Tuning my first antenna

So, I done gone and built my first antenna.

It's a simple directional antenna called a "yagi", designed to operate in the 2 meter amateur band (144-148MHz). The plans came from this QST magazine article. So far I'm out about $14 in parts and a couple hours' labor. (The plans are titled "7dB for seven bucks"; I chalk up the discrepancy to inflation and currency conversion).

The antenna works like this: there are three wire elements attached to a non conductive boom. The center element is the driven element; it is connected to the radio and is driven by it. The other two elements are called parasitic elements because they are not electrically connected to anything; they work by absorbing and reflecting the field produced by the driven element.

The longer element is called the reflector and the shorter one is the director. They work together to focus the field produced by the driven element, stretching it out along the axis of the boom and compressing it along the sides, in much the same way lenses and mirrors can be used to shape the light coming from a lamp.

But just like a musical instrument, building it is only half the job. Because an antenna is part of a resonant circuit, it must be tuned to match the radio to which it's attached. And the resonance changes with frequency, so the antenna must be tuned for the frequencies on which it will be used.

Now, I don't want to dive into a lot of electrical engineering here, partly because I don't completely understand it myself, so the short story is that a good measurement of how well your antenna is tuned is something called the standing wave ratio. Basically, if the antenna is out of tune, some of the power sent to it will be reflected back to the radio. The further out of tune, the more power is reflected. Too much reflected power will overheat and burn out the radio. The SWR is the measurement of how much power is reflected.

The resonant frequency is controlled by the length of the driven element. So tuning the antenna means changing the length of the driven element until the antenna resonates at the frequency you want. Since it's hard to make wires longer, I started with an antenna that was too long and trimmed the elements as necessary.

So how do you measure SWR? Why, with an SWR meter of course! Trouble is, I don't have one. However, a fellow operator was kind enough to lend me his antenna analyzer, which not only measures the SWR, but also the impedance, capacitance, inductance, and many other things. Since this is my first antenna, I'm not too worried about these things; I'm mainly concerned with building something that is an improvement over what I already have, without blowing up my radio, and an SWR below 2 will do that for me.

So I stood the antenna up on a step ladder in my back yard, hooked up the analyzer, and started tuning. Initially, the antenna resonated at about 130MHz, which is too low. I began trimming the driven element in 1/8" and 1/4" increments until I eventually got the antenna resonating at about 145MHz with an SWR of 1.6. This puts it nicely in the 2 meter amateur band.

How good is this? Well, before I packed everything away, I hooked the analyzer to the factory-built magnetic whip I have (A magnetic whip is an antenna which is magnetically stuck on the roof of your car). It resonates at 162MHz, has an SWR of 2.9 at 144MHz and an SWR of 2.4 at 148MHz. That doesn't sound very good, but if I were to give the manufacturer the benefit of the doubt, I would say that the poor readings are the result of the fact that a magnetic mounted antenna uses the surface to which it's stuck as a ground plane, which means it relies on a good electrical connection, something that is dependent on the type of paint used on the car (non-metallic paints will provide a poor electrical connection, which will hurt the antenna's efficiency.)

So while it's not the best antenna around, I'd say it's a good first try. Mainly I did it for the learning experience, and for that it was wildly successful, as it taught me an important lesson about antenna-feedline interaction:

The feedline gets in the way, electrically speaking. I originally had the coax feedline hanging down the mast in the center, which puts it inline and near to the driven element. On the advice of a fellow operator, I routed the feedline along the boom to the rear. This meant the coax was now running perpendicular to the antenna elements instead of parallel to them, which had a significant impact on resonance, impedance, and SWR. Future designs will take this into account from the beginning.