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October 2017

Circular Polarizating Antennas

I’m an avid VHF/UHF listener, and have always used a vertical non-directional antenna for receiving signals. However, after buying a circular polarizing filter for my Nikon, I realized that RF can be circularly polarized as well. How can I tell whether a signal is vertically or circularly polarized, without investing in a circular antenna? Do you know of any resources for how to construct circular polarized antennas?

#10171
Ed Moreno
Schaumburg, IN



Answers

Google diy vhf circular polarized antenna. There are quite a number of hits. For example: http://ka7oei.blogspot.com/2013/08/a-circularly-polarized-omnidirectional.html.

Ed Schick
Harrison, NY

You are very unlikely to encounter circularly polarized radio signals from local sources. Circular polarization is mostly used for receiving Low Earth Orbiting (LEO) satellites and stuff in the microwave and upper end of the UHF ranges. If you do find any of these signals they will have a distinctive rhythmic change in signal strength as the polarization of the signal changes; maximum as it matches your receiving antenna’s polarization, then minimum as it becomes opposite your antenna’s polarization.

If you have a receiver capable of covering the 145 MHz and 437 MHz ranges you may want to check out ham radio satellite sources such as https://www.amsat.org/fm-satellite-frequency-summary for information about receiving them.

As for receiving circularly polarized signals should you find any, if they are strong enough that the null in signal strength is still strong enough for you to hear it clearly, then you will likely not know the signal is circularly polarized and need do nothing special to receive it. For weak signals you will need to try both Right Hand and Left Hand circular polarization to determine which is needed, as the wrong circular polarization is worse than just using a linear polarized (in your case vertical) antenna.

If you want to learn more about circular polarized antennas https://www.e-education.psu.edu/geog862/node/1782 and https://rcexplorer.se/projects/2011/08/circular-polarization-explained have very good simple explanations of what is going on with circular polarization.

The source I recommend for anyone learning about antennas is The ARRL Antenna Book. They are currently selling the 23rd edition, but any edition from the last 30 years or so will likely suffice. Old and new versions are usually available at  local ham radio gatherings such as a “hamfest” or online at Amazon.com

Don Pitchford
Springfield, IL

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