How do you make 100,000 watts?
I'm curious as to how broadcast stations create their huge amounts of power. A local FM station, for example, mentions they broadcast with 100K watts. Do they use tubes? Many many tubes? Huge transistors?
I'm also curious as to what kind of antennas they use? An omni-directional vertical?
How do you make 100,000 watts? One watt at a time.
Lots of big parts able to take the amps, volts, and watts. The radio station a friend, who was chief engineer, ran used tube finals, and a grounding setup that had proven itself in practically every thunderstorm. And a vertical. Can't say I've seen a radio station using anything other than a vertical. But I could be wrong. My XYL tells me I'm wrong at least once an hour.
Last edited by KX8C; 08-04-2011 at 12:19 AM.
Reason: bad english, not that what is still there is better.
Originally Posted by W4AFB
To answer the OP's question some of that power comes from antenna gain.
Your 100KW station was most likely stating effective radiated power,.
With that said power levels in excess of 10 million watts CW are attainable using state of the art vacuum tubes and on lower frequencies 100KW can be achieved with transistors. Lots of them.
The "100,000W" is an "effective radiated power" figure, likely 50,000W to a 3 dB gain antenna system (although there are many ways to do this, including 25kW to a 6 dB antenna system).
FM broadcast stations are not vertically polarized. They're either horizontal, or a combination of horizontal and vertical.
Tubes reigned supreme for these transmitters for most of history, but lately solid state transmitters are taking over...mostly because not so many folks manufacture tubes anymore. Just a couple of tubes can run this much power. With solid state stuff, it's usually an array of amplifier modules combined together. I'd expect that "some day soon," it will all be solid state.
If you really want to see some "big" transmitters, check out what some shortwave broadcast stations use on HF. Some of those are well over 1 million Watts e.r.p., which could be 250kW output to a 6 dB gain system, or 100kW to a 10 dB gain system, etc. Many ways to do this, but in general those transmitters are real fun. I live very close to a 1.5 million Watt SW broadcaster here in southern CA. They use a 14.5 dB gain directional antenna, so the transmitter only had to run about 50kW output power or so, but it's real fun to go visit those sites and see what's going on. Usually, you can hold one end of a 40W fluoroescent lamp in your hand, and just hold it up in the air (connected to nothing but your hand) and it will light up just fine.
The solid state amp can be whole lot of lower wattage amp run into combiners. Think 50 or 100 X 1Kw amps into combiners. If you look at the schematic of modern SS amps (the IC-PW1 comes to mind as I have one) is really 4 X 250 watt amps into a hybrid combiner. That and water cooling make the amps handle the wattage.
I seem to remember that FM broadcasters were limited to 50kW erp,
BUT stations that were at higher power BEFORE the 50kW limit was established
were "grandfathered" at the higher power level.
WAY...WAY...WAY back when in the '80's, I was chief engineer of WAMO-FM
in Pittsburgh. It was one of the grandfathered stations as it was licensed for
72kW ERP operation.
It used just shy of 25kW from a Harris FM-25K transmitter to
an 8 bay circular polarized antenna at 400' agl. (I installed the FM-25K)
It used a single tetrode to generate the 25kW.
I do not remember the tube type,
but I THINK it was a 4CX20,000
(I may very well be WRONG too)
The driver stage was 4 solid state amplifier modules
combined to achieve a 1kW drive for the final power amplifier.
That was then, this is NOW...
I'm sure the state of the art now uses solid state devices
in transmitters up to the 25kW range.
Do a google search on HARRIS-RF, they were a BIG manufacturer of
high power broadcast equipment that IIRC was located in Quincy Illinois.
At one time, Harris manufactured High Power SW broadcast transmitters
in the 200-250 kW range. I suspect that they are still tube type since
acheiving that kind of power would take a BUNCH of PA modules to gat
that kind of power output.
Many years ago at Loral Corp. we built some 2.5KW S-Band (2.2-2.3GHZ)
Originally Posted by KB1NXE
There were eight modules containing four five transistor PAs using TRW custom made transistors.
Combiners in each module the eight modules ran into a "Magic-T". It was quite an amplifier for it's day two 6' racks bolted togather and four huge 28V power supplies to run it. This was air cooled and contained it's own wideband idolater, a waveguide switch and a waveguide dummy load.
At the time I was impressed.
A quick search of Harris
(I was curious just how much things have changed in the interviening years)
lists solid state transmitters up to 40kW for FM broadcasting.
It also lists tube type transmitters in dual configuration
up to 80kW.