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Old 28th Mar 2007, 10:06 am   #1
Panrock
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Default 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?

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Old 28th Mar 2007, 10:16 am   #2
ppppenguin
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Default 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.
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Old 28th Mar 2007, 11:01 am   #3
Ray Cooper
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Default 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:
This implies an antenna gain of 3dB. How was this achieved? Was it by reflection from the tower?
No, they took great pains to ensure that there'd be no support-tower radiation.
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.

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It would be interesting to learn the gain figures of the other 405-line main transmitting stations too.
Well, all of the other high-power sites (Sutton, the Moss, Shotts, Wenvoe) used the same basic aerial configuration originally: two tiers of four dipoles spaced about 0.4wavelength apart, driven in phase rotation at 90degree phase intervals. (This gave what a contemporary technical write-up called a 'rotating wave' - very confusing term, nothing to do with circular polarisation. The term died a death very quickly, I'm glad to say). All these TXs used around 50kW peak-white of vision power, giving 100kW of ERP. The aerial gain was rather more than 3dB, but this extra gain was effectively lost in the sound/vision combinery, and the much longer lengths of feeder used above that at AP.

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.

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I wonder what advantage there was, if any, when the high input power/low antenna gain setup (like at AP) was used?
Viewed from the standpoint of later practice, precious little. The design of the earlier arrays tended to dictate that you couldn't have too much in the way of vertical aperture: the early Band I high power setups were vertically polarised, and it paid to leave clearance between the ends of the (rather long, due to the low frequency) dipoles in each tier. These early aerials were very heavily built, too, as there was a preoccupation with the effects of severe icing in olden times. So it was all rather heavy, mounted on central spines of not very great cross-section: too many tiers and the thing would probably just buckle under its own weight and wind resistance. In later years, they got round the problem by using more than four dipoles per level, mounted on a much stiffer cylinder of much greater diameter (compare the original Sutton Band I aerials - see piccies on http://tx.mb21.co.uk/gallery/suttoncoldfield/index.asp
(sorry for the plug) and the replacement aerials on p28 of BVWS bulletin 31/3, if you've got it.

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And what was the practice used later for UHF?
On all except the lower powered relays, the universal parctice was high aerial gain, and lots of it, and as low a TX power as you could get away with. TX arrays of 16 to 24-tiers high were/are commonplace. It's all so much easier to do at these higher frequencies - one wavelength is small compared with the cross-sectional size of the support structure.

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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Old 28th Mar 2007, 12:32 pm   #4
arjoll
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Default Re: Television transmitting aerials

Quote:
Originally Posted by ppppenguin View Post
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.
AFAIK a dipole has a gain of 2.2dBi - based on a 2.2dB difference between dBi and dBd for the aerials we're using for the two Life fm stations in our region. Stacking two gives you roughly 3dB more gain, and if you go directional you can get more. We're running a very simple array of 2x 3 element yagi at our Mid Dome FM site and developing 2kW from a 300W transmitter, with 3dB beam width about 135 degrees.

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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