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

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 http://www.qsl.net/w5dxp

"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

kd5sak wrote:
"Cecil Moore" wrote in message
Wind some
wire around the tip top and extend the antenna by a few
inches. Then repeat your SWR measurements.

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

That would be good to do after adjusting the length.
--
73, Cecil http://www.qsl.net/w5dxp

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

Jimbo,

The most likely source of variation is inaccuracy of your SWR meter.
It doesn't measure SWR on the feedline anyway.

The questions you should ask is does it work? And is the transmitter
roughly loaded with the right load impedance?
----
Reg

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

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.

The matched line loss of 50' LMR240 at 146MHz is 1.5dB.

Jimbo tells us the source end VSWR at 146.2 is 2.3. One can make a
reasonable estimate that the VSWR at the antenna end of the line is
3.5 (not a very good match for an antenna that should use an
adjustable matching system). Total line loss is around 2.3dB, or about
0.9dB worse than matched line loss.

Now look at the tx output power, has it decreased because of the bad
load. Add that reduction (in dB) to the 0.9% above to get the overall
degradation of transmit perfomance.

A question: Is the proximity of other structures or feedline isolation
a cause of the high VSWR, and should you resolve that before tweaking
the matching?

Owen
--

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

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?

The VSWR in this situation varies smoothly from the value at the feed
point to the value at the source end of the line as dictated by the
(known) line loss.

In this scenario, knowing matched line loss and the VSWR at a point,
it is possible to estimate the VSWR anywhere else on the line (as I
have done in another post).

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?

There are two objectives:
1. delivering the transmitter its rated load impedance (so that it
safely delivers its rated power);
2. minimising line loss.

1. is achieved by a load approximately 50 ohms at the souce end of the
line.

2. is achieved by a load approximately 50 ohms at the load end of the
line (which will also satisfy 1.).

Owen
--