[SMCARA] Corrected: How to trim a dipole

[Tom Shelton]

Thanks to Cal and Tom for additional input.  Here's the corrected version.

 

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I was setting my wire antennas up for the CQ WPX CW contest this weekend and
had to make a few adjustments to my dipoles.  Here's an easy way to avoid
using the hit and miss method of wire adjustments.

 

First:  Why do this?  You might be thinking that since you have an antenna
tuner, if you're close - its good enough.  First off, I hate the term
"Antenna Tuner."  What you have is an impedance matching network - your AT
doesn't tune your antenna, it matches the nominal 50 ohm output impedance of
your rig to whatever it sees as the input impedance of your antenna.  If
your antenna has an input impedance of 100 ohms, your SWR will be 100/50 (or
2:1).  The impedance matching network adds capacitance and inductance as
needed to give you an apparent 1:1 match.  By adjusting your antenna to a
nominal 50 ohm impedance, and minimizing the amount of
capacitance/inductance your AT switches in, your Rig/Antenna system will be
more efficient and put more power into the air.  Like my first elmer told
me, "A dummy load is a perfect match - but not an efficient radiator."

 

So, lets say your 40 meter dipole is set up for the phone portion of the
band, with your best SWR at around 7.250 MHz.  You want to adjust it to
7.050 MHz to use it with CW or RTTY.  

 

The process is pretty simple.   First figure out what the length of your
antenna leg (we're still talking dipoles) is at 7.250 MHz.  (234/Frequency
in MHz) (234/7.25 = 32.28 Feet).  Now figure out what the length should be
at your desired frequency using the same formula (234/7.05=33.2 feet).  Find
the difference between the two (33.2 - 32.28 = 0.92 feet or ~11 inches).
Lengthen both legs by 11 inches and you'll find your best SWR has moved to
your new frequency.

 

Don't cut your wire - eventually you'll want to go back to the higher
frequency.  Just fold your wires back and wrap the extra along the length of
the dipole.  BTW - an antenna tuner will usually work for normal operations.
I have my 80 M dipole tuned for the CW/Digi portion of the band, but am able
to tune across 3.5 - 4 MHz with an antenna tuner.  For casual operations, no
need to pull your antenna down and adjust it.

 

One note - make sure you've turned off or by-passed your Antenna Tuner.

 

And for those of you who are wondering, yes, I used to do a lot of running
back and forth between my rig and my antenna.  Having to take a reading,
lower the dipole, adjust it, raise it back up, then checking to see what my
new SWR was - and repeating many many times before I figured out how to make
things easier on myself and be able to do this with a single trip.

 

 

 

Tom Shelton, ND3N

Ham Radio, Motorcycling, and a Loving Wife who lets me play with both

What more could a man need