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ChrisF_1

Dvb-t Reception Issue - Mt. Wellington, Auckland

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I've been trying to get reliable Freeview HD for a while now. I've played around with my aerial position, height, etc. and now have excellent Prime analogue reception but am still getting periodic breakup of digital. I have no direct line of sight to the transmitter, also there is a run of HT power pylons in my signal path about 1 km from me.

The main issue is on 538Mhz, to a lesser extent on 570Mhz and no problems at all on 666Mhz.

I had a medium gain yagi which I've just replaced with a Hills Ultimax 36 phased array. This runs through 1 metre of coax into a Kingray F type fully screened masthead amp. (No VHF aerial) From there I have a cable run of 26 metres to an F type fully screened 4 way splitter. Two of the outputs from the splitter go to tuners in my HTPC, one goes to my TV and the last one is unused and fitted with an F type 75 ohm terminator. All of the cabling (including to the TV) is new Belden RG6 quad shield, is free from bends and kinks, and well away from power wires. I'm using PPC radial compression F type fittings.

I hired a signal meter (Promax Prodig 5) for a weekend and found that my aerial seemed to be in a dead spot for the 538Mhz transponder. The field strength was jumping all of the place and not peaking above 47dBuv. Oddly, I found by taking it off the 3 metre mast and mounting it 1 metre above the roof I was able to lock the following specs (at the aerial)

Frequency (Mhz) 538 570 666

Power (dBμv) 58 59 63.5

Carrier/Noise (dB) 35 36 38

MER (dB) 32.9 32.9 32.9

MER Noise Margin (dB)14 14.1 14.1

Post V.BER <1.0E-7 <1.0E-7 <1.0E-7

All of the above seemed stable and I thought I'd fixed my issues. I tried other spots on the roof and got similar figures. At the TV (after the masthead amp and splitter) I was getting the following:

Frequency (Mhz) 538 570 666

Power (dBμv) 64 67 75

Carrier/Noise (dB) >33 >32 >39

MER (dB) >33 >33 >33

MER Noise Margin (dB)14.2 14.1 14.1

Post V.BER <1.0E-7 <1.0E-7 <1.0E-7

The amp is on minimum power (approx 19dB) and running off a DC power pack.

I found the next day I was seeing random dips in the MER noise margin and MER on 538Mhz and these coincided with huge spikes in Post V.BER erros which most of the time are zero. The spikes go past QEF and off the scale.

I had to return the meter before I could do more testing but have been running signal quality logging on my HTPC and am regularly getting drops below 60%.

I thought I'd done everything correctly with regards to my set-up and my understanding was the my reception figures weren't too bad. Especially with no post V.Ber erros. I'm now stumped. I've tried boosting the amp to med and also to full power (34dB) at which stage I'm probably overloading the front end of the tuners but I'm getting desperate.

Is my issue indicitive of periodic multipath arriving outside the guard interval or am I getting some kind of interference?

Any help or advice greatly appreciated.

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ChrisF_1

As you have found, antenna position at UHF in blocked signal conditions can be important. Sometimes small movements in any plane can have a large impact on readings.

Those signal readings at the antenna are quite good.

Having the amplifier gain at minimum is also satisfactory.

Your installation methods seem fine.

CBER readings are probably better than VBER, I'm not sure what that would tell me, anyway.

Did you look at the spectrum of the faulty channel? The appearance of this can be a good indication of the mechanism causing the issue. I realise you no longer have the instrument.

It sounds to me like some kind of interference, eg impulse noise. What is the periodicity of these dips?

As a matter of interest, what happened when (if) you pressed the TV KEY on the promax? Blank screen?

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Chris_F,

The masthead amplfier, what is the brand and model? I am trying to determine if the amplifer is wide band or contains a high pass filter.

For any further comment, firstly read Get the best reception post. Please examine the transmitter list link and tell me which transmitter site is yours.

I also need to know which channels the available analog channels numbers are available to you.

Please message me with your address, so I can plot the profile of the path you are using.

Lastly you are the first NZ installation I have commented on.

AlanH

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ChrisF_1

As you have found, antenna position at UHF in blocked signal conditions can be important. Sometimes small movements in any plane can have a large impact on readings.

Those signal readings at the antenna are quite good.

Having the amplifier gain at minimum is also satisfactory.

Your installation methods seem fine.

CBER readings are probably better than VBER, I'm not sure what that would tell me, anyway.

Did you look at the spectrum of the faulty channel? The appearance of this can be a good indication of the mechanism causing the issue. I realise you no longer have the instrument.

It sounds to me like some kind of interference, eg impulse noise. What is the periodicity of these dips?

As a matter of interest, what happened when (if) you pressed the TV KEY on the promax? Blank screen?

Hi, thanks for your reply.

CBER at the aerial were:

Frequency (MHz) 538 570 666

CBER 3.00E-04 1.20E-04 4.50E-05

This varied a bit on all transponders so the above is an approximate average.

The dips/dropouts are irregular. We can watch TV for an hour and have no issue, then another night we can have four dropouts in an hour at the same time.

Pressing the TV button on the Promax gave me the channel info but no picture. I believe the model I was using couldn't decode H.264 for display.

Here's a picture of the 538 Mhz spectrum - Spectrum

Edited by ChrisF_1

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Chris_F,

The masthead amplfier, what is the brand and model? I am trying to determine if the amplifer is wide band or contains a high pass filter.

For any further comment, firstly read Get the best reception post. Please examine the transmitter list link and tell me which transmitter site is yours.

I also need to know which channels the available analog channels numbers are available to you.

Please message me with your address, so I can plot the profile of the path you are using.

Lastly you are the first NZ installation I have commented on.

AlanH

Hi, thanks for your reply.

The masthead amp is a Kingray MHW34FS.

My transmitter site is Waiatarua.

Sorry, but I'm not quite sure what you mean by "I also need to know which channels the available analog channels numbers are available to you."

I've PM'd my address to you.

Thanks

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CBER at the aerial were:

Frequency (MHz) 538 570 666

CBER 3.00E-04 1.20E-04 4.50E-05

This varied a bit on all transponders so the above is an approximate average.

The dips/dropouts are irregular. We can watch TV for an hour and have no issue, then another night we can have four dropouts in an hour at the same time.

Pressing the TV button on the Promax gave me the channel info but no picture. I believe the model I was using couldn't decode H.264 for display.

I had sort of figured from your first post that the amplifier was such as you have now confirmed. Leaving it at the minimum UHF gain setting is appropriate, as well as ensuring the combining link is disconnected.

Those CBERs look OK from my perspective, as does the spectrum of the channel in question.

Unfortunately, that picture tells little as you would need to be watching the spectrum when the problem occurs, and if it is only fleeting, it may not be seen owing to the latency of the display.

It sounds to me like random impulse noise problems. Possibly originating from the power transmission network, although I don't have a cogent explanation for only one channel being affected.

I didn't think your instrument would show pictures, I was curious in case some special software had been developed just for NZ. I know the Explorer II & Explorer II+ would decode pictures.

There is a NZ DTV forum here, which may be of some assistance.

However, another expert has arrived on the scene, so I'll leave for now.

Thanks for the opportunity to comment on your situation.

Marc.

Edited by M'bozo

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Chris,

You are 4 km from the Remuera transmitter bearing 310 degrees. There is a clear view

The first thing you should try is no amplfier at all, I would also insert a VHF/UHF diplexer. Feed the antenna into the UHF input. The diplexer is being used as a high pass filter to remove the effect of the arcing. It should be inserted at the input to the receiver.

I expect the amplifier is being overloaded by the interference which will make it affect the received signals.

AlanH

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I had sort of figured from your first post that the amplifier was such as you have now confirmed. Leaving it at the minimum UHF gain setting is appropriate, as well as ensuring the combining link is disconnected.

Those CBERs look OK from my perspective, as does the spectrum of the channel in question.

Unfortunately, that picture tells little as you would need to be watching the spectrum when the problem occurs, and if it is only fleeting, it may not be seen owing to the latency of the display.

It sounds to me like random impulse noise problems. Possibly originating from the power transmission network, although I don't have a cogent explanation for only one channel being affected.

I didn't think your instrument would show pictures, I was curious in case some special software had been developed just for NZ. I know the Explorer II & Explorer II+ would decode pictures.

There is a NZ DTV forum here, which may be of some assistance.

However, another expert has arrived on the scene, so I'll leave for now.

Thanks for the opportunity to comment on your situation.

Marc.

Hi Marc, thanks very much for your time.

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Chris,

You are 4 km from the Remuera transmitter bearing 310 degrees. There is a clear view

The first thing you should try is no amplfier at all, I would also insert a VHF/UHF diplexer. Feed the antenna into the UHF input. The diplexer is being used as a high pass filter to remove the effect of the arcing. It should be inserted at the input to the receiver.

I expect the amplifier is being overloaded by the interference which will make it affect the received signals.

AlanH

Hi Alan,

I've previously tried to get reception from the Remuera and Skytower transmitters (they're both in pretty much the same direction from here) but the signals were very weak. (I did remember rotate my aerial to V pol) I believe Remurea is only outputting "9 dBW or 8W compared to 52 dBW - 150 kW of Waiatarua" (copied from another forum)

During the aerial set-up process, when I had the aerial optimised (or so I thought) I took measurements at the TV end of the cable i.e. after the splitter but without the masthead amp and got the following:

Frequency (MHz) 538 570 666

Power (dBμv) 45 47.4 50

Carrier/Noise (dB) 26 26 30

MER (dB) 22 24.3 24.6

VBER 2.40E-07 <1.0E-7 <1.0E-7

I deduced from this that I needed the amp but must admit I didn't try running without it. I'll give it a go and see what I get.

Just to clarify a couple of other points from your post, when you're talking about arcing what are you meaning? Are you thinking that my issue is impulse interference as Marc had suggested?

When you talk about putting in a diplexer near the receiver, should this go at the input to the splitter? Do I need to terminate the VHF input with a 75ohm terminator?

Thanks again for your inputs.

Chris

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Chris,

I have been unable to get a list of TV transmitters containing all their technical & location details in one place. Other than what I just saw from you I have not seen power ratings. Finding exact transmitter locations was a problem until I found a site which contained Google earth locations.

Arcing produces impulse noise.

The diplexer does not need the V input terminated.

Once you have tried the diplexer suggestion I made and if you found you have insufficient signal, you could try putting this diplexer between the antenna and the masthead amplifier as a trial.

There are further amplifier options if the above is not successful.

I would like to know where to find transmitter powers of NZ DTV transmitters.

AlanH

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Chris,

I have been unable to get a list of TV transmitters containing all their technical & location details in one place. Other than what I just saw from you I have not seen power ratings. Finding exact transmitter locations was a problem until I found a site which contained Google earth locations.

Arcing produces impulse noise.

The diplexer does not need the V input terminated.

Once you have tried the diplexer suggestion I made and if you found you have insufficient signal, you could try putting this diplexer between the antenna and the masthead amplifier as a trial.

There are further amplifier options if the above is not successful.

I would like to know where to find transmitter powers of NZ DTV transmitters.

AlanH

Hi Alan,

Thanks for the clarification. I'll try your suggestions over the next few days and report back.

The info on power output came from a reply posted in a NZ DVB-T forum but was not a compete list. The transmission sites here are managed by Kordia They may be able to provide you with the info you're after.

Chris

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Chris,

If the interference is in band and on channel, no amount of filtering will get rid of it.

The interference will preferably need to be stopped at the source.

I forgot to mention in my previous post, that one way I use to track down impulse interference from electric fences, power lines, and some rather esoteric sources at times, is a small portable am radio tuned off channel, and listening to see if there is correlation between noise heard and picture breakup. Then comes the fun of sniffing out the source.

RSM NZ has a database for transmitter & other information. Try looking for "prism". Cute name, btw.

Marc.

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Just to add something else into the mix, I've just been looking on Google Earth. I've marked the signal path (yellow line) with my house on the right of the picture. (Pic 1) There are some tall trees marked with "A," 270 metres from my house. Are these likely to be causing random diffraction issues with the signal where I am? My house is lower lying than the one in front of me at the street's edge.

There's another largish tree (Pic 2) very near my house (marked with "B") but the direct signal path just clears to one side of it. Is is close enough to cause problems?

Finally, the HT pylons are in fact only 500 metres away and run along the signal path for around 400 metres.

Thanks for the additional info Marc. I just need to try and find an AM radio!

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Chris F,

Which transmitter site are you trying to receive? The path is not in the direction I suggested.

AlanH

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Chris F,

Which transmitter site are you trying to receive? The path is not in the direction I suggested.

AlanH

Hi Alan, the only one I can get decent signal strength on is Waiatarua. The others (Remuera and the Skytower) although much closer are very low power and consistently break up. Waiatarua is the main transmitter for Auckland, all of the other sites are infill to deal with reception deadspots due to the hilly topography here.

Chris

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As part of trying to explore all avenues I contacted Hills here in NZ for any tips. They seemed stumped by my issue and very kindly sent one of their tech experts out to check my installation. He could find nothing wrong with anything I've done and took independent readings of my signal quality etc. confirming that it was well within the minimum specs for good reception. He too concluded that my issue must be interference.

It took me a little while to get hold of a diplexer (F type - fully screened) but I've now been testing with it for a few days and it does seem to improve the quality of the signal. I'm running the aerial straight down to the diplexer near the receiver with no amp or splitter in circuit. With the diplexer in the drops in quality seem less frequent and less severe based on logs of 20 hours at a time.

I've also dug out an old AM radio and with it off tune, went for a walk up the road. As I neared the power pole at the top of our driveway the level of static increased significantly and was detectable across the whole AM band. This persisted up the road and around the corner where the power lines are directly in the path of our aerial. It was even noticeable in our house with the radio near the mains wiring. As a cross check I did the same test at my Dad's house in another part of Auckland and there was virtually nothing picked up by the radio.

Is it possible there's a dodgy connection or insulator on one of the lines which is occasionally arcing and causing my interference issues? My understanding was that this was not likely to affect UHF but could cause problems on HF and VHF?

Unfortunately the interference is too sporadic to sit watching the TV with the radio on to see if the breakups coincided with audible noise spikes on the radio.

Again, any advice greatly appreciated.

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Is it possible there's a dodgy connection or insulator on one of the lines which is occasionally arcing and causing my interference issues? My understanding was that this was not likely to affect UHF but could cause problems on HF and VHF?

Chris,

Short answer, yes.

Generally speaking, the interference reduces as frequency increases. However, the harmonics of what is basically a spark gap transmitter, might be occasionally high enough in level at UHF to cause problems.

If the interference can not be tracked down, or dealt with, improving the signal to noise ratio of the receiving system, by experimenting with the antenna location, might be a way of overcoming it.

This type of interference, imo, can be difficult to quantify, unless the measuring instrument has the ability to display RS uncorrected errors. The interference can be so fleeting, it may not show up on other measurements.

(If the Promax decoded H.264, you might have been lucky enough to see picture breakup, since the explorer doesn't have a RS uncorrected function)

If it is the power line, it should be possible to go out at night, and see if the arcing is visible, with the unaided eye, or a pair of binoculars. It should then be a power supply authority responsibility to rectify this.

There could be another interference mechanism at play, but at least if one source is reduced or eliminated, it may make it easier to track the problem down, if it still exists.

(eg one bizarre one I struck here in Tassie years ago when working in land mobile comms was tracked to a loose coach screw holding the 2 metal stays of the cross arm to the wooden pole. Tightened the coach screw, problem went away.)

Marc.

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Chris,

Short answer, yes.

Generally speaking, the interference reduces as frequency increases. However, the harmonics of what is basically a spark gap transmitter, might be occasionally high enough in level at UHF to cause problems.

If the interference can not be tracked down, or dealt with, improving the signal to noise ratio of the receiving system, by experimenting with the antenna location, might be a way of overcoming it.

This type of interference, imo, can be difficult to quantify, unless the measuring instrument has the ability to display RS uncorrected errors. The interference can be so fleeting, it may not show up on other measurements.

(If the Promax decoded H.264, you might have been lucky enough to see picture breakup, since the explorer doesn't have a RS uncorrected function)

If it is the power line, it should be possible to go out at night, and see if the arcing is visible, with the unaided eye, or a pair of binoculars. It should then be a power supply authority responsibility to rectify this.

There could be another interference mechanism at play, but at least if one source is reduced or eliminated, it may make it easier to track the problem down, if it still exists.

(eg one bizarre one I struck here in Tassie years ago when working in land mobile comms was tracked to a loose coach screw holding the 2 metal stays of the cross arm to the wooden pole. Tightened the coach screw, problem went away.)

Marc.

Marc, thanks again for your inputs.

OK, an update. I contacted Radio Spectrum Management and reported a suspected power line interference issue. They seemed fairly sceptical that this could affect the digital UHF signal but nonetheless sent out an investigator. He's found a very noisy 11Kv pole up the road from us which, in his opinion would be definitely bad enough to cause issues to nearby VHF reception. This pole is well to the right of my signal path and I picked up the noise on the AM radio on the pole at the top of my drive which is to the left of the signal path. This means then that I've got a nice arc of radiating power lines in front of my aerial in all directions. The bad news is that although he's reporting it his week, he thinks it's unlikely to be fixed before the end of Feb.

I'm now keeping my fingers crossed that this is the problem and it be all ok when the maintenance is done.

In the meantime I'm keeping the diplexer in as quality results are significantly better than when it's out of circuit.

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Chris,

Thanks for the update.

The power of harmonics from arcing decreases with frequency, however with so many high voltage arcs, the total power will be high enough to cause problems. How is your FM and AM reception?!

I originally suggested a diplexer because they are cheap, however if the problem is bad enough, you may wish to try a filter.

There is no doubt you are on to the cause. I would be interested to know the ultimate result, if you get the time, I would like to know if the lower powered transmitters become receivable.

Alanh

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Chris,

Thanks for the update.

The power of harmonics from arcing decreases with frequency, however with so many high voltage arcs, the total power will be high enough to cause problems. How is your FM and AM reception?!

I originally suggested a diplexer because they are cheap, however if the problem is bad enough, you may wish to try a filter.

There is no doubt you are on to the cause. I would be interested to know the ultimate result, if you get the time, I would like to know if the lower powered transmitters become receivable.

Alanh

Hi Alan,

I don't listen to AM at all and FM very infrequently so can't comment. The problem has been so sporadic on the TV that it's only with me logging the quality for 20 hours at a time that I've been able to see if changes made have been beneficial. It's been a frustrating process but also a learning one and I'm very grateful that the tips passed onto me from this forum seem to have tracked down the culprit. (He says with fingers crossed!)

I had wondered whether a filter would be worth a try. Hills have a high pass in line one (FC658089) but I can't find any specs on it. My thinking is that this will be more effective than the diplexer but probably not a complete fix if the interference is spiking into the UHF band from time to time. Any thoughts?

I've actually borrowed the diplexer and need to return it after Christmas. So with a fix to the power pole not due until the end of Feb, either I need to buy a diplexer or if a filter is likely to be better I'll go that way instead.

I will post a final outcome when I have one. I'll try the other transmitters then again too but am not too hopeful. From everything I've read on them they are only good for a few blocks away and designed to fill specific known holes.

Thanks again

Chris

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