On Sat, 15 Feb 2014 22:29:31 -0800, "Sal" salmonella@food
poisoning.org wrote:
I can agree with the need for preventing feedline radiation but one thing
you and Terry say may be erroneous. I believe the radiating element of a
J-pole to be a half wavelength long, not a quarter-wave.
You're correct. See my follow up posting. I've been building J-poles
driving the short 1/4 wave element instead of the long 1/2 wave
element.
I looked at
Terry's EZNEC wires list and observed the long side (the radiator or driven
element) of the J is 57 inches and the short side (the stub) is 19 inches.
(These dimensions agree with my idea of a 2m J-pole. I've made a few.)
As I understand the action of the J-pole, net radiation is low or nil from
currents in the lower third of the antenna (bottom third of the radiator and
the adjacent stub).
Correct. The original Zepp antenna was designed to be lowered from a
hydrogen filled Zeppelin dirigible. Any sparks or arcing caused by
high voltage from the antenna to the dirigible would be considered a
really bad idea. So, the antenna was designed to have zero voltage
and probably zero current at the closest point of the antenna to the
dirigible.
The desired radiation comes from the top two-thirds (38
inches) of the radiator, which is very nearly a half wave at 2m. Allowing
for so-called "end effect," it's almost exactly cut to 146 MHz.
Well, here we disagree slightly. I once made a similar comment in
this group about the radiation from an end fed collinear antenna came
mostly from the bottom of the antenna. I was corrected by Roy
Lewallen (W7EL) who pointed out that the current through the length of
such an antenna is the same and therefore the radiation from all parts
of the antenna are similarly identical. Unless I'm missing something
(a real possibility), the situation is the same with a J-pole.
https://groups.google.com/forum/?hl=en#!original/rec.radio.amateur.antenna/DREJnRznluQ/bZyCgwa0JvwJ
This is interesting:
http://www.qikzepp.com/QikZepp_technical_information.html
It shows a 1909 German patent for a Zepp antenna. However, the
accompanying description once again makes the mistake of feeding the
1/4 WL section and not the longer 1/2 WL section:
Early fixed installation Zepp Antennas were a half-wavelength
long(or multiple) and fed with a 1/4 WL (or multiple) open
wire feed line which uses only one of the wires. The feed
line provided a matching section for the transmitter.
Please check my reasoning and math Your comments are welcome. Thanks.
I hate being wrong, but do I seem to be getting used to it.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558