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2016 Extra Class Study Guide: E9B - Antenna patterns

Posted: 27 Feb 2016 12:16 PM PST
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E9B Antenna patterns: E and H plane patterns; gain as a function of
pattern; antenna design

Many amateurs use directional antennas because they are said to have
“gain.� When this term is used, what it means is that a directional antenna
will output more power in a particular direction than an antenna that is
not directional. This only makes sense; You can’t get more power out of an
antenna than you put in. Assuming each is driven by the same amount of
power, the total amount of radiation emitted by a directional gain antenna
compared with the total amount of radiation emitted from an isotropic
antenna is the same. (E9B07)

To evaluate the performance of directional antennas, manufacturers will
measure the field strength at various points in a circle around the antenna
and plot those field strengths, creating a chart called the azimuth antenna
radiation pattern. Figure E9-1 is a typical azimuth antenna radiation
pattern.



The antenna radiation pattern shows the relative strength of the signal
generated by an antenna in its “far field.� The far-field of an antenna is
the region where the shape of the antenna pattern is independent of
distance. (E9B12)

From the antenna radiation pattern, we can tell a bunch of things about the
antenna. One of them is beamwidth. Beamwidth is a measure of the width of
the main lobe of the radiation pattern. To determine the approximate
beamwidth in a given plane of a directional antenna, note the two points
where the signal strength of the antenna is 3 dB less than maximum and
compute the angular difference. (E9B08) In the antenna radiation pattern
shown in Figure E9-1, 50 degrees is the 3-dB beamwidth. (E9B01)

Another parameter that’s important for a directional antenna is the
front-to-back ratio. In a sense, this is a measure of how directional an
antenna really is. The higher this ratio, the more directional the antenna.
In the antenna radiation pattern shown in Figure E9-1, 18 dB is the
front-to-back ratio. (E9B02)

A similar parameter is the front-to-side ratio. In the antenna radiation
pattern shown in Figure E9-1, the front-to-side ratio is 14 dB. (E9B03)

When reviewing an antenna radiation pattern, you need to remember that the
field strength measurements were taken at a particular frequency. When a
directional antenna is operated at different frequencies within the band
for which it was designed, the gain may change depending on frequency.
(E9B04)

Many different design factors affect these antenna parameters. For example,
if the boom of a Yagi antenna is lengthened and the elements are properly
retuned, what usually occurs is that the gain increases. (E9B06)

Because antennas radiate in three dimensions, the azimuth antenna pattern
tells only part of the story. To get a complete picture of antenna
performance, you also want to know what the antenna pattern is in the
vertical direction. This type of pattern is called the elevation antenna
pattern. An elevation antenna patter over real ground is show in Figure
E9-2. (E9B05)



In the antenna radiation pattern shown in Figure E9-2, 4 elevation lobes
appear in the forward direction. (E9B16) In the antenna radiation pattern
shown in Figure E9-2, the elevation angle of peak response in is 7.5
degrees. (E9B06) The front-to-back ratio of the radiation pattern shown in
Figure E9-2 is 28 dB. (E9B15)

Antenna design

To help design antennas, many amateurs use antenna modeling programs. All
of these choices are correct when talking about the information obtained by
submitting the details of a proposed new antenna to a modeling program
(E9B14):

SWR vs. frequency charts
Polar plots of the far-field elevation and azimuth patterns
Antenna gain


The type of computer program technique commonly used for modeling antennas
is method of moments. (E9B09) The principle behind a method of moments
analysis is that a wire is modeled as a series of segments, each having a
uniform value of current. (E9B10)

The more segments your simulation uses, the more accurate the results. The
problem with using too many segments, though, is that the program will take
a very long time to run. You don’t want to use too few segments, though. A
disadvantage of decreasing the number of wire segments in an antenna model
below the guideline of 10 segments per half-wavelength is that the computed
feed point impedance may be incorrect. (E9B11)

The abbreviation NEC stands for Numerical Electromagnetics Code when
applied to antenna modeling programs. (E9B13) This is different from the
more common definition of NEC, which is the National Electrical Code.

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