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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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28th Mar 2007, 10:06 am | #1 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
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Television transmitting aerials
In another thread Ray Cooper pointed out that the effective radiated power (peak-white vision) from Alexandra Palace was 34Kw, not 17Kw. Thanks Ray. I'd seen 34Kw listed in post-war publications but hadn't worked out how this was done. This implies an antenna gain of 3dB. How was this achieved? Was it by reflection from the tower?
We hear also that Crystal Palace used low input power with high antenna gain. It would be interesting to learn the gain figures of the other 405-line main transmitting stations too. I wonder what advantage there was, if any, when the high input power/low antenna gain setup (like at AP) was used? And what was the practice used later for UHF? Steve
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28th Mar 2007, 10:16 am | #2 |
Retired Dormant Member
Join Date: Dec 2003
Location: North London, UK.
Posts: 6,168
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Re: Television transmitting aerials
Antenna gain is acheived by concentrating the available power over a smaller solid angle. An isotropic radiator gives equal radiation over the entire sphere. A dipole has a 6dB(?) gain wrt isotropic radiator since there are nulls in the radiation pattern. If you can restrict the unwanted radiation up to the sky and down to the ground you can get more gain. If you can restrict the radiation to some points of the compass you can get even more gain.
I'm not an expert on antennae but I think the AP ones were stacked dipoles which concentrated the beam vertically. Antennae are reciprocal for TX and RX so they give the same gain in either direction. |
28th Mar 2007, 11:01 am | #3 | ||||
Retired Dormant Member
Join Date: Jan 2007
Location: Porthmadog, Gwynedd, UK.
Posts: 199
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Re: Television transmitting aerials
Interesting article on the CP Band I aerials: go here:-
http://www.bbc.co.uk/rd/pubs/archive/index.shtml - and get Engineering Monograph No.23. Quote:
The AP design was, to late eyes, a weird setup: but it worked well notwithstanding that. Unlike all later arrays, the upper half of the aerial was used for vision radiation only: the lower half, for sound radiation. All subsequent aerials combined the sound and vision and then split the resultant to feed the various stacks: this is what gives the vertical beamwidth reduction mentioned by ppppenguin. This vertical beamwidth reduction didn't occur at AP. I suspect that where the extra gain came from, was the fact that wire-cage reflectors (driven, rather than parasitic as is more usual these days) were used, between the radiating dipoles and the tower: these minimised the radiation towards the support-tower, and gave you the sort of aerial gain expected from an 'H' type dipole+reflector. Quote:
The medium-power sites tended to use 5kW Marconi TXs., but rather more in the way of aerial gain: this often manifested itself as more tiers to the aerial array. Quote:
(sorry for the plug) and the replacement aerials on p28 of BVWS bulletin 31/3, if you've got it. Quote:
For example, Sutton was running 50kW engine power for 1000kW ERP: Waltham, 10kW power for 250kW ERP: the Wrekin, about 6kW engine power for 100kW ERP. |
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28th Mar 2007, 12:32 pm | #4 | |
Dekatron
Join Date: May 2006
Location: Invercargill, New Zealand
Posts: 3,440
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Re: Television transmitting aerials
Quote:
If they developed 34kW EIRP from 17kW transmitter power then the 3dBi gain is very modest indeed. I did read in another thread that the tx was 5kW, which makes more sense, and not difficult to get from a few stacked dipoles. Another matter confusing issues is that it appears some authorities quote "ERP" which appears to be base on dBd, not "EIRP" as used in NZ which is dBi. We struck this with the Italian company Aldena who built our aerials - they used the dBd gain of the yagi as the starting point, and it was only while setting up that we picked that we weren't only putting out the 1.5kW they said we would be. EDIT: Have now read to the end of the thread and see that a lot of my post is not so much wrong, but just more 'modern' thinking than CP! Last edited by arjoll; 28th Mar 2007 at 12:38 pm. Reason: Really must finish reading threads before posting.... |
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