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
--

Owen Duffy wrote:
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
--

The numbers are confusing. 50 feet of LMR240 1.5 dB, (I understand
that.) Total line loss 2.3 dB. (Where did you get that?) .9 dB worse
than matched line loss. (Where did you get that number?)

There is wooden structure near since the antenna is located in an
attic. But I don't think there are any large metal structures close to
the antenna. The coax is in PVC pipe. It may go close to a metal AC
duct on it's way to the basement..

Thanks, jimbo

On Mon, 20 Mar 2006 16:51:44 -0700, jimbo wrote:

Owen Duffy wrote:
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
--


One step at a time...

The numbers are confusing. 50 feet of LMR240 1.5 dB, (I understand
that.)

Ok.
Total line loss 2.3 dB. (Where did you get that?)

It can be calculated knowing the line loss and source end VSWR. The
additional 0.8dB is increased line loss due to operation with VSWR1.
Try http://www.vk1od.net/tl/tllce.php for the calcs.

.9 dB worse
than matched line loss. (Where did you get that number?)

Sorry, my rounding error, call it 0.8dB. It is by subtraction of the
matched line loss from the total line loss.


There is wooden structure near since the antenna is located in an
attic. But I don't think there are any large metal structures close to
the antenna. The coax is in PVC pipe. It may go close to a metal AC
duct on it's way to the basement..

Is the roof sarked (does it have a metal foil lining under the tiles
or shingles), presumably it isnt a metal roof.


Thanks, 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.

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
--

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.

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
--

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.