I am still confused on this one.
I see a 1/4 wave radiating element joined to another 1/4 wave radiating
element that just happens to also act as a choke to reduce RF on the
return braid of the RG174 coax.

The point where they meet is supposed to look like the centre of a 1/2
wave dipole so the impedance must be around the 72 Ohms

If the Sleeve is acting as a choke(Stub) then does it not need a second
element as the return (ie is it not acting like a 1/4 length of coax
where the tube is the outer braid and the RG174 cable running up the
centre outer sheath is the dielectric and the outer braid of the RG174
becomes the inner conductor of the stub )?

If this is correct then the electrical length of the sleeve must be
modified by the presence of the RG174 coax outer sheath. The sleeve is
sort of performing a dual function.

As it is difficult to cut the tube once soldered in place, I will need
to make heaps of these to get the length right. At least if I had a
reasonable starting point it would minimize the number of iterations.

The other issue is that I read about cutting lengths for elements and
not that even an element in free space is trimmed down by about 3% to
account for some "end effect" ? Would this also apply to my sleeve ? If
so, then the length may be difficult to get right because if I make
allowance for the end effect then the choking effect of the sleeve will
not work as well.

I do not want to reinvent the wheel here as these antennae are sold
already being sold commercially and I am sure there are numerous people
who have successfully made these. Therefore if I can learn from other
people's mistakes and success, the starting point is much closer to the
solution.

I have a practical application but want to understand also. The
requirement is that I have a small antenna that can be built into the
end of a RF connector and plug directly onto a radio modem operating in
the 915MHz band. The power is 1 Watt max. The design needs to be
repeatable and not require trimming at every location (being portable I
have no control over where the unit is located), as there is no decent
ground plane, I need a unit that does not require radials or large
ground plane.
The low radiation angle and no requirement for large ground plane made
the J-Pole look like a good contender but it was physically difficult to
mount in this application.

After looking around and opening up some commercial units, I noted many
used the Choke Sleeve, 1/2 wave dipole antenna.

This is why I am now wanting to understand how these work and have a
good starting point for making them. The commercial units used molded
construction that was difficult for me to emulate. I have started with
RG174 Cable, 5/32 Brass tube all attached to SMA plug and used 1/4"
nylon tubing as a radome. It goes together well and looks good but does
not perform very well.

Thanks for any further info.


Dave VanHorn wrote:
With the tubing over a coaxial cable though it seems there is Vp related
to the dielectric formed between the sleeve and the outer sheath of the
coax cable but also the Metal has an Er value.


From what I understand, the RF will be traveling on the outside of the
sleeve, and won't "see" the internals at all.
So, the sleeve ends up as a 1/4 wavelength stub.

Is the diameter of the tubing critical (will it impact adversely

on SWR looking into this antenna).


It shouldn't be


The final point would be if I place some ferrite rings across the coax
below the sleeve, will this improve SWR ?


Not if the sleeve is doing it's job.
The sleeve's open end should look like a short at the operating frequency,
and keep RF off the feedline.



Thanks

Regards

David

David;

That's why they call it the ART of antenna design. First you make the
best mathematical calculations you can then you start cutting wire/tubing.
Make your lengths slightly longer than the math says then prune the
lengths until the antenna works the way you think it should.

There are so many variables in antennas that this is the only practical
way to make a new one. Once you find your optimal lengths you can make
more to the same specs. However remember that just like in housing
antenna's depend on three things: location, location, location and have to
be custom adjusted for each site.

Dave WD9BDZ