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Old February 16th 10, 01:48 PM posted to rec.radio.amateur.antenna
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Default OCF: Proprer type of balun (transformer)

On 15 f v, 19:22, Roy Lewallen wrote:
Sometimes the radiation from the feedline, mains wiring, and stuff in
the shack can be beneficial -- it might, for instance, fill in deep
nulls in the main antenna's pattern and result in a spectacular signal
strength improvement in particular directions.


I am slightly OT here, but the same argumentation could be used about
non-resonant antennas like the G5RV. Yes, ATU needed, but L-type
autotuners for example have very small insertion loss.

I am wondering about the real advantage of a razor cut dipole
(resonant anyway on a small region around a freq) versus a multi-band
dipole using a length of feed line matching section like the G5RV or
ZS6BKW/G0GSF Antenna System...

73 de Pierre VE2PID
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Old February 16th 10, 06:58 PM posted to rec.radio.amateur.antenna
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Posts: 644
Default OCF: Proprer type of balun (transformer)

On Feb 16, 4:48*am, ve2pid wrote:
On 15 f v, 19:22, Roy Lewallen wrote:

Sometimes the radiation from the feedline, mains wiring, and stuff in
the shack can be beneficial -- it might, for instance, fill in deep
nulls in the main antenna's pattern and result in a spectacular signal
strength improvement in particular directions.


I am slightly OT here, but the same argumentation could be used about
non-resonant antennas like the G5RV. Yes, ATU needed, but L-type
autotuners for example have very small insertion loss.

I am wondering about the real advantage of a razor cut dipole
(resonant anyway on a small region around a freq) versus a multi-band
dipole using a length of feed line matching section like the G5RV or
ZS6BKW/G0GSF Antenna System...

73 de Pierre VE2PID


As far as I know, the key advantage of a resonant half-wave dipole is
that its feedpoint impedance is a pure resistance, and generally low
enough to provide a decent match to a coaxial feedline. The radiation
pattern of a dipole changes very little with fairly large changes in
frequency, until it becomes somewhat longer than one wavelength. Even
then, it doesn't become inefficient; it just develops lots of lobes
and nulls as its length in wavelengths increases (which may or may not
be a disadvantage). Reading between the lines of what Roy wrote, a
bit of feedline radiation will at least change where those lobes and
nulls are. But even a relatively short antenna (e.g. a dipole that's
only 0.1 wavelength long) can be quite efficient--the loss is
typically mainly in the network (including the feedline) used to feed
power to it. Another factor to consider for electrically short
antenna is that you have to tune out a reactance that's large compared
with the radiation resistance, and that makes them narrow-band if the
matching network is efficient. You have to re-tune for even small
changes in frequency.

Cheers,
Tom
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Old February 16th 10, 07:31 PM posted to rec.radio.amateur.antenna
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Posts: 572
Default OCF: Proprer type of balun (transformer)

Something that hasn't been mentioned is that common-mode energy forms
into standing waves, e.g. on the outside braid of the coax. An
isolator (choke), as exists on a Carolina Windom, may only change that
point to a current minimum while simply moving the current maximum
point to a different location. Has anyone actually measured the common-
mode current on a Carolina Windom on all bands up and down the coax on
the shack side of the isolator?
--
73, Cecil, w5dxp.com
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