On 8/18/2017 7:34 AM, Grant Saviers wrote:
One not in the current handbook is the Witt/Leeson tuned coaxial
resonator feed method, see
http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1993/09/page27/index.html
Garry, NI6T, turned me on to this several years ago, crediting it to
W6NL. Dave, who teaches at Stanford, has long included it in his
classes, but denied credit for it, saying that it was far older than
him. And like me, he's no pup! I included the technique in a talk I did
some years ago on using SimSmith to design antenna matching networks.
http://k9yc.com/PacificonSmithChart.pdf
I showed that the technique is not limited to a one-wavelength matching
section -- indeed, a SimSmith model shows that it works fine with any
whole number of half-wavelengths up to about 4.
This technique does not, however, work for a multiband dipole to cover
80/75/40. I have such a dipole, so I worked long and hard at a model and
never arrived at anything that looked good on both bands. The problem
occur on 40M. Instead, I switch in stub matching networks in the shack.
To design them, I measure feedline Z with my DG8SAQ VNWA 3e, export the
data to SimSmith, and do the design work there. This is also included in
the SimSmith talk. Among other things, if line length and coax type are
known, SimSmith can show plots of feedline loss vs frequency. SimSmith
has data for many popular feedline types, and it's easy to measure the
electrical length of a feedline with the VNWA 3e.
http://sdr-kits.net/VNWA3_Description.html
Another point. Even if the tower is not resonant, it is still an active
element of any vertical antenna in the vicinity. N6LF alerted me to this
several years ago in an excellent study that he posted to his website.
I verified this by building a model of my 160M Tee vertical with a
grounded tower that wasn't even close to resonance and about 80 ft from
it, and saw a bump of several dB in the horizontal pattern.
The strength of the effect also depends on the impedance to earth of the
ground for the tower at the operating frequency. That impedance (mostly
R) shows up as a resistor in series with the base. I have two
quarter-wave wires sloping in opposite directions off the top of my 120
ft tower to a feedpoint about 60 ft from the tower. They are insulated
from the tower by a 10 ft length of 4-in PVC conduit. Each wire has four
radials elevated about 20 ft (this height recommended by N6BT), and I've
added 8 on-ground radials to the base of the tower to lower the Z of the
earth connection (because the tower is a passive element). NEC predicts
about 2 dB gain from this antenna, and on-air switching between the two
sloping verticals confirms the directivity (although not the gain, which
is FAR more difficult to measure).
73, Jim K9YC
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