Hi Tom,
To: <towertalk@contesting.com>
>Date: Tue, 29 Dec 1998 08:44:39 -0500
>From: "w8ji.tom" <w8ji.tom@MCIONE.com>
>
>Hi Ted.
>
>> Can you Towertalkians tell me the pros and cons of each antenna?
>
>
>A J-pole is not as ground independent as one might think or
>read. It is an end-fed 1/2 wl antenna , fed by a 1/4 wl
>stub. Because the end impedance of a 1/2 wl is not infinite, but
>rather some finite impedance between a few hundred and a few
>thousand ohms (depending on conductor diameter), the terminal
>current of the balanced line driving the actual radiator are not
>balanced. This causes the balanced feedline section of the
>J-pole to have common mode current adding destructive radiation
>to the pattern that raises the wave angle and lowers gain.
We did not observe any telltale indications of an excessive
feedline radiation problem with the one we used on 160 a few
years back. And we were looking for this effect during the
tuneup phase of the project.
We did isolate the coax at the feedpoint (turned out to be only a
few inches out from the short for 50 ohm feed) with a bunch of
ferrite beads. But we never had any bead temperatures above
ambient even when running significant power into the system.
Also, we had no "RF in the shack" types of problems and the coax
feedline dress or length had zero effect on the observed feed
impedance. And that was true both with and without the beads on
the coax.
There may well have been some small amount of radiation from the
balanced feeders. But I'm convinced that its effect on the TOA
for the system was small compared to the effects of proximity to
and characteristics of the local earth surface.
Bottom line is that it worked well for us and allowed us to use
the available support at near zero cost for a temporary antenna.
We made no field strength measurements but it was clear from the
reports we got while running less than 100W during testing that
the efficiency couldn't have been terrible. I suspect that was
due to having the bottom end of the radiator more than 1/4 wave
up in the air and the high current zone of the radiator about a
half wavelength above ground. Pretty good for an antenna that
only needs 50 foot of coax feedline.
The only real drawback was the relatively high Q of the system.
It was only about 60 KHz between the 2:1 SWR points. At
resonance, it was exactly 1:1.
Oh yeah, there is one other minor drawback. The requirement for
a 450 foot or taller climbable support... We had access to a 650
foot FM broadcast tower.
Snip...
73, Eric N7CL
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|