Circular Receive FM Antenna

[BobOnTheJob]

I've often wondered about this and I bet R. Fry and/or others may have the answer. (Some) FM stations transmit 100KW H&V, most often via a circular antenna. When you receive that signal, the effect of 100KW ERP is plucked from the airwaves via either an H or V receive antenna. If someone had a circular receive antenna (say an FM bay tuned to the desired frequency), would it "receive" both polarities at once and give you more signal than would an equivalent H or V receive antenna?

 

[frankberry]

Possibly ... but generally no. First you would need to know whether the circular polarization is clockwise or count-clockwise. Even if you get the correct polarization type, if either component (H or V) doesn't arrive at the proper time (multipath) the results will be worse than a simple H or V receive antenna.
This happens at microwave frequencies. We have found that the best performance is using either H or V but not both (circular).

 

[iyiyi]

Even one bay at VHF would be a great deal bigger than a breadbox!

 

[rfry]

... If someone had a circular receive antenna (say an FM bay tuned to the desired frequency), would it "receive" both polarities at once and give you more signal than would an equivalent H or V receive antenna?

Theoretically yes, for perfect c-pol with compatible antennas over a free-space path. And a benefit of using c-pol on both ends is that multipath reflections tend to be rejected by the receive antenna, because the sense of rotation is reversed (approx), at the plane of reflection.

But the quality of the c-pol radiated by a typical, non-panel FM antenna can be far from perfect, because the tower and other re-radiators in/near the antenna aperture affect some polarizations more than others.

The most practical benefit of c-pol FM broadcast transmission is to try to best serve linear, line cord and headphone wire receive antennas no matter how they are oriented in space.

 

[ironbear]

SCALA will be happy to sell you a FM circularly polarized yagi antenna...just specify HDCA-5CP or HDCA-10CP (or alternately, CA-5/10CP for higher gain.) And, don't forget to send lots of money.

De facto polarization among the FM transmit antennas manufacturers I'm aware of seems to be Right Hand Circular. I once improved a marginal link from an FM translator to another FM translator by setting up a SCALA transmit array to Left Hand, and the corresponding HDCA-5CP on the receive to Left Hand, improving an adjacent channel receive interference problem. I can't think of any other situations were RHCP vs. LHCP would be of use, actually, although you do theoretically get 3dB more signal at the receive end.

 

[Schroedingers Cat]

Once you are past line of sight, you are dealing with the result of at least one reflection, which will usually put the polarization into one plane, but not necessarily exactly H or V. It is more likely to be plane polarized at an odd angle in between H and V. If you are getting just a strong reflection off a conducting surface, which is line of sight, it may be circularly or elliptically polarized, but the sense will be changed from right to left or left to right. You would do better with separate H and V antennas, and the ability to change the sense of the receiving antenna, in such a case. A telescoping whip demonstrates some of these principles, and with no gain or height, often will allow cochannel and weak adjacent channel stations to be selected, and maximization of the signal.

 

[BobOnTheJob]

Fascinating information...thanks to all who chimed in so far. The furthest my experiments have gone is taking an FM yagi and mounting it at a 45 degree angle. As expected, the V only stations were dramatically better as the -20db cross polarity penalty was largely neutralized. Anyone who's never tried receiving a V only station on a H yagi would be floored at how poorly the V stations cover on an H yagi. The concept of allowing a lot more V ERP in the presence of a Channel 6 station because the H TV antennas reject the V is not only good in theory, it pans out that way in reality as well. Just ask any FM DXer how many times they've said "I'll never hear that station, it's Vertical only".