On Sat, 2008-04-12 at 12:27 -0700, Jim WA9YSD wrote:
> ----"Resonant antennas for each band/frequency is the way to go. An the
> coaxial choke balun properly constructed and installed very close to the feed
> point is and excellent performer. It handles lots of power without problems
> and is inexpensive to built."
>
> In addition installing the balun at the end of a 1/4 wave length of open feed
> line is a much better way to go. How ever single band, tuned DOUBLE BAZOOKA
> antennas is even a better way to go. Quieter (lower noise) than a folded
> dipole, lower RFI, and a bit more efficient than a Folded dipole because it
> eliminates the use of a balun.
>
The double bazooka is a sham. It can have a slightly increased
bandwidth, but that's from the fatter ends made of shorted twinlead, not
the connections at the center. And its made mostly of insulation, not
wire so it has poor mechanical strength, won't stand up to ordinary 50
mph spring freshets. The connections at the center do NOTHING for
balance on the coaxial feedline.
In reality the coax stubs are each a quarter wave long in coax, but two
are hooked in series in parallel with the feed point and that's too
little reactance change to affect bandwidth. If the two coax stubs are
reconnected so they are in parallel across the feedpoint, their parallel
admittance changes with frequency but in the opposite direction of the
antenna parallel admittance while the antenna parallel conductance is
practically constant over a band like 80 meters.
In fact while the two stubs in parallel make a 25 ohm stub an alternate
that I've made work for years uses a 3 meter stub of 50 ohm coax
resonated by a 2000 pf mica capacitor. The current in the capacitor is
rather large, so it takes a large transmitting mica to stand up to 100
watts of RF. This gives a 1.3 or better match from 3.5 to 4.1 MHz. Just
because the parallel resonant circuit in parallel with the feed point
compensates for the antenna's reactance change vs frequency. This was
known and applied for broad band VHF antennas during WW2. Some of the
details are in Volume 1 of Very High Frequency Techniques published
about 1948, report of the research of the Radio Laboratory at Harvard
during WW2.
Its quite practical to achieve added bandwidth on an 80 meter dipole by
forking the outer ends of the dipole, or wiring two dipoles, one cut for
80, one for 75 in parallel, or terminating the end of the antenna in a
triangle or a cross bar.
> Its draw backs for a DOUBLE BAZOOKA are just a bit smaller band width than a
> folded dipole, and about 0.1 less efficient than a folded dipole. With high
> grade coax, it might be more expensive to build.
>
> An advantage to a DOUBLE BAZOOKA is that if you use RG-8X for the feed line,
> the feed line is lighter then home made twin lead and easier to strain
> relieve the feed line. Because of no balun I feel that the Bazooka can
> handle more power without problems.
>
The double bazooka can't handle any more power than the feedline will
handle given the SWR of the antenna at the band ends. RG-8X isn't that
much bigger than RG-58 and often is made of second grade braid without
much copper. Its the copper cross section that makes it handle current.
Thin conductors overheat most easily.
> Keep The Faith, Jim K9TF/WA9YSD
>
>
>
73, Jerry, K0CQ
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