Brushbeater: Antenna Polarization and COMSEC

NC Scout at Brushbeater has an article on Antenna Polarization and COMSEC (communications security).

So you’re out there on a patrol, the commo window is open and you need to make a Cyril Report back to your TOC. Your RTO sets up the yagi getting ready to make contact, checking and rechecking the azimuth. He glosses back over the transmit and receive frequencies to make sure everything is set, double checking the report to make sure nothing was missed, and getting the approval from you, the Team Leader. You notice one small thing- the Yagi is horizontal, not vertical, and a slow grin grows on your face.

You’ve got a good man on the Team who paid attention.

There’s one element to small unit communications that usually gets zero attention- antenna polarization. Let’s take a look at our most common denominator at the basic level- the Baofeng UV-5R. Its a VHF and UHF FM two-way radio. FM nearly always uses vertical polarization, meaning in simple terms, the antenna is straight up and down.

In a conventional environment we do this for two reasons. First, pretty much everyone else is vertically polarized when communicating via FM, and second, there’s 9db of loss between a vertical polarization and horizontal polarization. Wait, what?

Like how we measure light in Lumens, signal strength radiating from an antenna is measured in decibels (db) of gain or loss. With each 3db of gain, we double our effective radiated power (ERP) in terms of signal strength. With each 3db of loss, we cut our strength in half. This is measured in orders of magnitude, meaning that with each 3db, the strength doubles on itself (4w x 2= 8w x 2= 16w, etc). Taking that into account, the difference in strength between horizontal and vertical polarization is 9db- quite a difference. If someone is using a vertically polarized antenna to attempt to intercept my transmission, they’d likely be using vertical polarization. After all, why wouldn’t you? Nearly all FM transmissions are vertically polarized, its common practice. But if I change my operating practice to account for this, now they’re going to have a harder time both intercepting and getting a bearing on me.

Not impossible, mind you. But much harder. And that’s on top of my other operating practices, such as transmitting on one frequency and receiving on another, keeping my transmissions as short as possible, and making sure I’m always using directional antennas. It goes without saying that your intended receiving station should be matched in polarization. Its a basic practice that, when coupled with my other techniques, turns inexpensive equipment into much more formidable gear for clandestine or unconventional forces operating in the field.

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