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March 21st 06, 02:40 AM posted to rec.radio.amateur.antenna

jimbo wrote:
wrote:

On Mon, 20 Mar 2006 17:51:28 -0500, Dave wrote:

Bob Miller wrote:

SNIPPED

I'm confused, wouldn't it be more important to have the right
impedance and swr at the end of the feedline, where the transceiver is
expecting 50 ohms?

bob
k5qwg

SNIPPED

If you have 50 ohms at the antenna, then you will have 50 ohms
regardless of the length of coax.

A 3:1 VSWR, assuming other responses to this post are correct, means
that your antenna connection could be anything from 16 ohms to 150
ohms and all sorts of combinations of resistance and reactance within
that range.

An antenna analyzer will let you determine antenna resonance, antenna
feedpoint impedance [hopefully 50 ohms], coax line loss, and allow
proper adjustment at the connection points for your coax.

Once again, find someone in your area with an antenna analyzer. I
repeat my offer if you are within 60 +/- miles of Concord NH.

It's possible to tune a Jpole with just a SWR meter. I've done ti
many times though it's more tedious than with an antenna analyser.

The simplest step is add some length to the long section, not alot
and see how the change affects it. Add more or less as needed
for a good minimum. If the minimum is still not on the mark (1:1
is ideal but anything under 1.3:1 flies well) then adjust the tap
point (very small movements) and resweep again. With patience
you (the person that asked) will get it right.

What happens with Jpoles is they often are not built identical
and small differences do show. Also like most halfwave and
larger antennas being close to "stuff" tends to affect tuning.
No harm or foul but learning to prune (aka tune) an atenna is a skill
and worth developing.

Allison
KB1GMX

Adding some length to the long leg would be relatively easy, BUT moving
the feed point would be a real challenge! The coax shield and center
conductor are soldered to the ladder line wires. When I tuned a j-pole
in my work shop, tiny movements made a change amd maintaining the
position while trying to get it soldered was nearly impossible. (At
least for my clumsy fingers.) Is there a better way to connect the feed
point?

Thanks, jimbo
Connect your feed points to the Jpole using clamps. Adjust the location
of the feed point and the characteristic impedance by moving the clamps.
You do not have to adjust the length of the feed line until you fine
tune the feed point impedance.
I just finished making the collapsible jpole antenna that was featured
in QST last spring. When I first tested the impedance it was way off.
All I had to do was loosen the clamps and bump the clamps up and down
the pole to obtain optimal settings. Due to the out of ham band use I
will be putting the jpole to the SWR at my xmit freq. is higher than I
would like the midpoint SWR is about 1.1:1.
This is why Antenna design is an ART not a SCIENCE.

Dave WD9BDZ