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Old September 15th 05, 02:41 AM
David
 
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Dave,

If I scale that up to 915MHz, the dimensions are almost perfect for
using RG58 coax instead of the copper tubing.The outer tube would be
5.5mm and inner conductor 1.6mm. Being 50 Ohm coax, I assume therefore I
do not need to slide the tap point up as their design did. I suppose I
should also add the choke coil at the antenna base for additional
isolation of ground currents.

The article mentions the top part as 1/2 wave and bottom at 1/4 wave. If
the bottom part of their design is 505mm then the antenna must be used
for 2m band. If this is the case, how come the radiating element is 1480
long ?

The Coaxial antenna mentioned in the article mentioned by Jerry looks
good. The version they mention with a GAP would be simple to construct.
It is a 1/4 wave radiating element at the top (earth braid stripped
back), then 1/4 wave of full coax, then a "small" gap, then the
transmission line. They do not mention the size of the gap but I assume
any small gap has the same effect of isolating the reverse current ?

Should the actual lengths of these sections be modified by "K" depending
on diameter of conductor or are they exact 1/4 wave length cuts ?

Thanks heaps.


Dave Platt wrote:
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.