From: K7GCO@aol.com
> Yuri: Right on again. For "Basic Review 101" once again a 1/4 wave
> open
> stub (open on both ends) and shorted stub (shorted on one end) have
> the "opposite reactance changes" with frequency. The Collins 1/4 Wave
> Bazooka is equivalent to a shorted 1/4 wave balanced stub across the
> feedpoint and a dipole is equivalent to a 1/4 wave open balanced stub.
> Above and below their resonant frequencies their reactance's vary in
> "opposite directions"--they can cancel if connected to each other. So
> you have a "variable load" in a beneficial way with frequency.
maxwell analyzed that well in Reflections. As a matter of fact Walt
thought the common misrepresentation.... that the stub in a
coaxial dipole was responsible for increased bandwidth.... was so
important he devoted an entire chapter to it in Reflections. It was
also published in September 1976 QST.
If you run outside to your Bazooka and cut the center conductor
and short it to the shield, there is no measurable change in
bandwidth!
It is quite well documented in many places the very small change
in bandwidth is mostly attributed to increased loss.
> middle of the band. A Bazooka will double the band width of a 75M
> dipole because the broadbanding is done "right at the feedpoint." NO
> LOSS IN THE FEEDLINE IS NEEDED.
I actually compared SWR bandwidth of a Double Bazooka sold by
IAC in Florida and a conventional dipole with the same conductor
size and height. I measured between 1 and 2% bandwidth
improvement in the coaxial antenna. By no stretch of the
imagination is 1 or 2% "double the bandwidth".
The ARRL Handbook says the bandwidth improvement is 1.5%,
and attributes that improvement mostly to loss.
Maxwell simply says "the reactance of the stub is ten times too
high----useless".
My Smith Chart agrees with the ARRL and Maxwell.
Now Frank Witt did design a stub system that works, but it also
adds even more loss. But at least Frank Witt's system has the
correct impedances, and the stub does have a worthwhile effect.
His antenna is in the ARRL Antenna Handbook in the section on
broadband antennas.
73, Tom W8JI
W8JI@contesting.com
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