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RE: Satellite Power Combiners?



Im trying to think why this *cant* be done with 6 2-way combiners, with
theoretically no loss.
(Using four-port type combiner).
(It would be much easier with a pic, but Im no good at ascii pics, so please
bear an explanation in words.)

The way I undersand these combiners is that if each port is (imagined) at
the corner of a square, then there is hi isolation between the diagonal
opposed ports, and ~3 dB split to the 2 adjacent ports, UNLESS the signal on
the diagonal port is identical and in phase, in which case it all goes to
one adjacent port, and none to the other adjacent port. (imagine a splitter
working in reverse)

That said, heres how my idea works.. we put 2 of the 4 signals we want to a
combine, into 2 (oppososite) ports of 1 combiner....now we get the 2
(identical but mis-phased?) outputs of its 2 'output' ports, and phase them
correctly so they are in phase, such that when we feed them into a second
combiner, they combine to only appear at one of the 2 output ports.

Now we do the same with the other 2 signals (combine to one), THEN we should
now have 2 physically separate signals, each one a linear combination of 2
signals. If we appaly the same theory to THESE 2 signals (ie combine with 2
combiners) , we can be left with 1 signal being a combination of 4 parts.

Im not saying this is the most efficient way of doing this, I suspect a
simpler circuit could acheive the same goal.
Realisation could possibly be done in stripline on high Er board. 

anyway..back to work.
DAve M. (VK5DSM)




-----Original Message-----
From: Bob Bruninga [mailto:bruninga@usna.edu]
Sent: Tuesday, 24 September 2002 02:57
To: amsat-bb@AMSAT.Org
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...)...

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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