Saturday, April 13, 2013

Antenna Tuners

I live in a small townhome, with no land to speak of. I would like to be able to talk on 80 m. Ideally, a quarter wavelength antenna is best, put at a quarter wavelength above the ground. That means 20m, or about 70 feet, which is longer than the length of my home! Given all of that, how can I get HF from my home?

One solution that I have is using an antenna tuner. There are some radios, such as the Yaesu FT-450D, which have a built in antenna tuner. While I plan on getting a radio like that some day, I've heard that a dedicated antenna tuner is far superior to a build in one in a radio. Having both should give me a bit more flexibility.

Enter an automatic antenna tuner. I've found details on how these work to be lacking, but I did find the article on eham to be helpful, as well as this one from TASRT, and this one from Ham Universe.

First of all, let me explain the purpose of an antenna tuner. If you don't care to know the physics behind this, feel free to skip the rest of this paragraph, along with the next two. An antenna works best if it is perfectly resonant with the frequency. For instance, using the middle of the 2m band frequency of 146 MHz, a half dipole antenna should be 3.4 feet tall. The edges of the band to to 144 and 148 are 3.43 and 3.34. The difference isn't that much, and in fact is sufficient to allow a standing wave ratio (SWR) of maybe 1.5, plenty good for any modern radio. An SWR is in effect how well the antenna is tuned, 1.0 being perfect, 2.0 being good enough, and beyond that usually needing some sort of correction.

Well, the ratios become much larger for higher frequencies. 80m covers 3.5-4.0 MHz, for instance. An antenna that tries to cover that entire spectrum will have great difficulties, because the impedance of the antenna changes. 50 ohms is the nominal impedance for most antennas, but the impedance changes depending on the frequencies.

Impedance can be corrected for, by using a system of inductors and capacitors. However, in order to do this, you have to limit the bandwidth, possible to only a very small section. These adjustments can be made by changing the impedance matching of the circuity to manage the correction. A device that allows you to adjust the peak signal of the antenna is called an antenna tuner. When you have a device that does this automatically, it is called an automatic antenna tuner.

So, you have an automatic antenna tuner, now what do you do? Well, I'm not going to go into a whole lot of detail, but I will say that you will need more than just the antenna and the tuner. Bottom line is, you need to calibrate your antenna before you just assume that the antenna tuner will allow you to work right. I'll leave you to look up the article previously posted on eHam, it will guide you to how to make sure everything is working right before you go and use the tuner.

Bottom line is, an antenna tuner will allow you to use a poor antenna without burning your radio out. An SWR meter will also be of huge user. They have their limitations, but they can allow you to talk on a broader range of bands than you otherwise would, using a poorer antenna. I'm probably going to get one, at least until I have enough land to actually build a decent one, but for now, it should do. If you really want to know more, take a look at the ARRL Guide to Antenna Tuners, the definitive source for antenna tuners.

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