[Antennas] Tuning Jpole question

[David Kelley]

Daniel,

I have never used a J-pole (yet!), but I think I can
answer your question.  To move the resonant point
up in frequency, you probably need to shorten the
main element a bit.  In fact, if I did the math
correctly, to move the resonant point from 144.01
MHz to 146 MHz, you should shorten the main
element by approx. 1/2 an inch.  If you cut off too
much, you can always solder a short piece of wire
to the top of the main element to regain some
length.

Moving the contact point of the coax along the
matching section (the two parallel pipes) primarily
changes the impedance match, not the resonant
frequency of the antenna.  You may need to
readjust the coax contact point after you shorten
the antenna.

Hope this helps.  Please let us know what your
results are.

73,
Dave NB4J

>   I just put up my first elevated external antenna... A 3/4" copper
>pipe Jpole for 2m operation...  I tried about 15-20 different position
>for the coax connection points and got it set at the point which gave
>the best overall swr.  The results are that I have an SWR of <1.1
>measured at 144.010mhz and an SWR of 1.7 at 147.990.  If I move the
>contact points up, the swr goes up across the board, move the contacts
>down and the swr goes up across the board.  My question is what should I
>do to move the low swr more towards the middle of the band or just lower
>it overall?  Do I need to chop some pipe off of the main  element or off
>of the stub or something similar?
>Thanks
>Daniel
>KC9DAG

--
David F. Kelley, Ph.D.
Assistant Professor
Electrical Engineering Dept.
Bucknell University
Lewisburg, PA 17837
dkelley@bucknell.edu