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

[Edward R. Cole]

Correction:

IN A:  the spacing is 1/4 wavelength to produce 90-deg shift.
IN B:  the quadrature hybrid provides the 90-deg shift so yagis can be
co-located on the boom.

Sorry for any confusion  - KL7UW

At 06:42 AM 11/21/2006 -0900, Edward R. Cole wrote:
>
>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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>>
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>73's,
>Ed - KL7UW  
>=========================================
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