[Amps] Power combiners and antennas???

[Rob & Terri Sherwood]

After 25 years in commercial broadcasting maintaining both 50 kW AM and 
50 kW FM transmitters, there was nothing I disliked more than combining 
two 25 kW FM transmitters to get 50 kW out.   When you use a hybrid 
combiner to sum two amps, you have to have a reject load that can handle 
at least 1/2 the power of one of the amps.  If for some reason one amp 
goes down, half the remaining power will go to the antenna and half to 
the reject load.  So in this case 12.5 kW to the antenna and 12.5 kW to 
the reject load.  So if you have two 250 watt 2 meter amps, and one 
faults, the reject load will have to dissipate 125 watts. 

K8LV is correct that you have to be able to adjust the delay to get the 
amps to combine properly.  Slightly different lengths of cable feeding 
the input will work, but it is tedious.  Considering the complexity of 
the Harris transmitter I dealt with, there were more failures in all the 
combining "junk" and logic to handle it than with the transmitters 
themselves. 

Personally I would rather have had one big transmitter and a lower power 
standby.

Since VHF and UHF brick amps are not especially clean on SSB, I suggest 
someone wanting high power on 6 or 2 meters consider a good tube amp 
that will put out the desired power rather deal with the headaches of 
trying to combine lower power "brick" amp.

Rob Sherwood
NC0B

RFlabnotes at aol.com wrote:

>Jim:
>       The idea of completely tandem RF chains is intriguing, and would avoid 
>most or all issues connected with simplex failure. However, one additional 
>requirement would be that you would have to insure precise phase delay in both 
>amps - I would not take this for granted, especially is they are tunable 
>amplifiers. Prepare to measure and have some small line stretching available.
>
>Eric von Valtier K8LV
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