[TowerTalk] "75-Meter Dipole Stretcher"

[Richard (Rick) Karlquist]

On 10/15/2017 7:24 PM, Paul Christensen wrote:

> What caught my attention was this statement in the article's second
> paragraph:  "The solution is to add an inductor at a multiple of a
> half-wavelength-long feed line, and its *effect will be the same* as a coil
> at the dipole feed point."
> 

I normally try to restrain myself from bashing QST technical articles,
because they are such easy prey :-)

Besides the loss problem you mentioned, a more fundamental problem is
that a half wavelength line is only a half wavelength at one
frequency, presumably the center of the band.  As you tune away from
that frequency, the 1/2 wave line is no longer just an attenuator
as you report, but it acts to reduce the bandwidth of the system.
The more multiples of a 1/2 wave, the more reduction.

The other issue is that he cut the dipole for the high end of the
band and then tuned it down with an inductor.  What I have been doing
for a long time is to instead locate a capacitor at the feedpoint
with the normally closed contacts of a relay across it.  The
dipole length is adjusted for CW, and the relay is energized to go
to phone.  An Omron MJN series relay from Mouser and a single 220 pF
1 kV mica capacitor are used.  The switchable capacitor is in series
with the center conductor of the coax at the "line" end of the common
mode choke.  The choke is just a big toroid with coax wound around
it.  I mount all this in a plastic box.  It works fine for inverted vees 
where there is a support at the center.  Even if the support is just
a 3 inch thin wall irrigation pipe held up by ropes.

Rick N6RK