View Full Version : Circularly Polarized FM Antennas

08-01-2015, 11:13 AM
The great majority of FM stations in the U.S. transmit circularly polarized signals. But ordinary linearly polarized receive antennas ignore half of the transmitted power. A circularly polarized receive antenna can recover it and provide up to 3 dB gain over a similar linearly polarized antenna. And when the incoming signal is linear but crosspolarized, a circularly polarized antenna can provide a much greater increase in signal level. For example, 39% of translators in California use vertical polarization. I hardly ever hear these stations on my horizontal Yagi, but they would be there using a circularly polarized antenna.

A unique property of circularly polarized antennas is their ability to attenuate multipath distortion. When a circularly polarized signal reflects from an object, the circularity sense changes. Right-circular transmitted signals become left-circular upon reflection. A right-circular receive antenna will attenuate them. The amount of distortion reduction depends on the complex reflection coefficient of the object, which varies with its composition and the reflection angle, and several characteristics of the receiver.

Rejection of opposite-circularity signals means that a right-circular antenna will attenuate ionospheric reflections. If you like to chase sporadic-E signals, this is not something you want. But if you're primarily interested in terrestrial DX or listening to distant FM stations for pleasure, it can reduce interference in summer. You can set up the crossed Yagis described below to switch between right- and left-circular polarization. If you switch to left-circular, you'll enhance most Es signals while attenuating local interference. I've never had a circularly polarized antenna up during Es season so I don't know first-hand how much circularity discrimination is possible. Much variation is likely since the propagation path involves ionospheric refraction, which can be highly irregular, combined with a second ground reflection.

I've made extensive updates to my circularly polarized cubical quad writeup. I've added new designs, including four elements on three spreaders with a boom length of 51". There's a lot of new data. The gain graph now compares more antennas, including the Körner 9.2 and crossed Yagis. I've added information on transmit polarization, including antenna pictures, to help you determine the circularity sense of a favored station. This required days of research and e-mails to several antenna manufacturers. (Since circularity sense doesn't matter for ordinary linearly polarized receive antennas, manufacturers can be somewhat lax about specifying it. I found data sheets for two antennas from one manufacturer that incorrectly specified the circularity sense. And I got an e-mail from a sales guy at another manufacturer who incorrectly claimed his antennas transmitted "mixed" polarization and were neither right- nor left-circular!)


The cubical quads offer high gain and very inexpensive construction from parts you can get at any hardware store. But their patterns are poor for interfering signals with unfavorable polarization. Few stations transmit left-circular signals, but propagation over irregular terrain can alter the circularity. So can the transmit tower when the antenna is improperly mounted. The quads rely on cancellation of the orthogonal signal components. When sufficiently altered in amplitude or phase, rear rejection can suffer. In addition, quad circularity sense is fixed and can't be switched. A solution to both problems is to phase two crossed Yagis with good individual patterns. When a signal with unfavorable polarization arrives, the inherent antenna directivity attenuates it. Signals with favorable polarization receive even further rejection. I wrote up the smallest practical array, one which uses ten elements total on a 60" boom. I include patterns for worst-case polarization and performance curves that show pattern variation with antenna height and ground quality. The simplest design uses one feedline and right-circular polarization. But if you add another feedline, you can switch among right-circular, left-circular, horizontal, and vertical polarization at the receiver. This is a highly versatile antenna system that lets you optimize reception for any standard polarization. And if you include a phase shifter and variable-gain amplifiers, you can exactly match polarization to maximize the strength of any desired signal or precisely crosspolarize to null any interfering signal.


A simple circularly polarized indoor loop offers several dB gain over a dipole.


Even rabbit ears can provide circular polarization. You can easily adjust them to null interference of any polarization. This writeup describes how to set them up and tune them.


Finally, simply tilting a dipole can provide forward gain from elliptical polarization. Two such antennas can be easily phased indoors for additional gain and directivity.


If you have any questions about a design, contact me by e-mail using the address at the end of the writeups.


Damon Hill
08-03-2015, 05:21 AM
Brian, you're too good to us.

The circularly polarized quad intrigues me, I just have to figure out how to build it with materials I can afford. The log-yagi is on indefinite hold for various reasons, but a two element quad should be doable.