The best reports I've seen published on broad banding antennas is in the
two volume set, "Very High Frequency Techniques" that was a report of
WW2 work at the Radio Research Lab at Harvard. Edited by Herbert J.
Reich. Published in 1947. Broad banding can come from multiple
techniques, fat or conical elements with reactance compensation circuits
at the feed point. I thought I invented one where I put a parallel
resonant circuit in parallel with the feed. They beat me to it and then
improved on it with a series resonant circuit in series with the
feedline for greater bandwidth up to 1.5 to 1 frequency range.
My scheme that has been used by a few is like this: Start with a dipole
cut for 3750 of plain wire, no fat ends or double wires. At the feed
point parallel it with a tuned circuit using about 2000 pf of capacitor
and about 3 meters of shorted stub of RG-58 in parallel. Resonant at
3850. The reactance of that capacitor is 20 ohms at 4 MHz so it sees at
least 3.5 amps at 100 watts power level. A transmitting cap is needed.
Then my particular scheme gets fed through a 50' length of RG-59 and
then at least 20 feet of RG-58. It has a SWR in this configuration of
about 1.3 from 3.6 to 4.1 MHz. It was designed for Army MARS interests
so is a little high in frequency. I didn't refine it all the way. The
SWR rises rapidly outside that frequency range and doesn't get better
than 1.3. The compensated antenna plots an impedance circle around 100
ohms (not quite resistive) on the smith chart which the 50' length of
RG-59 transforms to a circle around 50 ohms resistive.
One can use coax for the capacitor, essentially using a quarter wave
shorted stub tapped at the 3 meter point, or a couple quarter wave 50
ohm stubs in parallel at the feed point. That worked decently for me in
the basement testing the feed impedance of a VHF dipole. The rate of
change of reactance of the parallel tuned circuit or shorted stub can be
selected to match the change of parallel reactance (or admittance) of
the dipole to achieve quite a broad band match. In VHF Techniques they
got antennas decently flat from 100 to 150 MHz with the combination of
techniques.
My MARS station fanned dipole wasn't fanned from the feed point, only
from a point 6' from the outer end insulator. It was a single wire from
that fan point to the feed point on each side.
Most of the slightly improved bandwidth of the Bazooka antenna is from
the wider ends of twinlead compared to wire. The two stubs normally
connected in series across the feed point have little effect on the
bandwidth. If they are cut and reconnected so they are in parallel the
bandwidth can be significantly expanded. But the antenna suffers
structurally, with the coax and twinlead as radiator it has very little
copper for strength while it has considerable wind and ice collection
area. My radio club used to keep some on hand for FD and they needed
rebuilding every year because of broken wires.
73, Jerry, K0CQ
On 2/3/2011 5:06 PM, Richards wrote:
My research into this, including cage dipoles, suggests one may realize
little practical tangible effect on SWR bandwidth when replacing a heavy
wire antenna element with a small aluminum tube (where "small" = 1-2
inches in diameter.) Theoretically (I believe) it would have more impact
on higher frequencies a the size of the element increases relative to
wavelength.
There is some discussion of this in the ARRL Antenna Handbook. See page
9-4 and 2-3, et. seq. where it discussed the impact of larger diameter
elements on impedance.
Hope that points in a useful direction.
Happy Trails.
======================= Richards / K8JHR =========================
On 2/3/2011 5:30 PM, Dr. Gerald N. Johnson wrote:
That slow change of the resistive component and rapid change of the
reactive component is much more prominent if you look at the parallel
admittance than the series impedance.
73
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