Co-ax V 300 ohm flat twin.

[Terry]

Despite the universal use of coax for Band 1, I recall an early Bush Band I/III set (TV24A?) which had a socket for 300Ω balanced feeder* for Band III yet the Band III converter fitted to a number of Bush receivers, including the earlier TV24, had a 75Ω coaxial input.

Also, a number of early Bush FM radios had 300Ω balanced inputs but, as we were on the north bank of the Thames 20km due north of Wrotham, none of our customers needed external aerials.

* I only ever saw these on the workshop bench - I don't think many were sold compared with the TV24 so I've no idea what happened in the customers' homes. Also I don't recall seeing a drum of 300Ω feeder amongst the large selection in the shop.

[colly0410]

Looks like I was barking up the wrong tree (or aerial) with my vertical polarisation/co-ax post, it was just a random thought I had.... I love to read about how different counties evolved different ways of doing things. I presume once something is done a certain way (i.e. vertical polarisation for main high power TX's in the UK) then that is the way it gets done. I'm thinking maybe one advantage of VP use in UK band 1 was that when the continental TX's fired up in the mid 50's using HP they didn't cause as much co-channel interference as they would have done if they were not cross polarised, just another random thought...

[Terry]

Nice theory but it doesn't work! We had transmitters using different polarities as well for the same reason!

Tacolneston and Mendlesham spring to mind.

[Synchrodyne]

There was an interesting commentary on screened vs. unscreened TV aerial feeders in Wireless World 1955 August.

   

An example was given in which a change to screened twin feeder effected a major reduction in interference pickup as compared with the previously used coaxial feeder.

On the face of it, it was quite convincing.  However, I wonder whether the problem was not so much the original use of coaxial feeder, but the lack of a balun at the aerial end, which omission appears to have been customary UK practice.

Not using a balun to connect a balanced aerial to an unbalanced (coaxial) feeder was a recipe for allowing interference picked up by the coaxial screen to find its way to the coaxial inner and thence to the receiver.  That was explained in the W.T. Cocking Wireless World 1950 December article mentioned earlier in this thread, and attached here.


  WW 195012 Aerial Feeders.pdf (Size: 857.95 KB / Downloads: 3)


Cheers,

Steve

[Mike Watterson]

I'm sure it was lack of a balun and/or a mismatch.

However screened twin feeder is very expensive compared to the normal ribbon 300 Ohm twin feeder and more expensive than coax.

[Synchrodyne]

In respect of relative feeder costs, these Belling Lee advertisements from 1947 provide some clues:

           


L.336 unscreened 75-ohm twin was £0.031 per yard

L.600 coaxial 75-ohm was £0.062 per yard

L.1221 screened twin was £0.088 per yard

L.1221 was most associated with the Eliminoise radio aerial system, but was also stated to be suitable for TV use, so it looks as if it was Belling Lee’s contender in the 75-ohm screened twin class.

   

BICC also offered a full set of the three types, but I have not seen any pricing information:

   


Cheers,

Steve

[colly0410]

Nice theory but it doesn't work! We had transmitters using different polarities as well for the same reason!

Tacolneston and Mendlesham spring to mind.

Read somewhere that Tacolneston used HP to cut down on co-channel from/to Rowridge & Kirk-O-Shots, but there was a Dutch or Belgian TX already using that channel/frequency with HP & they had to have a very sharp null towards Holland/Belgium to placate them.... I was thinking of the early 50's when there were just the 5 (or 6, was Rowridge classed as high power?) main high power VP BBC TX's on air. I know as the late 50's progressed into the early 60's band one became stuffed to the rafters with BBC VP & HP TX's. If a frequency band could explode then the UK band one would have done...

[Terry]

That sounds like bad planning somewhere unless plans for the transmitters on both sides of the North Sea were being made at the same time with neither team having any idea of what the other was up to. However, Tacolneston is well to the North East of its service area so power to the east would have been relatively low, anyway. Was the other transmitter at Goes in Zeeland, I wonder? That is right on the south-east corner of the Netherlands, so many aerials directed at Goes would have also been directed out across the North Sea beyond!

Mendlesham was also HP and was also on Ch. 11, co-channel with Chillerton Down.

[Mr Hoover]

The transmitter was at Liege in
Belgium on Ch E3 with horizontal polarisation.
There was also a Ch E2 main station
at Aalter in Belgium with HP and
a small relay somewhere near the
coast using VP on E2.
The BBC transmitter at Swingate on Ch B2 was very directional,aiming up into
the East Kent area.
Brighton also used Ch B2 with VP
and the service area was much more 
to the West than East.
There were very low power relays on Ch B3 HP at Rye and Bexhill but doubt if these were much of a co channel threat to anyone.

[colly0410]

IIRC Tacolneston planned to use band three but for some reason couldn't, so was shoehorned into band one after the continental co-channel station was already on air. Seem to remember reading somewhere that band three channels 6, 7, 12 & 13 could not be used till into the 60's, ITV was already using ch's 8, 9, 10 & 11 so no room for the beeb there. Belmont BBC1 eventually fired up on ch 13, I suppose if Tacolneston could have used that instead of ch 3 it would have done, but then what ch would Belmont have used? I sometimes think "could I have planned the channel plan any better?" But don't think I could have made any more improvements to the system, kudos to the channel planners for doing the job they did...