Radio antenna systems have a different impedance, or resistance to electrical flow, on the antenna beams and the feed-point cable connected to them. This causes interference and limits the quality and intensity of reception. Gamma matches are a type of transformer that match the impedance difference between the antenna and the coaxial cable. You especially need to match this difference in impedance, when you work with antennas with multiple elements -- because, in this kind of set-up, the impedance at the feed-point cable is particularly low.
The method to adjust the gamma match of an antenna will vary depending on the model and configuration you use for your antenna. However, a general description of the method may help you adapt the technique to your set-up.
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Set your antenna's driven elements to their calculated length. Every antenna is designed to receive or transmit particular wavelengths and intensities. Set your antenna to its basic configuration, so you have a baseline to work with.
Choose a tap point for your set-up. The tap point determines the level of impedance you transmit to the coaxial cable. Impedance levels of 50 and 20 Ohms are not uncommon.
Tune your antenna's capacitor to mid-range.
Sweep the antenna's reception band and search for the smallest standing wave ratio, or SWR. The SWR is measure of the impedance match between your antenna and you feed line. The SWR of an antenna is often represented graphically by waves. Tune your radio for waves with the smallest amplitude -- or difference between peaks and troughs -- possible. Write down the tuning frequency for your SWR at mid-range.
Tune your capacitor to the smallest SWR possible. If your SWR worsens, adjust in the other direction.
Repeat steps 4 and 5 for as long as you have time, or until your SWR is at a reasonable level. If you are not achieving the SWR curve you want, adjust the length of the antenna's elements.
Tips and warnings
- You can also adjust the impedance of an antenna, by adjusting the spacing between the antenna's tubes. Don't do this unless you have a good understanding of the physics and mathematics that determine impedance in a multi-element antenna.
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