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R: Satellite Downlink Power Combiners?




----- Original Message -----
From: Bob Bruninga <bruninga@usna.edu>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, September 23, 2002 7:27 PM
Subject: [amsat-bb] Satellite Downlink Power Combiners?


> Question for the Experts:
>
> How does one combine four 2m satellilte transmitters to a common antenna?
> A Wilkenson power combiner comes to mind...
>
> But if the transmitters are NOT all phase coherent and are on seperate
> in-band frequencies, isn't half the power from each transmitter lost in
> the loads?  The text books always seem to use the example of combining
> multiple parallel amps (all with the objective of combining them all in
> phase...)...
>

Bob,

For the above problem look at the article "Wilkinson hybrids " by
Ernie Franke, WA2EWT published in Ham Radio Magazine January 1982
pages 12 to 18

Chapter "amplitude and phase unbalace" at page 16 shows equations
to be used to compute losses in dB both for amplitude and phase
difference.

Equation 9A if the input signals are in phase but differ in amplitude.

Equation 9B if amplitudes are balanced ,but the phase of the input
power differs.

Look also at page 58 of the article "The hybrid ring" by the same author
WA2EWT published in Ham Radio Magazine august 1983


73" de i8CVS Domenico





> We want to have 4 separate 5W 2m XMTRS connected to a common downlink
> antenna. These XMTRS will be on different in-band frequencies so are not
> coherent or in phase.  A four-way wilkenson combiner is what we are
> considering, but not if we end up only getting 1.2W ultimately delivered
> to the antenna from each one's class C final.
>
> The text books say the power lost in the resistors is zero if all
> impedances are balanced, but when XMTRS are non phase coherent, then
> who knows what their impedance looks like on the other's design frequency.
>
> If not a wilkenson power combiner, then what do you use so that with four
> 5W xmtrs, each one gets its own 5W's delivered to the antenna
> independently?  I fear the answer may be a multicoupler where you have to
> have very High Q cavities such that each one only sees the antenna at its
> own frequency.  But then this is impossible to put into a small satellite,
> and besides all the freqs are within 200 KHz of each other....  Too close
> to get any isolation with cavities or helical resonators....
>
> Or combine at low level and then use a very linear amp?  ANd pay a big
> penalty in DC power?
>
> Expert opinions solicited...
>
> de WB4APR@amsat.org, Bob
>
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