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The Doctor Is In: More Power for EME Stations:

from The ARRL Letter on January 24, 2013
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The Doctor Is In: More Power for EME Stations:

Carl Hasbargen, KN0WS, of St Paul, Minnesota, wrote to the ARRL's Doctor, wanting to know how to use more transmit power for the new Earth-Moon-Earth (EME) station that he plans to build later this year. In the past, he has used a portable generator to power a 180 W, 432 MHz linear amplifier into a quad Yagi antenna system. While Hasbargen was successful using that setup to make a contact via the Moon, he figures additional transmit power would be helpful. He told the Doctor that he would like to be able to use his current generator.

Hasbargen asked "If I have two identical model solid state linear amplifiers, each capable of amplifying a 25 W input into a 180 W output, can I simply take the 50 W from my transceiver, divide the power using a commercial antenna power divider to drive each linear amplifier, then combine the two 180 W outputs from the pair of amplifiers back through another power divider (used backwards as a coupler) to produce a final 360 W to the antenna?"

Here's what the Doctor had to say:

If you pull off the covers of many high power solid state amplifiers, you will see that is exactly how they are constructed internally (see photo). While it seems like a straightforward idea, the main issue, especially at EME frequencies, is relative phase delay. While a single dual-module amplifier is relatively easy to build and test as an integrated system, your separate amplifiers were not built with that in mind. At 70 centimeters, a wavelength is 70 centimeters long, but 1/10 of a wavelength is only 7, about 5 centimeters, or 2 inches in coax.

I would think you would want tolerances to accumulate to well less than that for reasonable operation, and with independent assembly and device selection (not selected as matched components) and alignment, I would be surprised if two randomly selected amplifiers would be that close. If you have the capability to phase match them, the difference, if any, could be accommodated in a short length of coax, preferably on the input side.

If the amplifiers are linear and can amplify at very low levels, it would be drive them with an input splitter and measure the power at each output. If they are the same and you put them in a combiner and the level is twice each, less the attenuation of the combiner, you are probably good to go. I would bring up the power slowly and watch for heating and other undesired effects.

Of course, in real life, each of the power divider/combiners will introduce a little loss, so don't expect the full 360 W -- still it should be close.

Thanks Doctor! Do you have a question or a problem? Send your questions via e-mail or to "The Doctor," ARRL, 225 Main St, Newington, CT 06111 (no phone calls, please). Look for "The Doctor Is In" every month in QST, the official journal of the ARRL.


The ARRL Letter

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