Thanks for the clarification Larry. I was pretty sure that modern
transmitter technology wasn't something I had in a dream. I was up
in Fairbanks a couple years ago and I suspect that keeping
the finals cool isn't much of a problem there
My father was
the engineer for a UHF station back in the late 50's. He told
stories of having to run out and get bottles of Nitrogen to keep
the finals going. Life is much simpler now.
Doug, k3qt
s/v Callista
"Larry W4CSC" wrote in message
...
Er, ah, Bruce? There are no "finals" any more. The 50KW transmitter
is just a switching power supply from Harris.....take a look at:
http://www.geocities.com/aaron_white/inside.html
It's all done by that big board switching the modules on and off at
the audio rate, in sequence. See those two little powertab
transistors on the copper heat sink? THOSE ARE THE FINALS!
If a module fails, the computer simply bypasses it and uses another
module (there are spares) until someone shows up at the transmitter to
look for the failure lights. They don't even shut down the
transmitter to swap them out, any more. Hot swaps just like a
computer...(c;
The two PC boards on the door are the "modulator", the drivers for the
modules creating the RF. It's so efficient it's cooled with small
FANS! To produce a 50,000 watt carrier, it requires only 55,000 watts
of AC power from the power company, not the 120KW we were accustomed
to. No massive filament power, no blood-red plates being cooled with
water, no cooling pond.
The RF modules (there are 144 of them in a Harris DX-50, 50KW
transmitter) look like:
http://www.geocities.com/aaron_white/dxmod.html
Some are 100W, 500W, 1KW, 2KW. The audio is simply converted into how
many modules run at this instant in time. The modules, themselves are
switching on and off at the carrier rate. The output filter takes out
the 3rd harmonic caused by the square wave switching up. Nothing
analog to make heat. It's a RF synthesized sinewave just like your
boat inverter.
You'll be glad to know from the Harris out it's still RF....(c;
Here's the ATU at WFAN:
http://www.geocities.com/aaron_white/atu1.html
75KW dummy load:
http://www.geocities.com/aaron_white/load.html
50KW isn't what it used to be...(sigh).
http://www.geocities.com/aaron_white/maintx.html
The modulator and final are inside the right doors. The other end is
the power supply. Kinda a non-event unless the RF output flashes
over, which my friends in broadcasting says is also a non-event for
the digital transmitter. It simply clears it, resets and tries again.
If it continues to flash over, the transmitter comes up in low power
(10KW) until the techies show up.....
The computer that operates the transmitter is in that rack to the left
of the Harris at WFAN. The whole thing is in a corner of the
transmitter shack which is mostly taken up by the darkened old 50KW
monster you're thinking about.
Tom Ray, Director of Engineering over at WOR in NYC, sent me some cool
pictures of their IBOC Digital AM experiments on their Harris DX-50.
If you listen to WOR or WLW's IBOC on a regular AM radio you hear a
faint hissing sound that is the digital subcarriers. The two
stations, 10 Khz apart, are experimenting with interference caused by
the IBOC carriers. Most interesting.....AM that sounds like mono-FM
but at great distances.
On Fri, 30 Jan 2004 21:20:46 GMT, Bruce in Alaska
wrote:
In article ,
"Doug Dotson" wrote:
and dispatches licensed techs
Nope Doug, no License Required anymore. Knowledgable Tech's recomeended
but not even required. That's why I got out of the Broadcast Business
35 years ago. To many Carneigie, and Elkins, First Phone's showing up,
and getting OTJ Training on my license as Chief Engineer. There's no
such thing as an Automated 50,000 Watt AM Transmitter. When those
puppy's decide to melt down, they do it in a hurry, and usually with
lots of sparks and steam from the liquid cooling systems. Usually
the finals are pulled at 50% of rated time, but nowdays with skimpy
maintaince money, owners are pushing that out to 80% and catastophic
failure is happen a lot more often. Reminds me of a very cold January
Night when KRAB-FM's antenna took a hell of a wet snow load, and
unbalanced the two 5Kw finals explosivly. I came around the corner
and the DJ on duty was out in the street with a bunch of steaming holes
in the snow where hot final Parts had melted down through the snow.
Took the consulting engineer and I, 8 hours to rebuild the final cage
(Lots of plumbing parts) and get it back on the air.
Bruce in alaska
--
add a 2 before @
Larry W4CSC
No, no, Scotty! I said, "Beam me a wrench.", not a WENCH!
Kirk Out.....