On Apr 1, 1:34*am, Richard Clark wrote:
On Thu, 31 Mar 2011 18:46:22 -0500, tom wrote:
http://www.arrowantenna.info/osj/j-pole.html
This design works moderately well. Drive the 19.25 inch element. The 51
inch element is the radiator on 2m, the 6.xx inch element makes the
19.25 inch one radiate on 440. The thing isn't great on 440 because the
51 inch portion is there. It is rugged though.
I built a duplicate, which you can do if you look at all the parts pages
on the site. The main problem, same as all J poles, is common mode
current issues on the feedline.
tom
K0TAR
Sorry, missed the part where you already have this antenna. *My fault
for not reading the whole post until after I responded.
Hi Tom,
Your suggestion would have been my choice too - except for Tom's first
purchase choice. *The open stub has always seemed to be a more natural
feed method.
The addition of the parasitic radiator is still an option for any
design. *
73's
Richard Clark, KB7QHC
Richard
I didn't ignore choking the feed line but I will readily confess that
I did not choke it twice. Starting immediately below the bottom of
the matching stub I followed the recommendation of the various authors
and wound a multi-turn coax balun with a six inch diameter coils of
coax. They call for ten turns if I recall correctly. I was not aware
of the need for a second choke at one quarter wavelength away. Do you
have the energy to explain why that is necessary? How critical is the
length between the two chokes. Do I use the middle of the two chokes
as my measuring points? Could I substitute a one to one current balun
built of ferrite beads? That would have a less intrusive appearance
and accumulate less ice in the winter.
If you check the link that I gave for Ed Fong's dual band j-pole;
available here
http://f1.grp.yahoofs.com/v1/
kL6QTZLk1DQjM_Cn3vuvnsLUIuEsvRHSqUZyX2mw294a7mYKkc
\FBIXXlRY_6QxreqNWVpn0b7Dogiw9LafU63W429yoO/DBJ2_port_art.pdf or just
look it up in the files section of the Yahoo reflector; you will see
that he did test it in the field with fairly sophisticated
instrumentation. He uses a trapped radiator to obtain similar gain on
seventy centimeters as a simple J-pole without the trap gets on two
meters. His work was published in QST and I didn't find any
authoritative repudiation, or even strong criticism, of his design.
Why would the presence of the trap in the lower half wave of the two
meter collinear half wave J-pole wreck the tuning on two meters.
I'm only trying to learn here.
--
Tom Horne, W3TDH