[Mldxcc] Fwd: Dipole VSWR

[Emileigh Starbrook]

Alan asked:

 ---  How did you measure the SWR vs height? ---

OK this is a very good question.  I use an MFJ-259C antenna analyzer -
fresh batteries.  I start by setting the analyzer frequency to the band
center - let's say 18.110 and the pole at minimum height.  I adjust the
antenna length to "dip" the VSWR to it's lowest point.  Then I raise the
mast to it's maximum height and then change the frequency to see the lowest
SWR - which is typically 1 : 1.1 or better.  Next I bring the pole down,
subtract the full height best SWR frequency from 18.110, multiply by -1 and
add it to 18.110.  I then set the Analyzer to that frequency, adjust the
antenna to perfectly resonate at that frequency on the ground, then raise
it back up and in the past, the perfect match happens at full height.

I know this sounds tedious and time consuming, but after doing it for many
years, it's usually about a 2 minute process.  And good exercise :-)

This year, I cannot get the dip at maximum height.  Instead of being able
to get any reasonable match at full height, it happens much lower. That is,
if you watch the analyzer while you are raising the mast, it goes down to 1
: 1.0 which it always did.  But instead of staying at 1.0,  the higher you
go, the SWR goes back up linearly.  The higher it goes the worse it gets.
At full height it's like 1 : 1.5 or greater regardless of frequency.

I checked it on 5 different bands including 6m and the results are the same.


--- I assume you have a pulley system. ---

No pulley needed - it's a 5 section aluminum mast - you push up each
section manually.


--- Did the antenna change shape as you pull it up? ---

No - the ends are unsupported (though they are older and might be drooping
more - though it's negligible.) so they have the same droop at 6' as they
would at 25'


--- What's the SWR difference between this new optimum point and the max
height (or where it was optimum before)? ---

At maximum height of the mast I could get a perfect 1:1.1 or better.  Now -
1 : 1.5-1.7

At the shorter height I can still get 1 : 1.0-1.1.


Hope this explains it more.

Emily N1DID