[Antennas] Vertical coaxial endfed dipole...

[George, W5YR]

The impedance of the coils of coax affects only the current flowing on the
outer braid of the coax. By placing the added inductance of the choke coil
at the proper point the "end"of the antenna sees effectively an insulator,
not perfect but close enough to work. Coiling the coax is one approach;
placing ferrite beads over the coax a la the W2DU balun will work also, and
perhaps better and easier to handle. But the principle is the same with
either approach: form an electrical "end" to that half of the dipole.

The conversion from the balanced feed at the center of the dipole (where
the center conductor connects to or actually forms one half of the dipole
and the *outer* braid from the center to the choke coil input side forms
the other half) occurs at the point where the choke is placed. On the
antenna side of the choke, current is flowing on both the inside
(differential mode) and outside (actual antenna current)of the coax braid.
On the other side of the choke, the choke inductance eliminates or reduces
the current flowing on the outer braid surface (now a common-mode current)
while having no effect on the current flowing on the inner braid surface
(differential-mode or transmission line current).

In operation, transmission-line or differential-mode current flows on the
inner braid surface until it reaches the center point where the outer braid
is connected to form the other half of the dipole. Antenna currents then
flow on the extended center conductor and the outer braid as with any
dipole antenna. But the portion of the feedline "within" the braid-half of
the antenna is acting as the transmission line while the outer braid is
acting as one-half of the dipole.

This antenna is deceptive in its appearance, but if you keep in mind that
the coax braid is really two conductors, inner surface and outer surface,
it is less confusing. Actually, this antenna is quite a clever utilization
of this property of coax. But remember that the is electrically not any
different from the classic "coaxial vertical" antenna dating back to the
30's. This antenna was widely used on 10 meters after the band was
re-opened in 1945 since it was quick and easy to build and surplus
brand-new coax was available for pennies/pound.

72/73/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771
Icom IC-756PRO #02121  Kachina #91900556  IC-765 #02437

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. 

James Duffer wrote:
> 
> >The ARRL handbook describes the construction of an and fed horizontal
> >coaxial dipole. This antenna is nothing more than a L4 radiator wire
> >soldered on the center conductor of a L4 coax cable. On the feed side of
> >the coax several loops of the same coax cable are present to creat a 1:1
> >balun.
> 
> This puzzles me also, how does "several loops of coax cable" creates a
> "balance to unbalance" condition???  Wouldn't it just create a current
> choke?.  How does the balance to unbalance conversion take place with just a
> few loops??
> 
> >Anyway to the questions:
> >
> >Has anyone had any experience with such an antenna ?
> >
> >How would this antenna rate to a center fed dipole ?
> >
> >I was thinking of building this antenna for 10m, the antenna would be
> >housed inside a fiberglass fishing rod in order to use it as a vertical...
> >
> >Any tips ? comments ?
> >
> >73
> >
> >Demetre - SV1ENS