AV Forum Member

  • New Member
  • 2 posts

Posted Yesterday, 07:38 PM

Thanks for the comments.

 

Good on you. A TV is no good for comparing the sensitivity of antennas. 

You need the spectrum analyser that good antenna installers use. The gain should be compared with a dipole whose length is equal to 1/2 wavelength of the channel under test. The dipole needs to be at right angles to a line between it an the transmitter with no blockages in the path. The polarisation must be the same. The antenna under test should be similar to that and then rotated through 360 degrees in horizontal and vertical directions to get the directivity.

 

Hi Alan, thanks for the information. However, I think you may have missed the point. It was not my intention or requirement to perform analysis or optimisation on the antennas. I am not an installer, and I do not have access to expensive spectrum analysers to perform these tests.

 

In fact, my intention was quite the opposite: to provide some basic info for other people who may be interested in making their own TV antenna, that may otherwise be hesitant because they think that they do require expensive equipment to do so. In actual fact, for the majority of cases, you do not need to perform testing, or require expensive equipment, and I believe a signal strength function built in to many TVs would be sufficient to give a basic indication of the performance of the antenna (especially in a comparitive test, between 2 antennas).

 

JoeyJojo

 

There are all sorts of people in this world, some are creative, have natural talent, are inquisitive and have the potential to turn a need into reality.

Some other people are quick with advice but in reality could not produce a single original thought.

You are one of the former, absolutely very well done.

 

James

 

Hi James, thanks for your kind words. I love to learn new things, and I think the most rewarding part is being able to put that new knowledge to use. The internet is a wealth of information (I wish it was around when I went to school...), but it's a 2-way street: it only works when people give back too (even if it is just a basic DIY).

 

Well done! If you ever do it again, might want to get a simple plumber's pipe cutter for the elements. The sort used to cut copper tube, they work well with Aluminium, too. You twirl them around the tube, gradually tightening the cutter roller blade. Much nicer finish, stops that "rough-cut hacksaw edge" you get otherwise.

 

I made an old yagi into a 3 element VHF antenna when our restack happened a couple of months ago, getting rid of the UHF channels. But I'm only 7km from the transmitter, so a low gain antenna is fine for me. Used the QuickYagi program to calculate the element lengths and spacings required. Now the J-pole boom bolted to the house fascia doesn't sway in the wind anymore since the antenna is now much smaller.

 

Hi DP, good point on the pipe cutter. Would save a lot of sandpapering. Also good point on the smaller antenna requiring less bracing -> I may consider removing the front 2 elements on my Yagi to reduce the wind load, since it will be mounted on a very high pole.

 

(Don't worry too much about anything Alan says

 

I think he has good intentions, but probably not the best communication skills.

 

 

- There are no channels broadcast in UHF frequencies, so I only needed a VHF antenna, and not VHF/UHF combined.

 
Wellll.... :unsure: :huh:

 

Sorry, I forgot to mention 31 QCTV, which is UHF 529.5 MHz. However, in order to reduce the complexity of my antennas, I decided that I would live without it.

 

 


You sure it's LLDPE and not HDPE? Most of the ones I've seen and cut up are HDPE (though it may depend on whether you buy the "3 for $10" ones from Kmart, or the "3 for $5" ones from Waynee's or the Reject Shop ;)). Either way, the stuff is an antenna-maker's (& general hobbyists') dream - hard enough for light structural uses but easily worked (even if it is filthy stuff to machine or cut!), excellent and stable dielectric properties, good-to-excellent excellent resistance to weathering / chemicals / UV, etc. Hardest part is finding decent thicknesses these days; the manufacturers seem to shave a mm or 2 off each year...

 

Yeah, LLDPE from Ikea.

 

Thanks again guys.


Edited by JoeyJojo, Yesterday, 07:40 PM.


#7 alanh

alanh

    AV Forum Member

  • Senior Member
  • 12,680 posts

Posted Today, 09:36 AM

I would not worry about "Channel 31" as all community TV stations will be going off air in 2015 http://www.malcolmtu...of-community-tv

 

The signal strength meter on most TVs is just about as good as the oil light on a car for determining the oil level in the sump.

 

There are good installers who sometimes read this forum but don't post who have these meters.

 

 

 

Alanh


Edited by alanh, Today, 09:39 AM.


#8 MLXXX

MLXXX

    AV Forum Member

  • Senior Member
  • 5,440 posts

Posted Today, 11:41 AM

...
and I believe a signal strength function built in to many TVs would be sufficient to give a basic indication of the performance of the antenna (especially in a comparitive test, between 2 antennas).

I would agree.

 

The signal strength meter on most TVs is just about as good as the oil light on a car for determining the oil level in the sump.

The oil light is either on (inadequate oil level/pressure) or off (adequate oil level/pressure), whereas television signal strength indications are typically displayed on a multi-value scale, permitting an indication of comparative signal strength over a range of signal strengths, though not of course the absolute signal strength.

For example a cheap Soniq 42" set I acquired a few years ago will display signal strength on a scale from 0 to 100, and the type of modulation (64QAM, etc).