The Alexandra Palace Service Area

[Panrock]

Yes, it's gratifying to see my map shows a reception patch in the Coventry area, though this is at the extreme fringe, equivalent to a signal strength of about 50µV/M.

Steve

[FERNSEH]

If a set like the Cossor 902 was available at the time the AP coverage map was drawn reliable reception would have been possible in the Coventry area.
Cossor 902 sensitivity was quoted as: The input signal required to give a 30 volt deflection should not be more than 67 db below 0.1volt at 45mcs.

The 1947 Cossor had five vision RF stages which employed the new SP6 (6AM6) pentode.

DFWB.

[Panrock]

I've been told by someone who knows far more about these things than me, that for a 75-ohm dipole operating at 45MHz, it just happens that the numerical value of field strength expressed as V/M lies very close to the value of the actual signal at the aerial terminals.

So in this case a 50µV/M field strength and a simple dipole aerial would theoretically result in a near-as-dammit 50µV signal at the aerial terminals. With an 'H' we'd get more.... somebody can perhaps fill us in with the gain of a typical vintage 'H'?

67dB down on 100 mV is about 45µv, so yes, our dear old Cossor should definitely have picked something up in Coventry!

Steve

[Nuvistor]

Is there are relatively easy way of working out from the mV/M, aerial gain, feeder loss and impeadance of the circuit what the terminal voltage at the receiver will be?
Frank

[Nuvistor]

From WW May 1953 "The Belling Lee Page" gain of an H aerial over a diople 5db, a 3 element aerial, i.e. H plus director 8db.
http://www.americanradiohistory.com/...ld-1953-05.pdf
Frank

[Panrock]

Originally Posted by Nuvistor

Is there are relatively easy way of working out from the mV/M, aerial gain, feeder loss and impeadance of the circuit what the terminal voltage at the receiver will be?

I don't know. I was told by this gent that the relationship is determined by frequency, aerial gain and terminating impedance. The way to work it out will be in an advanced text somewhere. Sorry I can't be of more help.

Steve

[P.Pilcher]

The telly and radio shop where, when a kid I used to buy some of my bits often proudly told me of the time in 1947 they put a mast on the roof of their premises and built a "green screen wonder" to receive good pictures from the transmitter at Alexandra Palace. This was at Coleshill near Birmingham.

P.P.

[Panrock]

Somebody over on the 'Holme Moss' thread has mentioned the MB21 site - an excellent resource.

From there, there is a link to THIS, a pre-war map of the sound transmitter coverage 'estimated for roof level (i.e. 10 to 15 metres above ground)'. The field strengths on my own map were for peak-white-vision at a reception height of 9.5 metres (31 ft).

Steve

[Panrock]

I've just heard from someone whose mother told him "they had television in Chippenham just before the war". Looking at the map, this does indeed seem feasible.

I'll try to get round to generating another map, for the crowd over on the "Holme Moss" thread sometime today.

Steve

[chipp1968]

AS I was saying to Steve earlier .

From Childhood my Mother told me that they had a Television at Chippenham prior to the war .

My Grandad worked as head engineer at Bulsons Radio shop . The set was almost certainly one of the HMV Marconi table sets with screen in the RH Corner as that is how Mum described it to me long before I ever saw one .

I know there was a very tall mast set up with the Aerial system on top .There was talk of the picture fading away and coming back .

Whether they actually got pictures before the war though or actually in 1946 we will never know . I know they had the set in the war as Mum remembers it being used for shortwave .

She said she remembered the lords mayor show ..I know there was a post war
showing of that early on ..

Certainly by 1948 they had a twelve inch Murphy .

[colly0410]

Would channel 1 be less affected by co-channel interference that the other band 1 channels? The system B (625 lines) channel E2 was on 48 to 54 mhz so wouldn't affect ch 1. I can't think of any other system that used 45 mhz except Ireland IA, & AFAIK that was never used..

[irishtv]

I can confirm Channel A in Ireland was never used for broadcast, it was only used in cable relay systems. It was originally assigned for Maghera, but concerns on interference to BBC TV from Divis on Channel B1 meant that Maghera was assigned Channel B instead. Channel C, the only other Band I System I channel was used for low power relays only.

[FERNSEH]

For pre-war and early post-war fringe area TV reception would necessitate having an efficient aerial system and possibly the addition of a preamplifier. Cossor made a three stage amplifier for the 54 and 1210 models.
Circuit diagram attached.
The pre-amplifier shown in the second attachment was removed from my 1938 Baird T23 TV receiver. The T23 employs timebase and power suppliers similar to those fitted in the T5. The principle difference is that in the later model the vision receiver unit is similar to the one fitted in the model T20. The unit employs three TSE4 secondary emission RF tetrodes.
It's possible the Baird T23 was used in a fringe area hence the fitment of the extra RF amplifier unit.
The third attachment shows the specifications of the Pye D16 TV receiver.
Vision sensitivity is quoted as 30microvolts. Even better than the Cossor 902.

DFWB.

[Synchrodyne]

Originally Posted by colly0410

Would channel 1 be less affected by co-channel interference that the other band 1 channels? The system B (625 lines) channel E2 was on 48 to 54 mhz so wouldn't affect ch 1. I can't think of any other system that used 45 mhz except Ireland IA, & AFAIK that was never used..

Insofar that there were fewer TV channels used that aligned more-or-less with channel B1, then yes, I think you’re right that it would have been less subject to long-distance interference than channels B2, etc. But on the other hand, such co-channel transmitters that could interfere probably had a bigger reach than was the case for the higher channels, so in that sense, B1 may have been more susceptible to interference. Approximately co-channel with B1 were channels F1 (when it was still used in the 1950s) and F2 (its lower portion, anyway). Outside of Europe, channel NZ1, 45 to 51 MHz, was probably the lowest frequency analogue TV channel used, and it surely would not have been an interference problem in the UK.

NZ1 was used for the main (100 kW erp) TV transmitters at Te Aroha, Wellington and Hedgehope (Southland). In the Wellington case, channel NZ1, with horizontal polarization, was evidently chosen because it was seen as having maximum “bendability” in what is very hilly terrain. I’d guess that channel NZ1 was chosen for Te Aroha because it had a very large intended footprint and the lowest possible frequency would provide maximum reach along say the southern lobe where it was not much limited by obstructing terrain. Apparently Wellington did interfere with Hedgehope at times, although not the other way around, but I don’t recall that Te Aroha suffered from Wellington interference.

I guess you could argue that in the UK case, from a network viewpoint, the London transmitter was not the best use of channel B1, which might have been better deployed for Wenvoe (hilly territory) or Holme Moss (for maximum reach). But then back in 1936, what later became channel B1 was all that was assigned for TV, and higher frequencies might have been a technical stretch. Band I itself was not defined until the ITU 1947 Atlantic City meeting, and the UK Band I TV channels were not announced until 1949, following a decision to adopt vestigial sideband transmission. And at the time channel B5 may not have been usable, as initially the UK had slightly truncated Band I to 41 to 66.5 MHz for TV use. (Maybe that's why B5 went to Wenvoe, the last of the five main transmitters to be built, by which time B5 had become available. In that case one could work back to Sutton Coldfied and deduce that it got B4 rather than B5, which probably have suited its terrain well enough, because B5 was not then available.) So by the incidence of history, London used channel B1 and the rest of the network had to be fitted in around it.

Cheers,

[SteveCG]

I got the impression that to the North of London the service area of Crystal Palace was about the same as AP - even though the ERP of CP was a lot higher. The extra distance and the earth's curvature no doubt explains this.

In the late 70's reception of CP on B1 in the Cambridge area was as much affected by radio comms from the Eastern Bloc as by ignition interference!

[Nuvistor]

Originally Posted by SteveCG

In the late 70's reception of CP on B1 in the Cambridge area was as much affected by radio comms from the Eastern Bloc as by ignition interference!

Do you refer to the "Woodpecker"
Frank

[SteveCG]

Frank,

Yes but also there were voice comms - I remember the nickname the 'monkey men' because they just spoke numbers. It added 'background' to some of the bands performing on Top of the Pops !!

[Nuvistor]

The numbers stations, didn't know they transmitted so high, but it could easily be an harmonic of the transmitter. I don't suppose whoever was transmitting them cared what harmonics were distributed, the woodpecker certainly covered short waves effectively.
More on topic I posted some photos of Band 1 aerials in Haverhill in the 405 aerial thread.
Frank

[Panrock]

Originally Posted by SteveCG

I got the impression that to the North of London the service area of Crystal Palace was about the same as AP - even though the ERP of CP was a lot higher. The extra distance and the earth's curvature no doubt explains this.

Yes, the earth's curvature would have formed an approximate natural limit under normal conditions. The advantage with CP was, within that limit, the areas with a weak signal were proportionately smaller.

Steve

[ntscuser]

Was there a lot of disruption for Londoners whose aerial had been pointed at AP but now had to be pointed at CP?

Wasn't there also a change from DSB to VSB at the same time?