[SMCARA] Re - "How to trim a dipole"

[Tom Shelton]

Cal,

 

You are correct - I wrote this on the fly without aid of a technical editor.


 

Tom Shelton, ND3N

 

From: Cal Spreitzer [mailto:n3cal at md.metrocast.net] 
Sent: Thursday, May 28, 2015 11:42 AM
To: gl1800winger at verizon.net
Subject: Re - "How to trim a dipole"

 

Tom,

 

After reading your email I believe there is a error in your post listed
below??  Wouldn't you have to add 11 inches to your dipole to get it
resonate on the lower portion of the band in the example you provided?   The
antenna needs to be longer vice shorter as you go down in frequency...  

 

73

 

Cal/N3CAL

 

 

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.