Fall 2003 Issue

Photo 1. AMSAT Mode L antennas mounted for circular polarization.

AMSAT “Mode L” Antennas

By Kent Britain, WA5VB

This month we are going to cover AMSAT “Mode L” antennas. These antennas are like 435-MHz AMSAT antennas, but they are centered on 1270 MHz (photo 1).

Normally when a Yagi gets this long, its bandwidth becomes very small, which makes construction very, very critical. I’ve backed off about 1 dB from maximum gain to come up with an antenna that works across the entire 1240-MHz to 1300-MHz band. If you miss it by 30 MHz, it still works at 1269 MHz. (See figure 1.)

Measured gain at just over 16 dBiC is what you can expect from the typical 3–3.5-foot dish system on L-Band. Mathematically, the formulas say that you can get 20 dBiC from a 3.5-foot dish, but you are not going to get that 60% theoretical efficiency with the feed blocking much of a small dish.

Wood

When you mount a Yagi element to a metal boom, you have to adjust its length slightly to allow for the effects of the metal boom. Normally we just consider wood to have no effect and go on. As Ed Manuel, N5EM, unintentionally showed me several years ago, the effect is not zero! In this case, an L-Band ATV antenna was built using a 1" ¥ 1" wood boom, which makes nearly one third of the element inside wood! This detuned the antenna nearly 50 MHz. Even the 1/2-inch width boom used in this month’s design lowers the center frequency of the elements about 10 MHz. Therefore, don’t substitute a stronger/wider wooden boom. One ham built this antenna using plastic “hot water” pipe, but I haven’t had that antenna on the antenna range to measure how much the frequency was pulled.

Construction

The boom is made from 1/2" ¥ 3/4" wood. You might try 1/2" ¥ 1", but I think 1/2" ¥ 1/2" would be too weak. Do not substitute a thicker wood where the elements pass through!
The elements were made from 1/8-inch diameter silicon bronze welding rod, which is relatively cheap from your local welding supply. However, 1/8-inch hobby tubing or 1/8-inch aluminum ground wire can also be used for the elements. Make sure the ends are flat and square. Now is a good time to buy that pair of calipers you’ve been meaning to get for some time. You’re going to have to be very careful with the measurements for the elements. You need to be within .05 inch (approximately 1/16 inch). It’s better to be a few thousandths short than a few thousandths long for the elements. I used a bench grinder to bring mine into tolerance.
 

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