[SMCARA] How to trim a dipole

Thu May 28 11:09:41 EDT 2015

Clear and Simple!  Thanks, Tom.

One other thought; "It is easy to cut an antenna shorter, but impossible to cut it longer"!

GL in the contest OM!

73 de Tom/W4OKW

-----Original Message-----
From: SMCARA [mailto:smcara-bounces at mailman.qth.net] On Behalf Of Tom Shelton
Sent: Thursday, May 28, 2015 11:02 AM
To: smcara at mailman.qth.net
Subject: [SMCARA] How to trim a dipole

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).
Shorten both legs by 11 inches and you'll find your best SWR has moved to your new frequency.

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

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