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I'll try to clear up the confusion. Any LNB (without feed) is going to be linear. Feeds designed for circular polarization reception will convert circularly polarized signals into linearly polarized signals. There are several ways to do this. One can machine flats or squish a normal linear feed in a vise at 45 degrees to the LNB probe(s). One can also use a dielectric slab or a real 1/4 wave plate of the proper shape and length, also inserted at 45 degrees. Which direction with respect to the 45 degrees will determine whether the linear LNB will receive a left or right circular signal. None of these are perfect - they work over limited frequency frequency ranges and some cause significant losses in SNR - not just gain.

Such a feed is a reciprocal device - in other words it will convert L&R to H&V, but also H&V to L&R. But the latter conversion is only effective if the plate/slab/flat is at 45 degrees with respect to the transmitted signal. If the plate/slab/flat is parallel or perpendicular to the linear polarization direction, the signal will pass through directly without being altered, albeit with some loss. This is the principle for the Chaparral International feed. One aligns the dielectric plate so it does not interfere with linear polarizations. Because the polarization is selected with a probe driven by a servo motor, there are four different positions required to receive L&R and H&V.

Unfortunately an orthomode feed, like the Bullseye, can only get two of these positions. If you were to construct a proper slab/plate, you could receive L&R, but you would lose H&V. I have been struggling with what to do on the next BUD to go up - I have a lovely ADL pure L&R othormode feed looking for a home, but I also want H&V. I've looked at designing a combined feed, but am concerned the losses will be unacceptable. At the moment it looks more likely that I'll simply cut up a couple of scalar plates and sidecar an orthomode H&V next to the L&R. Because off-axis gain drops fairly slowly for small angles on 'normal' f/D prime-focus dishes, I expect this will perform better.

One can consider a circularly polarized wave as a superposition of two orthogonal linear polarizations, one parallel and the other perpendicular to the slab/plate, with one delayed by 90 degrees with respect to the other. The component that is parallel to the slab/plate is not affected when passing through the feed.

However the perpendicular component must pass through the slab/plate, and because the slab/plate is denser than air, this component is 'slowed down' by approximately 90 degrees. Depending on whether the signal is left or right polarized, the net effect will be that the exiting components will be in-phase or at 180 degrees to one another. Recombining these components will generate a linear signal 45 or 135 degrees to the slab/plate, which can be received by H&V probes.

pendragon said:

One can consider a circularly polarized wave as a superposition of two orthogonal linear polarizations, one parallel and the other perpendicular to the slab/plate, .....

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Quite a bit off topic, but slightly related to the above... something I've argued about with dozens of people over the years...
You can look at CP as coming from the orthogonal linear polarizations like you mention above, and you can also create equations for the wave rotating as it goes through space. Both are pretty much the same thing, however, what if you have an almost instantaneous pulse of CP RF. The question is, does that pulse continue to rotate, as it goes through space. The equations for the wave seem to say that it will, and everyone I've argued with say that it will, but I contend that it's only the source that's rotating, and an instantaneous pulse is really linearly polarized, and won't rotate as it goes through space.
Not sure if I made that clear, but I'm curious what your opinion would be?

Sorry for being off topic.

off topic: ADL Ortho feed

pendragon said:

I have a lovely ADL pure L&R othormode feed looking for a home...

Click to expand...

I just got one... or most of one, and should start a thread on getting it to work, some day.
Unfortunately, at this time I don't need the distraction.
Hope you keep some pictures as you go, as I may have some questions for ya later. -

B.J. said:

You can look at CP as coming from the orthogonal linear polarizations like you mention above, and you can also create equations for the wave rotating as it goes through space. Both are pretty much the same thing, however, what if you have an almost instantaneous pulse of CP RF. The question is, does that pulse continue to rotate, as it goes through space. The equations for the wave seem to say that it will, and everyone I've argued with say that it will, but I contend that it's only the source that's rotating, and an instantaneous pulse is really linearly polarized, and won't rotate as it goes through space.
Not sure if I made that clear, but I'm curious what your opinion would be?

Click to expand...

With CP there will always be a one-to-one mapping of the instantaneous phase of the signal and its instantaneous polarization angle. So I guess if you want to visualize "jumping" on part of the wave and following it through empty space, the spot you would be standing on would remain constant in phase and polarization angle. However in a stationary frame one would "see" a propagating wave rotating. Does that make sense?

Anole said:

I just got one... or most of one, and should start a thread on getting it to work, some day.
Unfortunately, at this time I don't need the distraction.
Hope you keep some pictures as you go, as I may have some questions for ya later. -

Click to expand...

If you've got the same one I do, it looks impressive. From what I've read it does not use a dielectric slab, and its 1/4 wave plate has a significantly lower loss. Peering down the feed, I'm willing to accept this claim.

I've staked out a spot on the roof for the 7.5' Winegard that is going to point the thing, but I'm going to have to move my 1.2m somewhere else to make room. NSS 806 is pretty low even in Denver, and that is the real target. I guess you'll probably have to live with the more western birds.

With a little luck I'm hoping to get this dish ballet started on the weekend. My other near-term project is to rewire my switch matrix to accommodate 40-50 LNBs. If I don't get that done soon I'll turn into a prune when the sun starts baking the attic where most of the wiring will be. Then it's on to C-Band CP. I'll try to keep a record and possibly start a thread if it looks like something of interest to others.

pendragon said:

With CP there will always be a one-to-one mapping of the instantaneous phase of the signal and its instantaneous polarization angle. So I guess if you want to visualize "jumping" on part of the wave and following it through empty space, the spot you would be standing on would remain constant in phase and polarization angle. However in a stationary frame one would "see" a propagating wave rotating. Does that make sense?

Click to expand...

Yeah. I think you agree with everyone I was arguing with.
Actually, while composing a reply, I think that I may have finally figured out where my logic was going wrong, after decades of arguing the topic, but I'll have to think about it a bit more.