Wednesday, 8 August 2018

70cms, television coverage and TVI

Well, my dual band 2m/70cms beam is now aloft, and with a bit of mixing of cables I am now on 70cms. The antenna has 7 elements on 2m and 12 elements on 70cms with a boom length of almost 3 metres. Thus it should in theory match my previous 2m beam with the added advantage of giving me 70cms as well. Claimed gain figures are 12.6 dBi for 2m and 14.6 dBi for 70cms. I might have got more gain from a separate 70cms antenna but another boom is not really practical on my mast.
New Antennas-Amplifiers 2m/70cm dual band beam with pre-amps above the 4m beam at GM4FVM
I have bought from Goran at Antennas-Amplifiers in Serbia before. While this antenna is not cheap, as a dual band antenna it is cheaper than two separate antennas. High quality materials such as driven elements made of brass are impressive. The extra weight of the strong boom raises the overall price once you add the postage, but I am very pleased with it.

Results so far are good and I will go into that in a later post. Well, "good" if you mean beating my previous 2m DX record by a very large margin. That tale is for another day.

Given that I was using some less than brilliant coax on 70cm and also that I have little faith in the Icom IC-7100 as a DX receiver on that band - though it is fine for what it is - I decided to fit a moderately priced 70cms mast head pre-amp.

My experience with the SSB Electronic 2m pre-amp showed me how useful these things could be. However, SSB Electronics products have a high price to go with the high performance. For 70cms I decided to trade down a bit. Another German company, SHF-Elektronik of Darmstadt make a more affordable range of pre-amps for both 2m and 70cms. The 70cms one, "Mini-70" looked just fine and that was the one I went for. To save cabling I used a Bias-Tee arrangement, this time from DK4MM.
Bias-Tee, SHF Mini 70 pre-amp and fitting kit

I know the theoretical benefits of having the amplifier at the mast head, though I am sure that in many situations it is a luxury many people will do without. I have been using shack pre-amps for years. However, as I was doing a new set-up putting the pre-amp at the mast head seemed worth it. The results are that all the signals I have copied so far disappear if I unplug the pre-amp.

Although the pre-amp has RF VOX switching I decided to fit a sequencer. I have bought the Down East Microwaves one. It is very compact in the supplied enclosure. Although in theory all I need is a breakout cable I have decided to mount it in an ABS box with the type of plugs I use on the outside of the box. I have all the plugs and sockets to do it now, but even though it is small it does not fit into my standard ABS box unless I leave the DB-15 plug case off it. Not wanting to be a bodger more than I have to, I have ordered a bigger box. The initial plan to mount it in a plastic food container which came with a Chinese meal has been rejected - but it was the perfect size.
DEM sequencer with 50 pence piece for size purposes
The mast head pre-amp does need to be sequenced, and some day I may add a linear amplifier too. Just with the pre-amp working in VOX mode I can see the SWR spike in the IC-7100 before the RF VOX works. The pre-amp is rated at 150W but even with my 20W I fear that one day the pre-amp will burn out due to the RF VOX delay.

Anyway except for the sequencer, which took a very long time to arrive in the UK and for the tax to be paid so that it could be delivered, everything was up and working. Then I found that my efforts to prevent TVI on 70cms had not gone far enough. Ooops. TVI was still there on 70cms (though not as bad as it was).

This is not really 70cms fault. For a variety of complex reasons the "Freeview" terrestrial TV signal we get locally is limited to a handful of channels. Not enough channels for the FVM household. Whilst more channels are available from the main transmitter at Selkirk, the minor ones are transmitted from there with 3dB less power. Our location is outside the Selkirk service area, and it is officially only served by the Eyemouth TV repeater, which does not carry most of the channels.

The underlying problem stems from the rather strange way the UK implemented the changeover to digital television. If you live outside the UK I suspect that this will sound very odd indeed. I'll not go into it further here. Suffice to say that after years of trying to get a reasonable signal from Selkirk I have given up on terrestrial TV. If like me you live in an area of the UK served by a TV repeater (rather than a main transmitter) your choices are very limited.

We could sometimes get a signal on the minor channels from Selkirk but for much of the time we had patchy reception. Often we could fill in the gaps with internet streaming. The final straw though was Mrs FVM trying to watch programmes recorded from Selkirk which had bad picture break up and finding that these programmes are not available via internet streaming. So we were sitting watching the television equivalent of a migrane headache appearing on the screen, with no alternative at hand. I mean, who doesn't want to watch "The Good Fight"? That Christine Baranski is a bit of favourite of mine ... not that my personal viewing is very important here.

So, simply to improve the number of channels we could see reliably we have installed a satellite system called "Freesat". In the absence of the correct wall bracket, my old T and K brackets which used to hold up the 6m beam and rotator are now in use to hold up the satellite dish. Given that the dish is very light weight it hardly needs such elaborate metalwork. The pole is a bit longer than necessary too but there wasn't the time to cut it. There might be room up there for a vertical on top someday ...
New Freesat TV receiving dish at GM4FVM on oversized brackets and with long pole...
More TV channels.

That's the reason. The fact that the previous very weak signal was overwhelmed by my 70cms signal was not the reason for spending good money getting us a satellite service. Oh no, amateur radio could not justify doing all that. Of course, this money was spent to improve the minor TV station reception. The fact that I can now run full power on 70cms on any beam heading (impossible before) has got nothing to do with it.

Oh no, nothing at all.

I wonder how many amateurs have been blamed for TVI when a weak TV signal was the real cause.




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