[Antennas] SWR dipole

Fri, 6 Sep 2002 17:28:05 -0500

Yes, but not necessary to to fold the shield back, as the outer surface
of the shield carries the common mode or antenna current down to the
point of the choke coil. In fact the antenna can be tuned by adjusting
the position of the choke coil, more or less along the feedline, approx.
a 1/4 wave from the vertical dipole center. Very little common mode
current gets past the choke coil because it (if 7 turns or more on 6"
dia.) forms a high impedance trap being near parallel resonance with the
distributed capacitance of the turns. I would not worry about common
mode currents past the choke coil, as very doubtful you could even detect
them. 
For further info see the excellent article by James Taylor, W2OZH, QST,
August 91, or "More Wire Antenna Classics", Vol.2 by ARRL. Taylor was
not the first to use this principle of a "tuned transmission line trap"
with coax end feed of dipole antennas. 
73, Mike, ac5p 
On 2002年9月06日 11:48:14 -0500 [email protected] writes:
> George,
> I could be wrong but the way I read Terry's post, he is using
> coax as the vertical dipole with the shield folded back as
> the lower half???? That would make sense to me as I read it..
> otherwise, it does seem confusing :)
>> Chris
> WB5ITT
>> "George, W5YR" wrote:
> > 
> > Terry, I am not sure that I have the right picture here, but are 
> you
> > feeding the *end* of a vertical dipole directly with 50-ohm coax 
> through a
> > coiled-coax choke balun?
> > 
> > If so, how are you handling the enormous impedance mismatch 
> between the
> > antenna (probably 1000 ohms and possibly much more at the end) and 
> the coax
> > (50 ohms)?
>> > Terry Fletcher wrote:
> > >
> > > Yesterday I put up a 20M half wave vertical dipole with a coax 
> choke
> > > balun at the bottom end of the shield side. . It's 32' long and 
> seems
> > > to work ok. The bottom end is off the ground about 15'. Good sig 
> reports
> > > and it's 1.7:1 or less across the entire band. I don't 
> seem to
> > > have any rf on the feed line, no way to tell.
> > >
> > > I wanted to use ferrite beads as a means of decoupling the 
> shield (ala
> > > W2DU) from the bottom end of the dipole element. But I had some 
> rg-8x on
> - - - 
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