David wrote:
I am a little confused with calculation of appropriate length of metal
tubing used in a sleeve antenna design (the 1/4 wave section).

I understand that as RF travels through various mediums, Velocity of the
wave slows down relative to Er of the material. So for coax the
wavelength would be say 0.66 Lambda.

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.

1. Do you need to count for both these when calculate how the tube
should be.
2. Is there relationship that needs to be considered between sleeve
length and distance between sleeve and coax (ie. If the sleeve is made
larger diameter does it effect the length required for the tube or is
this still defined strictly by Er) also, Does the diameter of the tube
effect the impedance ?

The specific example is RG174 Coax cable vertical dipole. The top
radiating element is 1/4 long. At the bottom of this element is a length
of RG174 Coax. 1/4 wave length of this coax has a 5/32 Brass sleeve. The
sleeve is soldered to the RG174 coax at the point where the coax meets
the top 1/4 wave element.
I need to understand what needs to be used to calculate the length of
Brass tubing sleeve given the tubing diameter is 5/32 and besides air,
the outer sheath of the RG174 is also between the sleeve and Earth
braid. Is the diameter of the tubing critical (will it impact adversely
on SWR looking into this antenna).

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

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