"Serge Auckland" wrote in message
...

"Arfa Daily" wrote in message
...

"tony sayer" wrote in message
...
I guess you would get it on and off capacitively. Not too difficult to
make
a cap of a few decimals of a uF at several hundred kV working, I would
have
thought. You'd then use these to bridge any high inductances in the way,
such as tranny windings, and to get the data on and off. Depending on
the
frequencies involved, you may actually get away with a few hundred pF.

Perhaps but I don't seem to remember seeing much else apart from longish
glass insulators big switch gear and fecking big trannies!...


I'm sure that such caps must have existed when megawatt main UHF TV
transmitter sites, such as Sandy Heath, used valve transmitters at the
bottom of the mast ( perhaps still do ?? ).

Klystons 'tho solid state may be used there now.

Do note that the megawatt etc is radiated power, much more to do with
aerial gains. They don't have megga-watt UHF TV tx'es....


--
Tony Sayer


Yeah, know all about ERPs, but do the flat panel transmission antennas
actually have much gain ? Thought they were just a bunch of co-phased
dipoles. Bet the launch power from the tx is still a good 2 or 300 kW,
though, and I'd bet that there was a fair whack of volts on the klystron
to get there ...

Arfa
The high-power UHF TV Transmitters in the UK are 50kW, mostly
PyeTVT/Harris with a number of Marconi as well. The difference between the
Tx power and ERP is entirely made up of antenna gain. UHF antennas have a
very narrow beam, tilted downwards, as there's no point sending power off
into space. The beam is sufficiently narrow so that the signal close to
the mast is actually less than at a distance, also avoiding people's
receivers from overloading.

I don't know how many Klystrons are left in UK service, I would guess a
fair few. Channel 5 was engineered right from the start with Solid State
transmitters, of , if I remember correctly 10kW power, Channel 4 was
klystron originally.

S.

Thanks Tony and Serge. All interesting stuff. When S.H. UHF was first put
into service, I was an apprentice in the TV trade, and the Rediffusion
branch that I worked for, was chosen to do field strength monitoring by the
IBA, I think it was. Their boys left a receiver with us, with a chart
recorder attached. It was my job to check the tuning of it every morning.
Then it got hit by lightning as I recall, and the very expensive feeder from
Sweden or some such place, got fried. I seem to think that a substitute
piece of pipe from somewhere was put in place to get the site going again,
but there was some VSWR issue with it, because it was a few feet too short
and had to be joined, and whenever the antennas iced up a bit, the SWR seen
by the tx would go just a bit too far out of spec, and the auto protection
would pull it off the air. That tx was never as good after ... Happy days
!! ;-)

Arfa