For many types of antennas, matching the impedance of the antenna to the impedance of the feedline, normally coax, is essential. When a feedline and antenna are mismatched, some of the power you are trying to transmit will be reflected back down the feedline or dissipated in the feedline. The ratio of the amplitude of the reflected wave to the amplitude of the incident wave, or the wave that you’re transmitting, is called the reflection coefficient, and it is mathematically related to SWR.
QUESTION: What parameter describes the interactions at the load end of a mismatched transmission line? (E9E07)
ANSWER: Reflection coefficient
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. It’s called a delta match because when connected this way, the feedline and antenna look like the Greek letter delta.
QUESTION: What system matches a higher-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)
ANSWER: The delta matching system
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. The purpose of the series capacitor in a gamma-type antenna matching network is to cancel the inductive reactance of the matching network. The gamma match is an effective method of shunt feeding a grounded tower so it can be used as a vertical antenna.
QUESTION: What 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)
ANSWER: The gamma match
QUESTION: What is the purpose of the series capacitor in a gamma-type antenna matching network? (E9E04)
ANSWER: To cancel the inductive reactance of the matching network
QUESTION: Which of the following is used to shunt-feed a grounded tower at its base? (E9E09)
ANSWER: Gamma match
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. 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.
QUESTION: What 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)
ANSWER: The stub match
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.
QUESTION: How must an antenna’s driven element be tuned to use a hairpin matching system? (E9E05)
ANSWER: The driven element reactance must be capacitive
Lengths of 75-ohm coax can also be used to match impedances. For example, 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. Note that this only works on one band as the length of 75-ohm coax you use will only be 1/4 of a wavelength on one band.
QUESTION: Which of these feed line impedances would be suitable for constructing a quarter-wave Q-section for matching a 100-ohm loop to 50-ohm feed line? (E9E06)
ANSWER: 75 ohms
QUESTION: Which of these choices is an effective way to match an antenna with a 100-ohm feed point impedance to a 50-ohm coaxial cable feed line? (E9E10)
ANSWER: Insert a 1/4-wavelength piece of 75-ohm coaxial cable transmission line in series between the antenna terminals and the 50-ohm feed cable
Another use for coaxial cable is as a phasing line for antennas that have multiple driven elements. The theory here is that by feeding the driven elements out of phase with one another, you can create a directional radiation pattern. A common application for phasing lines is a phased, vertical array.
QUESTION: What is the primary purpose of phasing lines when used with an antenna having multiple driven elements? (E9E11)
ANSWER: It ensures that each driven element operates in concert with the others to create the desired antenna pattern
Finally, unless you’re going to be doing microwave work, you probably won’t need to know about Wilkinson dividers, but here’s the information anyway. Wilkinson dividers divide power equally between two 50 ohm loads while maintaining 50 ohm input impedance. They’re used mainly in microwave systems.
QUESTION: What is a use for a Wilkinson divider? (E9E08)
ANSWER: It is used to divide power equally between two 50-ohm loads while maintaining 50-ohm input impedance
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