TV interference from the continent

[ppppenguin]

This used to be a routine problem in southern England in the days of analogue TV. Casued by high atmospheric pressue and temperature inversions. It was even worse in the days of VHF 405. With digital TV I thought this was all in the past but I was wrong: https://www.bbc.co.uk/news/technology-50945421

I haven't seen the effects, but like most digital problems I suspect there's a very fine margin between working perfectly and a complete mess.


When designing any transmitter network the engineers have to make assumptions about propagation and weather. If you design for everything to work come hell or high water then you've probably restricted the number of channels you can use. If you allow for too much network capacity then the wrong kind of weather will cause trouble too often.

I wonder if the relinquishing of Band V has made things worse.

[Richard_FM]

I read that one of the home tapers who audio recorded many now missing episodes of Dr Who lived in Exeter & sometimes had problems with the sound from French television breaking through when the atmospherics were right.

[Old Sparky]

Many years ago, when I was staying in Letchworth, Herts. whilst on my Electronics training course, during an atmospheric 'lift', French TV sound broke in over BBC1, whilst the pictures were disturbed by interference from the French transmissions. This of course was back in the analogue days, when BBC & ITV were using 405 lines on VHF Channels, and French TV used 819 lines.

[ppppenguin]

819 line signals would display on 405 sets with 2 pictures side by side. Both were +ve modulation. The sound couldn't be received at the same time as it was on a wildly different frequency to 405 sound.

[Terry]

Ah yes - memories of seeing two 819 line test cards side by side on a 405 line TV!

Being only about 20 miles from Crystal Palace and Croydon, Sporadic E didn't cause us any problems but when we got the first dual standard sets with a dual standard IF strip, we had great fun if we had one in the workshop.

The first was the Bush TV115L, the end of the TV115 run and its successor, the TV125. No UHF tuners until much closer to the BBC2 launch but ideal for VHF 625 line reception! Anyone familiar with these sets will know that the IF strip is vertical. The bottom two sections of the system switch connected HT and IF input to the relevant tuner so it was a simple matter to stick two screwdriver blades between the appropriate connecting tags to do the conversion which, of course, could easily be reversed instantly if needed!

Reception always seemed to rotate in a clockwise direction. Norway first thing, then moving on to Russia, then to West Germany by late morning. Afternoons were virtually always Spanish TV.

We once had virtually continuous reception from TVE one Saturday afternoon - lots of sport, of course, but not a single trace of a bull fight!

[colly0410]

Ah yes remember the days of BBC1 on channel 4 from Sutton Coldfield being totally wiped out by Sp E, we'd watch a snowy but better than nothing BBC1 on channel 13 from Belmont as that was never affected being band 3. I remember being a young kid living in Skegness & BBC1 (presume it was Holme Moss on channel 2) was unwatchable nearly all the time, this was before Belmont fired up...

[jimmc101]

I believe that the Rowridge  (On the Isle of Wight) is the only main transmitter to use dual polarisation to alleviate this problem.
When it was horizontal polarisation only, I used to suffer from this at some time every summer.  Touch wood no problems since turning the aerial to vertical.
Analogue was bad enough with severe patterning but digital disappeared completely.
I'm located north of Portsmouth behind Portsdown hill, so the aerial points towards France.
It is strange to see a random selection of horizontal and vertically polarised aerials all pointed in the same direction.

Jim

[colly0410]

I suppose if they'd foreseen the co-channel interference problems that Rowridge had they could have used vertical polarisation from the get go. But I don't think they expected it to be such a big problem as it was, & still is for some with horizontal aerials. Good solution to the problem to go dual polarisation me thinks...

[colly0410]

What was the worst/best UK band 1 channel for foreign Sporadic-E interference? I know channels 2 & 4 were bad as I experienced the interference when I lived in Skegness (ch2) & Nottingham, (ch4) but can't remember seeing any patterning when I lived in Chester-Le-Street that was on channel 5, but not far from Pontop Pike. I'm thinking channel 1 would only be troubled by French TV's 819 system channel 2 which was on almost the same frequency, none of the 626 line systems (B & C) where near channel 1. I'm wondering how channels 3 & 5 were affected by 625 signals with positive/AM sound & negative/FM sound coming in?

[Mike Watterson]

Co. Clare in Ireland had 625 Line on Band I from 1962 till 1999 and I believe there was interference from Norway once.

This week I was testing a 3 m whip for 70MHz and after VNA tests went on air. As expected it couldn't be used to transmit on 50MHz ( too close to a half wave for the FC40 tuner which is bypassed on 70Mhz). However a Western Sahara Amateur station (approx 50.15x I think) was busy with UK stations that I couldn't make out. He was 5 & 3, very clear (SSB). I also heard a Norwegian that day. So obviously North/South propagation. The aerial was on a "workmate" type bench clamping a 1.5m pole as a mount and only about 2m from the gable end of the house near the middle.

After twenty years of an FT101ZD as main station rig I splashed out on an FTdx10 last September, but not used it much yet. The approximately 3/4λ whip (306 cm) with 3off 1m radials deliberately has no matching at the base so it can be used as an HF vertical on the FC40 remote "tuner". A transformer match for 6m (50MHz) works, but then 70Mhz and HF doesn't work. Putting an exact lenght (possibly 1/4λ?) coax between the whip and the tuner makes 50MHz sort of work (likely about 700V RF at whip base at 100W on 50 MHz) . It works OK for 3.5MHz. The 1.8MHz would need a bigger base loading coil than most ATUs provide. Maybe the answer is remote relay switches when it's put on the chimney. Contact rating would be challenging.