I agree with Herb 1000 % I also switch to cage feed antennas here on the high
Desert because the ground out here is very poor .
My verticals are 70 foot top loaded ( t ) with 62 radial on each of them I have
6 cage wires 8 foot across the cage . My bandwidth is 1.8 thru 1.933 SWR less
than 1.2 1 The unipole is easy to install requires little tuning . No high
price vacuum caps required .
The base of my unipole can be seen at my web page shown on QRZ. Com
73 Randy W6OAR
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My change from a single wire to a three wire cage unipole certainly made
my antenna bandwidth useable. I think several NAB tech papers on this
suggest that the copper wire cage also increases the overall efficiency
of the radiator. Before the changed to the cage feed I could only work
15 Khz with a CLC network. The antenna performance was mediocre. The
three wire and finding the sweet point changed everything here.
One very important issue not mention yet is the fact that the "apparent"
bandwidth due to a poor ground system can cause us to make harmful
assumptions. The poor ground and relative ground loss scenario can make
any shunt fed tower appear to have greater bandwidth. We feel good
because we have a large bandwidth and the VSWR is against the pin to the
left! The bandwidth is real but the performance can be poor, sort of
like connecting non-inductive resistors from the feed point across the
coax. resistors. I think Earl can prove this theory from his models. It
is so very important to begin these test with a very good grounding and
radial system,. With a poor ground system you could have a low VSWR over
a good portion of the band but have an antenna that loses nearly half
its power applied in the ground below. But yet you feel good about the
antenna for all the wrong reasons. I am sold on a cage wire feed shunt
feed especially for a short tower like I have.
73
Herb Schoenbohm, KV4FZ
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