On Mon, 20 Mar 2006 15:37:26 -0500, Dave wrote:

In your set up the VSWR tells you very little!!

That 50 feet of LMR240 is modifying the actual VSWR as seen at your
meter. It is not telling you the VSWR at the feed point.

First, the antenna should have a balanced feed. With coax you need a 1/2
wavelength [coax corrected for velocity factor] matching stub to keep
the line balanced. This will minimized coupling antenna current on the
outside of the coax cable.

Second, you should measure the antenna feed point impedance at the
feedpoint, or 1/2 wavelength from the feed point [coax corrected for
velocity factor].

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


Third, adjust the feedpoint location to get 50 + j0 ohms on your
preferred frequency using an antenna analyzer [one or more members of
your radio club should have one].

I'm located in southern NH, USA and if you are reasonably close I can
assist in tuning your antenna.

DD

jimbo wrote:

OK, I have a new 2 meter j-pole antenna installed in my 3rd floor attic.
I have 50 feet of LMR240 coax running to the basement. I measure SWR at
the following frequencies on simplex.

144.2 2.5
145.2 2.4
146.2 2.3
147.2 1.9
147.9 1.7

Can I conclude that the antenna is electrically short for the 2 meter band?

Thanks for any insight, jimbo