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2016 Extra Class Study Guide: E9E - Antenna matching

Posted: 02 Mar 2016 04:36 PM PST
http://feedproxy.google.com/~r/kb6nu...m_medium=email

E9E Matching: matching antennas to feed lines; power dividers

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 ratio 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 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.

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 a shunt inductor. (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.
One use of a Wilkinson divider is that It is used to divide power equally
between two 50 ohm loads while maintaining 50 ohm input impedance. (E9E13)

The post 2016 Extra Class Study Guide: E9E – Antenna matching appeared
first on KB6NUs Ham Radio Blog.


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Posted: 02 Mar 2016 11:55 AM PST
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