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I
did a lot of computer modeling of the 4 bay antenna and others
but I didn't learn nearly as much as I did when I was able to
do
some actual field testing of antennas built from the computer models.
One of the biggest
mysteries I was trying to solve was some people had good luck with
their DIY builds and others didn't. It
appears it has mostly to do with how the elements and phase lines are
mounted to the frame and what material is used for an insulator.
Mounting antenna elements and phase lines directly to a wood structure
appears to be a bad choice.
Range
Testing
The range testing was
done outside at a
height of 25ft. the transmitting antenna and receiving (test) antennas
were spaced several wave lengths away but not so far as to introduce
ground bounce. I used a series of folded dipoles as reference antennas
and all of the other measured antennas were compared to the folded
dipole readings. For this test I used a spectrum analyzer with it's own
tracking generator as the transmitter. Each antenna was tested at least
twice to double check the readings and the same balun was used in all
testing.
Field testing
The field testing was done in an open field with all antennas at a
height of 20ft. The antennas were all hooked to a Channel Master CM0064
mast amp and then run through 50ft of coax where the signal
was
split to a Zeinth DTT900 converter box and a B&K spectrum
analyzer.
I checked for the ability to lock the signal with the DTT900
and
to verify what station I was receiving and used the B&K
analyzer
for signal readings.
One of the most
interesting things I learned was that the range and field testing
didn't follow the computer models as close as you might think. Most
antennas overall
didn't
perform as well as the computer models show and many antennas didn't
show max gain at the same place as the computer models. On the flip
side there were a few times where the antennas actually beat the
computer models.
The 4 bay style antenna frequency response curve appears to peak at a
lower frequency than the computer models show. This is not a surprise
since the area where the whiskers and phase lines attach is a sensitive
area and contact with most anything in that area will detune the
antenna.
Here
are the results of some
of my range and field testing.
This
test was
performed on my antenna test range
Here are the frequency curves of some DIY 4 bays, one particular note
is
how poorly the antenna built with wood stand-offs performed (brown plot
line). The wood stand-offs were 1" x 2" wood blocks and were poly
coated before the wires were attached. The exact same wires were
removed from the wood frame and attached to a PVC fitting
frame,
the aqua blue plot line shows the PVC frame. I then cut the
whiskers to 9" as shown in the yellow dashed line plot, all these
examples peak at a frequency below the computer modeled frequency. On a
positive note even though the antennas built on plastic stand-offs
peaked lower in frequency they did hold peak gain figures very close to
the computer models.
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