Winter 2008 Issue

ANTENNAS

D-Star Antennas

By Kent Britain, WA5VJB

Photo A. D-Star 1200-MHz antennas.

As many of us know, the VHF/UHF repeater slots are pretty full in the metropolitan areas. At the same time, it takes a lot of local activity to support new technologies. The end result is that many of the new D-Star repeater systems are showing up on 1200 MHz. This month we will cover the three antennas designed for these new D-Star systems (photo A). Keep this quiet, but they also work on AMSAT L-band, 1200-MHz ATV, and SSB, and the little one makes one heck of a quick dish feed.

Mobile Antenna

Photo B is a simple 1200-MHz mobile antenna. Have you priced any of the commercial 1200-MHz mobile antennas yet? Have you gotten over the shock? The one shown in photo B will cost you an old mag-mount antenna and a length of stiff wire. Vintage 800-MHz cell-phone antennas work well here as a source of parts. This is a simple vertical collinear antenna with about 5 to 6 dBi gain.

Start with about 16 inches of stiff wire. I used 1/16-inch bronze welding rod, but an old stainless-steel whip can be used—if you can bend it!

You do not want to use a magnetic mount that contains any loading or matching networks in the base. Just use something that is a plain magnet. Also, it doesn’t have to be a magnetic mount; it can be a threaded new Motorola type antenna mount, but watch the total height.

After you form the whip per figure 1, mount it such that the bottom of the phasing section is 3.5 inches above the ground plane. I’ll bet you never had to allow for the thickness of your mount or magnet before. Welcome to 1200 MHz!

Tweaking SWR

Built per the dimensions, SWR should be well under 2:1. However, if you are one of those people who can’t stand any SWRs crawling around on your antenna, and you can measure SWR (we tend to call it return loss on these frequencies), Do your tuning by expanding and compressing the coil. The coil is really a delay line that keeps the top and bottom sections of the antenna in phase. It’s not a loading coil like you would see on HF.

The one place to be very careful is in the choice of your magnetic mount. Many CB mounts have tuning caps in their base and stand too high. You want a mount with a pretty low profile. You also want your coax long enough to do its job and not much longer. Small-diameter coax has a lot more loss at these frequencies than it does at 2 meters or 440 MHz. So again, just long enough to do the job is best.

Simple Yagis

If you are a fair distance from the repeater, a little gain and getting the antenna up a bit higher can help a lot. These Yagi antennas are part of a family of simple and inexpensive antennas we affectionally call “Cheap Yagis.” The antennas are designed a bit differently. We start with the driven element, which has about a 150-ohm impedance. As other elements are brought close to the driven element, they load down the driven element. By spacing the other elements at just the right distance, we end up with a 50-ohm match.
 

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