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?
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Circularly or elliptically polarized antennas are used to transmit to both vertically or horizontally polarized antennas to reduce flutter to or from a moving receiver, as mentioned by Don Pitchford. I've seen some local HDTV stations in the Washington D.C. area are now using elliptically polarized antennas and I've found I can receive these stations using either a horizontal or vertical antenna. So, in a moving car receiving the TV signal using a vertical antenna will work just fine. After all, it is much easier to install a vertical antenna on a car than a horizontal antenna is.
There is probably no need for you to install a circularly polarized antenna to receive circularly polarized signals, use either your vertical or a horizontal antenna, either should work just fine.
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.
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