Does anyone know of where I could find an example of a homebrew open stub
fed j antenna. I want to construct a unit for 915 MHz using SMA plug and
coaxial cable.

Thanks in advance

David wrote:
Does anyone know of where I could find an example of a homebrew open stub
fed j antenna. I want to construct a unit for 915 MHz using SMA plug and
coaxial cable.

An open stub J-pole is the same as a Zepp. 1/2WL element
end fed by 1/4WL balanced series section. The coax connects
to the end of the series section. Arrow Antenna makes one
for 2m. Plans are available that could possibly be scaled
to 915 MHz.
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil,

Thanks for the info. The open stub J looks like it should be suitable
for my application being ground independent, omni directional and having
low radiation angle.

Cecil Moore wrote:

David wrote:

Does anyone know of where I could find an example of a homebrew open stub
fed j antenna. I want to construct a unit for 915 MHz using SMA plug
and coaxial cable.


An open stub J-pole is the same as a Zepp. 1/2WL element
end fed by 1/4WL balanced series section. The coax connects
to the end of the series section. Arrow Antenna makes one
for 2m. Plans are available that could possibly be scaled
to 915 MHz.

Dave

Have you considered making a coaxial version of the J-pole?

http://www.ansoft.com/news/articles/04.05_MWJ.pdf

Jerry





"David" wrote in message
...
Cecil,

Thanks for the info. The open stub J looks like it should be suitable for
my application being ground independent, omni directional and having low
radiation angle.

Cecil Moore wrote:

David wrote:

Does anyone know of where I could find an example of a homebrew open
stub
fed j antenna. I want to construct a unit for 915 MHz using SMA plug and
coaxial cable.


An open stub J-pole is the same as a Zepp. 1/2WL element
end fed by 1/4WL balanced series section. The coax connects
to the end of the series section. Arrow Antenna makes one
for 2m. Plans are available that could possibly be scaled
to 915 MHz.

David wrote:
Thanks for the info. The open stub J looks like it should be suitable
for my application being ground independent, omni directional and having
low radiation angle.

Be sure to include a choking function at the feedpoint.
--
73, Cecil http://www.qsl.net/w5dxp

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Thanks for the feedback everyone.

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).




Cecil Moore wrote:
David wrote:

Thanks for the info. The open stub J looks like it should be suitable
for my application being ground independent, omni directional and
having low radiation angle.


Be sure to include a choking function at the feedpoint.

In article ,
David 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...df/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 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
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