impulse interference: masthead filter worth trying?

[delback]

Hi, my first post on this forum (and of course it's about a problem).

Our Freeview reception is generally good. Rooftop Group A aerial pointing at Crystal Palace (about 20 miles away), Labgear masthead amp, CT100 downlead. We have a Topfield TF5800 which reports consistent 99%/100% signal quality on all MUXes, although signal strength is perhaps a tad low (typically around 65-70%).

We get occasional pixellation and sound dropout. I believe this is probably impulse interference. Might a masthead filter (like the MHF2137 on http://www.maxview.co.uk/acatalog/MASTHEAD_BANDPASS_FILTERS.html) help? Does impulse interference from things like passing traffic and other motor/ignition sources typically lie outside the UHF range?

 

[mrfreeview]

Hi, my first post on this forum (and of course it's about a problem).

Our Freeview reception is generally good. Rooftop Group A aerial pointing at Crystal Palace (about 20 miles away), Labgear masthead amp, CT100 downlead. We have a Topfield TF5800 which reports consistent 99%/100% signal quality on all MUXes, although signal strength is perhaps a tad low (typically around 65-70%).

We get occasional pixellation and sound dropout. I believe this is probably impulse interference. Might a masthead filter (like the MHF2137 on http://www.maxview.co.uk/acatalog/MASTHEAD_BANDPASS_FILTERS.html) help? Does impulse interference from things like passing traffic and other motor/ignition sources typically lie outside the UHF range?

Firstly don't worry about your signal strength but in any case you don't actually know the signal strength that you are receiving as percentage is part of a known quantity and you don't actully know the quantity.

Secondly you don't say what type of group "A" aerial you have. Is it a dipole contact (the coaxial cable terminated directly onto the dipole) or does it have a balum fitted? A balum type will help to reduce impulsive noise.

At 20 miles from Crystal Palace I am also supprised that you need an amplifier - do you know its gain? I am at 37 miles from Crystal Palace and can receive DVB-T without an amplifier - however of course fitting helps the fade margin factor!

Also if you think the impulsive noise is from passing traffic have you tried tilting your aerial upwards about 10-20 degrees?

Saying all of that I had problems with intermittent impulsive noise and my aerial is a group "A" bench mark 1, is tilted upwards slightly and I found that fitting a A/E diplexer so using the diplexer as a Group "A" band pass filter before my amplifier was the solution.

P.S. mine also fitted with CT100

I hope my post has been of use

 

[aekostas]

is tilted upwards slightly

How does one achieve that? Is there a standard fitting that allows it? I suffer from trees and need retuning as they go in/out of leaf.

Thanks!

 

[mrfreeview]

How does one achieve that? Is there a standard fitting that allows it? I suffer from trees and need retuning as they go in/out of leaf.

Thanks!

On the bracket that fixes the aerial to the mast you should find that one of the fixing bolts is via an angled slot in the bracket so that you can position the aerial at other then 90 degrees to the mast.

 

[delback]

Many thanks for your comments:

Secondly you don't say what type of group "A" aerial you have. Is it a dipole contact (the coaxial cable terminated directly onto the dipole) or does it have a balum fitted? A balum type will help to reduce impulsive noise.

To be honest I can't remember the make. It's a 48-element (the type with 12 "X-shape" pieces). As far as I remember the cable terminates directly onto the dipole. I'll have to get it down to check.

Am I correct in thinking that a balun turns the signal into a balanced one, with two out-of-phase copies thus giving some common mode noise rejection? (I understand balanced audio, but RF is a bit of a mystery to me). What I don't understand is how a balanced signal can go down a signal COAX cable - does one of the pairs run down the shield? And presumably if I do fit a balun, a matching one is needed at the other end to convert back to single-ended?

At 20 miles from Crystal Palace I am also supprised that you need an amplifier - do you know its gain? I am at 37 miles from Crystal Palace and can receive DVB-T without an amplifier - however of course fitting helps the fade margin factor!

It's a 10dB amp. About a year ago I called in a local aerial specialist. He said that we happen to live in a difficult reception location (Carpenders Park, South Watford) and was surprised it was as good as it was. It was he who suggested getting a masthead amp. It's installed, but I'd say it hasn't really helped that much - siganl strength went up a couple of percent, quality was 99/100% before fitting the amp anyway.

Also if you think the impulsive noise is from passing traffic have you tried tilting your aerial upwards about 10-20 degrees?

No, but I'll give that a go.

... I found that fitting a A/E diplexer so using the diplexer as a Group "A" band pass filter before my amplifier was the solution.

Sorry, what's an A/E diplexer? Is it something that splits a mix of group A and E into two band-limited signals?

 

[mrfreeview]

Many thanks for your comments:


To be honest I can't remember the make. It's a 48-element (the type with 12 "X-shape" pieces). As far as I remember the cable terminates directly onto the dipole. I'll have to get it down to check.

Am I correct in thinking that a balun turns the signal into a balanced one, with two out-of-phase copies thus giving some common mode noise rejection? (I understand balanced audio, but RF is a bit of a mystery to me). What I don't understand is how a balanced signal can go down a signal COAX cable - does one of the pairs run down the shield? And presumably if I do fit a balun, a matching one is needed at the other end to convert back to single-ended?

The dipole is a balanced device with a natural impedance of 300 ohms.

The coaxial cable is 75 ohms unbalanced so of course there is an impedance mismatch if directly connected to the dipole, leading to reflections and as you say potential for noise. The impedance of the receiver is a nominal 75 Ohms and so a further balun is not needed.

It sounds like your aerial could verywell be wideband - Wideband is never going to be needed for Crystal Palace and the performance of a wideband aerial is with respect to forwards gain at its worse at the lower end of the UHF band - just in the band that Crystal Palace transmits.

A physically smaller sized group A aerial would give just as much if not better forwards gain in Group A over any wideband.

It's a 10dB amp. About a year ago I called in a local aerial specialist. He said that we happen to live in a difficult reception location (Carpenders Park, South Watford) and was surprised it was as good as it was. It was he who suggested getting a masthead amp. It's installed, but I'd say it hasn't really helped that much - siganl strength went up a couple of percent, quality was 99/100% before fitting the amp anyway.?

So perhaps its not adding much value, but it will be adding noise to your signal.

Sorry, what's an A/E diplexer? Is it something that splits a mix of group A and E into two band-limited signals?

an A/E diplexer allows signals from one aerial being recieved in group A to be combined with the signals from another aerial being received in Group E onto a single coaxial cable. In reality what you have is: on the Group A input to the device the signal is fed via a low pass filter with the filter high frequency cut off being somewhere at channel 37 and then a Group E input into the device with the signal fed via a high pass filter with the filters low pass cut off at about channel 39. The outputs from the two filtered sources are then combined.

How I have used the diplexer is bascially connected backwards and so as just a low pass filter. I have taken the signal from the aerial and feed this into the diplexers output (the connection for the diplexers combined signal output) then place a 75 ohm resistor on the Group E input ad take my output from the group "A" input - hope that makes sense!

But a group A band pass filter should be just as good. I only used a diplexer backwards as it was cheaper and was easier to get my hands on.

 

[delback]

The dipole is a balanced device with a natural impedance of 300 ohms.

The coaxial cable is 75 ohms unbalanced so of course there is an impedance mismatch if directly connected to the dipole

Ah, right. Now I understand.

It sounds like your aerial could verywell be wideband

No, it's definitely a Group A aerial, although I've just taken another look and it's 32 element (8 groups of 'X-shape' parts) rather than the 48 I said before.

But a group A band pass filter should be just as good. I only used a diplexer backwards as it was cheaper and was easier to get my hands on.

But given I have a group A aerial, would a bandpass filter actually do anything?
[edit - sorry, I've just re-read your first response and see that you found a filter on a group A aerial does help, so may try that as well]

I think I'll try tilting the aerial upwards and fitting a balun as a first step.

Thanks again for your help.

 

[HeadBanger]

Before tilting the aerial fit some ferrite rings on the coxial cable (nearest to the receiver) like these. I've fitted a few of these and they do seem to have helped quite a bit.

Since fitting only the noisiest of passing mopeds causes any breakup on a weak mux.

 

[delback]

Before tilting the aerial fit some ferrite rings on the coxial cable (nearest to the receiver) like these. I've fitted a few of these and they do seem to have helped quite a bit.

Since fitting only the noisiest of passing mopeds causes any breakup on a weak mux.

I've got a couple of clip-on ferrites in my box of odds & ends, so I'll give them a try. Thanks for the tip.

 

[mrfreeview]

I think I'll try tilting the aerial upwards and fitting a balun as a first step.

well actually you can't "fit a balun" as such to an existing aerial. The aerial is either designed and built with a Balun or the coaxial wiring will go straight onto the dipole - with all the mismatch that it causes!

I suggest that you try and identify your actual aerial, find its maufactuer and then look up the specification on the WEB.

FYI: If you have really good eyes or a telescope at the front of the aerial on the rod that the reflectors (it forms the main structure of the aerial) are fitted there should be a plastic bung. If this is RED it indicates a GROUP A aerial.

O and just one other point noise could be coming into your decoder via the mains supply. And as you have a masthead amplifer that has a power supply connected to the mains if may just be getting in through that from the mains.

One other thing that I have found was that if I put my multimeter on AC volts and measured from the sheath of the coaxial cable (actually going to the aerial) with respect to earth I was getting about 150v ....but of course the current was bascially nothing... this I found was coming from the sheath connection of the decoder...no doubt through a switched mode power supply in the decoder. I earthed that down as well!

 

[delback]

well actually you can't "fit a balun" as such to an existing aerial. The aerial is either designed and built with a Balun or the coaxial wiring will go straight onto the dipole - with all the mismatch that it causes!

OK, understood. So if my aerial doesn't have a balun and I wanted one, it would be necessary to change the aerial, right?

FYI: If you have really good eyes or a telescope at the front of the aerial on the rod that the reflectors (it forms the main structure of the aerial) are fitted there should be a plastic bung. If this is RED it indicates a GROUP A aerial.

I do know for sure that it's group A, because I bought it from a local aerial supplier specifically for Freeview from Crystal Palace, and I asked for a group A one. It does have a red bung. I just can't remember the make/model. I do recall that it *isn't* an Antiference or Blake.

Perhaps I should just change the aerial anyway: presumably for a standard 1 benchmarked one with a balun?

O and just one other point noise could be coming into your decoder via the mains supply. And as you have a masthead amplifer that has a power supply connected to the mains if may just be getting in through that from the mains.

I've heard the mains-borne interference theory before. (Incidentally, the masthead amp is powered from the PVR (a Topfield 5800) - there is no separate power supply for it).

Many thanks for your help.

 

[aekostas]

On the bracket that fixes the aerial to the mast you should find that one of the fixing bolts is via an angled slot in the bracket so that you can position the aerial at other then 90 degrees to the mast.

Thanks

I will get the installers to check next time they are around, but they told me it's "as high as it goes". It's a DAT45 and it has its own (10 deg, I think) adjustment; they may have missed the bracket trick.

Or is it not standard for these brackets?

 

[mrfreeview]

Thanks

I will get the installers to check next time they are around, but they told me it's "as high as it goes". It's a DAT45 and it has its own (10 deg, I think) adjustment; they may have missed the bracket trick.

Or is it not standard for these brackets?

I have installed a couple of Televes DAT45 and the fixing bracket that comes with the antenna to fit it to a mast has the option to set it so that the antenna can be pointed "slightly" upwards. If I recall you could actually set (I don't know why you would want to!) the aerial at 45 degrees above the horizontal.

I hope my post has been of use

 

[mrfreeview]

OK, understood. So if my aerial doesn't have a balun and I wanted one, it would be necessary to change the aerial, right?.

Yes I believe so.

I just can't remember the make/model. I do recall that it *isn't* an Antiference or Blake.

Perhaps I should just change the aerial anyway: presumably for a standard 1 benchmarked one with a balun?

Well that is the 1,000,000$ question isn't it - will it make things any better?

Myself I would before embarking on replacement at least try and find out what I was dealing with. Perhaps even a trip back to where purchased for assistance - if only to ask "what did you sell me"!?

There appears to still be only one antenna in Group A with CAI bench mark 1 acreditation. The Blake DMX-10A which is what I have fitted.

http://www.blake-uk.com/page/aerial_benchmark/aerial_dmx

It was my selection because of it bench mark 1 status and I knew that the performance of a group A antenna for receiving signals in just group A would outshine the performance of any wideband.

You should note that the bench marking process only deals with the electrical performance of an antenna and does not take account for physical construction - but the quality of construction by Blake is good - but perhaps not, in my view, quite as good as Televes!

I've heard the mains-borne interference theory before. (Incidentally, the masthead amp is powered from the PVR (a Topfield 5800) - there is no separate power supply for it).

Perhaps if you haven't done so already check that the supply that the Topfield can deliver is suffient to power your masthead. I think you have already stated that adding the masthead amp didn't really improve signal strength.

 

[delback]

Many thanks once again for your help. Sorry for the delay in responding - it was half-term last week and I've been away.

Myself I would before embarking on replacement at least try and find out what I was dealing with. Perhaps even a trip back to where purchased for assistance - if only to ask "what did you sell me"!?

I may well do that, but in the meantime I have seen a picture of an Antiference DX8A, and it looks suspiciously identical to the one I have. So perhaps it was some kind of OEM version that the aerial installer sold me from his trade stock (it came in a plain brown cardboard box)? If it is a DX8A, then according to Antiference it does have a balun.

There appears to still be only one antenna in Group A with CAI bench mark 1 acreditation. The Blake DMX-10A which is what I have fitted.

That's the one I was considering. Question is, for receiving Group A from about 25 miles away, will it be better or worse than a Televes DAT75? The chap two doors down the road has a DAT75 and tells me he has no problems with Freeview. (I realise the DAT75 is a wideband aerial).

And then the lady across the road has what looks like a "traditional" UHF aerial (simple cross-pieces rather than "X-shape" elements) and she says her Freeview reception is fine. The difference here is that my aerial points over tha road towards Crystal Palace, whereas hers is on the other side of the road, so maybe mine is vulnerable to passing ignition interference and hers isn't.

Perhaps if you haven't done so already check that the supply that the Topfield can deliver is suffient to power your masthead. I think you have already stated that adding the masthead amp didn't really improve signal strength.

The amp is a Labgear PUM111. The Topfield supply is 5V, while the Labgear PSU is 12V. I tried both supplies, and according to the Topfield's signal strength display, both gave the same reading (which was in turn a few percent higher than without the amp). My guess is that the amp uses a commodity chipset that is happy to run from a wide range of supply voltages. I know for certain that the Topfield is powering it: if I switch off the Topfield's DC supply, there is no signal at all. I therefore chose to use the Topfield supply rather than the Labgear one in order to avoid another pair of F-connectors in the signal path.

 

[mrfreeview]

I may well do that, but in the meantime I have seen a picture of an Antiference DX8A, and it looks suspiciously identical to the one I have. So perhaps it was some kind of OEM version that the aerial installer sold me from his trade stock (it came in a plain brown cardboard box)? If it is a DX8A, then according to Antiference it does have a balun.

At http://www.antiference.tv/high-gain-dx8-0 you can read about the DX8

That's the one I was considering. Question is, for receiving Group A from about 25 miles away, will it be better or worse than a Televes DAT75? The chap two doors down the road has a DAT75 and tells me he has no problems with Freeview. (I realise the DAT75 is a wideband aerial).

And then the lady across the road has what looks like a "traditional" UHF aerial (simple cross-pieces rather than "X-shape" elements) and she says her Freeview reception is fine. The difference here is that my aerial points over tha road towards Crystal Palace, whereas hers is on the other side of the road, so maybe mine is vulnerable to passing ignition interference and hers isn't.

The Televes DAT75 has only about a 3dB increase on forwards gain over the DAT45.

What you can do with the DAT series is fit the Margin Raising Device (MRD) rather than the passive dipole. The MRD is powered from a PSU located in the house and for all purposes operates like a masthead amplifer except that it is located right behind the diople so is less susceptible to noise.

Antiference quote the DX8A with a forwards gain of 14.2dBi

Televes quote the DAT45 with a forwards gain of 16.5dBi

Televes quote the DAT75 with a forwards gain of 19dBi

Blake quote the DMX10A with a forwards gain of 14.65dBi

However the gain on the DAT wideband is not linear and the figure quoted is at the best point in the antenna's performance which is at about channel 62.

For the forwards gain in Group A on the DAT series, if I recall correctly from memory, (the slope graph is provided in Televes documentation on the www) is about 4dB less. So the DAT75 forwards gain would in Group A be about 15dBi 0.8dB better than the DX8A - so nothing!

However if the MRD is also fitted it will give another 13dB of gain. His this guy with the DAT75 got the MRD? - if so there will be a power supply for it in the house.

The DAT75 is really a big aerial, as you would have seen, and personally I would never consider this if I could manage with the DAT45.

Mathematically I think the DAT75 with MRD in Group A should give a forwards gain of about 28dBi

and the Antiference DX8A Plus a 10dB Masthead amplifier should give a comparable forwards gain of about 24.2dBi

4dB difference, just over twice power.

Increase the masthead with the DX8A to 13dB and they are both just about the same - from a forwards gain perspective.

If the noise is being introduced into the downlead, that is the level of impulsive noise is higher than the level of the signal then increase of the signal level in the cable could be the solution - so to improve the Signal/Noise ratio. - Fit a bigger amplifier at the masthead!

Also if you have unscreened wall plates upgrade these, solder the centre core of the coaxial to the center core of the BellingLee plug.

If the noise is being picked up by the aerial then tilt the aerial up, add a filter before the amplifier. Other option could be to try another location on your house for mounting the aerial.