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Author Topic: Pawsey Stub  (Read 1602 times)
VK2FAK
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Posts: 87




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« on: December 04, 2012, 07:06:19 PM »

Hi all..

A couple of questions... feeding a 70cm beam..

does a coaxial Pawsey stub have to run parallel to the feedline..?
If I needed  to match the ant. and had 2 lengths of coaxial cable, taking the Velocity factor into account to make 1/4 wave sections...can I just attach a stub section ( not taking the Vel. factor into account ), so will be a little longer than the 2*1/4 wave sections.. at the grounding point.

And for that matter can I take the Stub section in the other direction and ground it to the boom.

John
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G8HQP
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Posts: 120




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« Reply #1 on: December 05, 2012, 04:01:12 AM »

Yes, it must run parallel. This is because the Pawsey stub acts as half of a shorted quarter-wave twin line. The other half is the final quarter-wave of the feed line.

You may be able to do some impedance transformation in the feed line too, as that will happen on the inner currents. The Pawsey stub uses outer currents.
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WB6BYU
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Posts: 13038




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« Reply #2 on: December 05, 2012, 08:26:35 AM »

The second piece needn't be coax - it can be any conductor.  It forms a quarter
wave shorted transmission line stub with the coax, which looks like an open
circuit to RF, so reduces the common mode currents.  So it is best if it runs
parallel to the incoming coax.

The length of the Pawsey stub should NOT include a correction factor for the
coax velocity factor - that's because the current is flowing on the outside of
the shield rather than in transmission line mode inside it.  The stub of feedline
will, of course, have its own velocity factor based on the dielectric between
the two conductors - if they are tightly bound together then it might drop
down to 0.9 or so, but generally with air spacing it doesn't make a practical
difference.  If you are using a quarter wave matching stub it may make more
practical sense to ground the coax shield at the junction point, which would
make your Pawsey stub on the short side.

If the stub is made shorter than 1/4 wave it will look like an inductor:  this
inductance can be used as part of an impedance match system such as a
beta match.  I seem to remember at least one design where the beta
stub was made using tubing elements with the far end shorted to the boom,
then the coax was run in through one of the tubes to the feedpoint.
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G3TXQ
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Posts: 1464




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« Reply #3 on: December 05, 2012, 11:30:59 AM »

To achieve best CM attenuation, the CM path impedances looking from the feedpoint along the two "conductors" should be identical; that argues for them being the same length, having the same CM velocity factor, and the same diameter. The obvious and simplest solution is to use a piece of coax the same as the main feedline.

From a CM attenuation perspective they can actually be any length you like, but because you have inadvertently formed a 2-wire TL S/C stub that appears in parallel with the feedpoint, it needs to be an odd number of quarter-waves long for minimal effect on the feedpoint impedance. As WB6BYU pointed out, when making that calculation it's the Differential-Mode Vf of the "twin wire" TL that matters, not the Differential-Mode Vf of the individual coax.

73,
Steve G3TXQ
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