[amsat-bb] Re: Relative merits power divider vs power divider/combiner

Edward R. Cole al7eb at acsalaska.net
Tue Nov 21 07:42:23 PST 2006

Scott, Joop:

The question was feeding two yagis to produce circular pol.  The impedance
is tranformed by either 36-ohm or 70-ohm sections of coax.  However, in A
the yagis must be physically offset one half wavelength to produce the
90-degree phase shift required for circular-pol (that can be accomplished
by one leg of the phasing harness being longer by half-wavelength instead
of physical offseting the yagis).

Approach B is using a quadrature hybrid which takes care of both impedance
transform and phasing shifts.  If one separates Rx and Tx on two of the
input ports and connect each yagi to the two outpot ports, there is no
loss.  The input ports are isolated from each other depending on how well
the hybrid is built.  This is the common approach used for 1296-eme where
RX and Tx are opposite sense of circular pol. (Tx-RHCP, Rx-LHCP).  For
satellite use one would terminate one input port with a 50-ohm load to use
the same sense CP in Rx and Tx.

There is some transmission-line loss thru either approach.

73's Ed - KL7UW

At 09:58 AM 11/21/2006 -0500, Scott Townley wrote:
>As long as all ports are terminated properly, they both are reciprocal
devices--they work the same on RX as they do on TX.
>If ports are mismatched, different things happen between A and B (and
that's true for both RX and TX)
>Device A will show mismatched impedances at all other ports, and may no
longer evenly divide power.
>Device B will show no mismatch at the 3 input/output ports, but power will
be dissipated in the port 4 load.
>Note that a properly designed Device A will not be mismatched on RX at the
split ports.
>The main advantage of Device B is the relaxation of the requirements of
precise impedance control.
>Scott Townley NX7U
>Gilbert, AZ  DM43di
>---- Joop & Tineke Verdoes <joop.verdoes at nameplanet.com> wrote: 
>> OM-s
>> As I got so much truely valuable and usable feedback on my hybrid power
>> divider/combiner questions, I now dare to ask the following.
>> For making circulary polarised yagis one needs to split 'things' over
>> two yagis
>> There are basically two ways to split power
>> A- use a 36 ohm power splitter, often a piece of waveguide with one
>> connector on one end(for the TRX) and two connectors (for the Yagis) on
>> the other end
>> B- use a 70 (75) ohm hybrid power splitter/divider, which has 4 ports,
>> one extra for a terminator. The TRX sees just 50Ohm.
>> A- and B- are both fine for SPLITTING power. In both cases the TRX sees
>> 50ohm, although there are two 50ohm connected in parallel and the other
>> side of the divider. So both are fine for transmit.
>> But......
>> A- Is ONLY a power splitter seen from the transmitter's side, it does
>> present a mis-match as seen from the yagi, so on receive things are less
>> than optimal
>> B- Does work both ways, it presents a nice 50ohm port to the yagi, but
>> the RX signal is split between the RX and the other yagi, so the signal
>> that goes to the RX is 3db down.
>> Does anybody have a view on how "bad" A- is vs B-?
>> Have fun
>> Joop, PA1JAV
>> _______________________________________________
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>Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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Ed - KL7UW  
http://www.qsl.net/al7eb - BP40iq 
144-EME: FT-847, mgf-1801/1402, 4xM2-xpol-20, 170w
432-EME: FT-847, mgf-1402, 1x21-ele (18.6 dBi), 60w
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