Re: Open Stub fed J antenna

In article <Lk3We.47558$FA3.24590@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
David <groups@xxxxxxxxxxxxx> wrote:

>The article Jerry pointed me to starts looking like the commercial
>antenna I was initially studying. Their unit though only had a 1/4 wave
>above the coaxial sleeve.
>
> From the SMA plug, the coax coiled around the radome must have been the
>choke to help reduce RF currents radiating from the earth braid.
>Then the 1/4 wave length coax forms the match followed by the expose
>radiating section.
>
>It looks like an open stub fed J-pole where the stub is enclosing the
>inner element rather than being constructed as a rod next to it.
>
>Does this seem reasonable ? (PS. I'm a beginner with regards to antenna
>theory and would like to understand what is happening so that I can
>experiment with some kind degree of success).

Yes, it does. Take a look at the following:

http://download.antennex.com/hws/ws1002/sperrtof.pdf

A Sperrtof, in effect, is a J-pole whose matching section is a coaxial
tube rather than a single rod or wire. It sounds rather like what
you're describing.

As with all such (I think), the radiating section is 1/2 wavelength
long, give or take a smidge, and behaves as an end-fed 1/2-wave
dipole. The matching section isn't supposed to radiate significantly.

The overall radiation pattern would, I expect, be essentially the same
as other J-poles and other end-fed 1/2-wave radiators - similar to a
center-fed 1/2-wave dipole, but tilted a bit "upwards" away from the
feedpoint.

You can distinguish a Sperrtof-type antenna from one of the coaxial
dipoles Jerry referred you to, by the length of the single-wire
radiator - it's 1/2-wave for a Sperrtof and 1/4-wave for a coaxial
dipole (which is really a center-fed dipole).

There's an interesting dual-band 2m/440 antenna which was written up
in QST in October 2000 - ARRL members can get the article at
http://www.arrl.org/members-only/tis/info/pdf/0010050.pdf

It's interesting because it's _called_ a J-pole, _looks_ like a
J-pole... but electrically it isn't. It's actually a center-fed
vertical, not a Zepp. The stub at the bottom acts as a
choke/decoupler, not as an impedance transformer.

--
Dave Platt <dplatt@xxxxxxxxxxxx> AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
.

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