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Winter 2008 Issue |
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ANTENNAS D-Star Antennas By Kent Britain, WA5VJB |
Photo A. D-Star 1200-MHz antennas. |
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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. 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! 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.
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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