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
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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
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