Cecil Moore wrote:
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?


That's what I would conclude from the data given. Wind some
wire around the tip top and extend the antenna by a few
inches. Then repeat your SWR measurements.

I am not sure what you mean by "wrap some wire around the tip top." I
could splice some wire on the long leg of the ladder line?

kd5sak wrote:
"Cecil Moore" wrote in message
om...

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?

That's what I would conclude from the data given. Wind some
wire around the tip top and extend the antenna by a few
inches. Then repeat your SWR measurements.
--
73, Cecil


One might also try adjusting the feed point location and see what effect
that has.

Harold
KD5SAK



I have done that on a j-pole that I made, but that antenna was in my
shack, where it was accessible. This antenna is in a third floor attic
where I have to wriggle my feet through 14 inches of insulation to
find a rafter to stand on. Not conducive to much tweaking.

Thanks, jimbo

Dan Richardson k6mheatadelphia wrote:
On Mon, 20 Mar 2006 08:12:51 -0700, 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?



Have you a common mode choke on the feedline? J-poles are nortorious
for feedline common mode problems.

Danny, K6MHE


Maybe a 4-5 loop coil of coax close to the feed point?

Thanks, jimbo

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].

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



Well, Utah is a little too far away for your offer of help. But, why
shouldn't I measure SWR at the transmitter? Doesn't a SWR of 1.0 at
the transmitter indicate 50 + j0 ohms which is what the transmitter
wants?

Thanks, jimbo

jimbo wrote:
I am not sure what you mean by "wrap some wire around the tip top." I
could splice some wire on the long leg of the ladder line?

The antenna appears to be too short. Lengthen the antenna,
normally accomplished at the tip top.
--
73, Cecil http://www.qsl.net/w5dxp

On Mon, 20 Mar 2006 15:01:53 -0700, jimbo wrote:

Dan Richardson k6mheatadelphia wrote:


Have you a common mode choke on the feedline? J-poles are nortorious
for feedline common mode problems.

Danny, K6MHE


Maybe a 4-5 loop coil of coax close to the feed point?

Thanks, jimbo

Yes, that would help.

Danny

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.

Owen Duffy wrote:

On Mon, 20 Mar 2006 21:31:02 GMT, Bob Miller
wrote:



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].


Why the half wavelength?



A 1/2 wavelength transmission line repeats the load impedance at the
antenna with minimum loss, assuming you cannot get an analyzer at the
antenna itself.

IMO, measuring directly at the antenna and solving the matching/load
problem there is the easiest and best method. You are free to disagree.

jimbo wrote:

SNIPPED


Well, Utah is a little too far away for your offer of help. But, why
shouldn't I measure SWR at the transmitter? Doesn't a SWR of 1.0 at the
transmitter indicate 50 + j0 ohms which is what the transmitter wants?

Thanks, jimbo

It IS what the transmitter wants! But you may be introducing
un-necessary power losses in your coax from any true VSWR 1:1.

It is possible to have a VSWR 25:1 at the antenna and still have close
to 1:1 at the transmitter! REALLY !! Your transmitter is happy. But you
are wasting power heating the coax, not radiating it at the antenna!
This is caused by impedance transformation along the coax. VSWR ~ 1:1 at
the antenna provides best power transfer from coax to antenna.

BTW, I used to live in Kaysville and Layton [ZIP=85015] in the olden
days [30+ years ago]

On Mon, 20 Mar 2006 17:54:19 -0500, Dave wrote:

Owen Duffy wrote:

On Mon, 20 Mar 2006 21:31:02 GMT, Bob Miller
wrote:



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].


Why the half wavelength?



A 1/2 wavelength transmission line repeats the load impedance at the
antenna with minimum loss, assuming you cannot get an analyzer at the
antenna itself.

IMO, measuring directly at the antenna and solving the matching/load
problem there is the easiest and best method. You are free to disagree.

Ok, but again you can measure the impedance anywhere and knowing line
parameters, calculate Z at any other point.

Owen
--