## Extra Class question of the day: matching antennas to feedlines

For many types of antennas, matching the impedance of the antenna to the impedance of the feedline, normally coax, is essential. Mismatched lines create high SWR and, consequently, feedline losses. An SWR greater than 1:1 is characteristic of a mismatched transmission line. (E9E08)

When a feedline and antenna are mismatched, some of the power you are trying to transmit will be reflected back down the feedline. The ration of the amplitude of the reflected wave to the amplitude of the wave you are trying to send is called the reflection ratio, and it is mathematically related to SWR. Reflection coefficient is the term that best describes the interactions at the load end of a mismatched transmission line. (E9E07)

To match the impedance of the feedline to the impedance of the antenna, we use a variety of different techniques. The delta matching system matches a high-impedance transmission line to a lower impedance antenna by connecting the line to the driven element in two places spaced a fraction of a wavelength each side of element center. (E9E01)

The gamma match is the name of an antenna matching system that matches an unbalanced feed line to an antenna by feeding the driven element both at the center of the element and at a fraction of a wavelength to one side of center. (E9E02) The purpose of the series capacitor in a gamma-type antenna matching network is to cancel the inductive reactance of the matching network. (E9E04)The gamma match is an effective method of connecting a 50-ohm coaxial cable feed line to a grounded tower so it can be used as a vertical antenna. (E9E09)

The stub match is the name of the matching system that uses a section of transmission line connected in parallel with the feed line at or near the feed point. (E9E03) What the stub does is to add reactance at the feed point. By varying the length of the stub, you can change the reactance that the stub provides to whatever value is needed. An effective way of matching a feed line to a VHF or UHF antenna when the impedances of both the antenna and feed line are unknown is to use the universal stub matching technique. (E9E11)

Inserting a 1/4-wavelength piece of 75-ohm coaxial cable transmission line in series between the antenna terminals and the 50-ohm feed cable is an effective way to match an antenna with a 100-ohm feed point impedance to a 50-ohm coaxial cable feed line. (E9E10) Note that this will only work on one band as the length of 75-ohm coax you use will only be 1/4 of a wavelength on one band.

Many directly-fed Yagi antennas have feedpoint impedances of approximately 20 to 25 ohms. One technique often use to match these antennas to 50-ohm coaxial cable is the hairpin match. To use a hairpin matching system to tune the driven element of  a 3-element Yagi, the driven element reactance must be capacitive. (E9E05) The equivalent lumped-constant network for a hairpin matching system on a 3-element Yagi is an L network. (E9E06)

Some beam antennas use multiple driven elements in order to make them multi-band antennas. The primary purpose of a phasing line when used with an antenna having multiple driven elements is that it ensures that each driven element operates in concert with the others to create the desired antenna pattern. (E9E12)

I’m not sure that Wilkinson dividers are used much in antenna systems, or why this question is in the section on feedline matching, but here it is. The purpose of a Wilkinson divider is that it divides power equally among multiple loads while preventing changes in one load from disturbing power flow to the others. (E9E13)