Summer 2007 Issue

ANTENNAS

Weak-Signal Mobile Antennas

By Kent Britain, WA5VJB
 

Photo 1. A 2-meter halo for horizontal polarization and a 2-meter vertical for vertical polarization. (Photos by the author)

Back in the late 1940s and the early 1950s, there was considerable discussion about the advantages of horizontally versus vertically polarized antennas for long-haul VHF/UHF QSOs. When considering propagation, it doesnít really matter if the signals are horizontally or vertically polarized, but there are mechanical advantages to each. As a result, SSB and CW generally are used from home stations and the antennas are horizontally polarized. Mobile stations usually find it much easier to put up a vertical whip antenna, so most mobile station antennas are vertically polarized.

Therefore, when it comes to mobile SSB/CW operation, you have the challenge of coming up with a mobile horizontally polarized antenna. However, 6 meters is rather an exception. When the 50-MHz SSB signal bounces off a few E-layer clouds during an opening, the signal that bounces back has a confused polarization. Thus, a vertical antenna works almost as well as a horizontal antenna on 6-meter skip. I worked more than 30 states with an ICOM IC-502 (3 watts on a good day) and a 2-meter 5/8-wavelength mag-mount antenna on top of a 1977 Chevette. Iíll bet, though, that I got something mixed up there. No, a 5/8-wavelength antenna is electrically 3/4 wavelength long, but is shortened to physically be 5/8-wavelength long. Thatís why there is a tuning loop in the base of a 5/8-wavelength antenna, to get that extra 1/8 wavelength. Therefore, the 2-meter 5/8-wavelength mag mount is also a 1/4-wavelength whip on 6 meters. It works great with some of the new multiband rigs as well.

The Halo and Squalo

In Photo 1 we have the most common VHF horizontal mobile antenna, the halo, and a 1/4-wavelength mag mount. In its simplest form, the halo is just a 1/2-wavelength dipole bent almost into a circle. Capacitance coupling between the tips can be used to shorten the antenna even more, but at the expense of a narrower bandwidth. This halo certainly has seen a rough life, and it uses a beta match, or a shortened stub, to get the feedpoint impedance back to 50 ohms.

In Photo 2 is my veteran 2-meter squalo. It was a used antenna when I got it 25 years ago. I added another 100,000 road miles, and forgetting to take the squalo off several times before pulling into the garage has created its share of wear and tear. However, it still works. This squalo uses a gamma match, which is an offset feed with some series capacitance. Personally, I am not fond of gamma matches, but they do work. The half of the dipole with the match on it has the most RF current, which makes the maximum radiation off the back corner of the squalo on the side with the match. Thus, gamma-match squalos are not exactly omnidirectional.

I dug out my 222-MHz halo antenna, but it seems that it didnít survive an avalanche in my garage last winter! Well, it was tuned to 220.1 MHz when I first used it mobile. However, I digress. Terry, W5ETG, has spent a lot of time tweaking mobile antennas, and he suggested I use a balanced feed on my 220-, 432-, and 1296-MHz halos. In Photos 3 and 4, I show my 432-MHz halo using the balanced feed. It has a 4:1 coax balun and equal-length matching arms on both sides. Tuning with Terryís match was smooth and broadband. Itís certainly worth the extra effort.

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