[SMCARA] How to trim a dipole

Fri May 29 22:09:13 EDT 2015

Antennas ... Then and now

Once upon a time, the old vacuum tube rigs had a pi network output and you
could easily match the plate circuit of the final amplifier to a 75 ohm
coaxial cable.  And ... the local cable TV company threw away miles of good
RG-59 in short to medium length hanks.  This cable almost perfectly matched
the center impedance of a half wave dipole (about 72 ohms at resonance).
You could also build your Heathkit AM-2 SWR Bridge for compatibility with
either 50 or 72 ohm lines.  I can't remember anyone in my town who built
there's for 50 ohm line.  Given those long obsolete conditions, it was
possible to trim a dipole until there was precisely zero reflected power.

Today, the center impedance of a half wave dipole is still about 72 ohms;
but most of us have radios that are built to match 50 ohms and have no means
(in the final tank circuit) to match the amplifier to anything else; you
also don't see much RG-59 or RG-11 on the market. We feed our dipoles with
RG-8X or some other 50 ohm line and we automatically end up with an SWR of
something near 1.5 to 1 (at resonance).

Having grown up in those earlier times, I really miss the dead nuts, letter
perfect dipoles that even beginners could successfully construct.  I have
even wondered about the commercial feasibility of building and selling a
dipole kit with a 1.5 to one balun that lived at the feed point and
transformed the antenna's 72 ohms to something close to 50.  Why would I
think that?  Because, there must be other old timers out there who, like me,
can remember when you could trim a dipole to "perfectly flat" on your
favorite frequency (or your transmit crystal frequency). It was very
satisfying; all that power going off into space and none of it coming back.
In 1959, no one would have considered "tuning out" a 1.5 SWR with a tuner at
the transmitter end of the coax.  It was considered heresy to operate a coax
line with SWR present; while fooling yourself and your radio into thinking
that the antenna was perfect.

I miss those days!  DX engineering has my balun ... but it's rated at 5KW
and costs more than I want to pay.

Now, full disclosure before I close ... so no one takes me wrong here:  an
SWR of 1.5 to 1 isn't going to hurt anything ... nothing.  Only a small
fraction of your power will be dissipated in the feed line (far less than
anyone is going to notice on the far end S-meter). Broadcast engineers focus
on phase (when feeding a multi element array) and don't worry much at all
about SWR (something that really surprised me).  So carry on . I just wanted
to share what this thread brought to my mind.  Actually, I've been thinking
about building a truly flat dipole (2015 style) for a long time; and just
because it feels good.

73

Sam

-----Original Message-----
From: SMCARA [mailto:smcara-bounces at mailman.qth.net] On Behalf Of Tom
Shelton
Sent: Thursday, May 28, 2015 12:01
To: 'Clarke, Tom AIR4.0P NATOPS'; smcara at mailman.qth.net
Subject: Re: [SMCARA] How to trim a dipole

Tom,

Thanks...   Yes, 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.  

Tom Shelton, ND3N

-----Original Message-----
From: SMCARA [mailto:smcara-bounces at mailman.qth.net] On Behalf Of Clarke,
Tom AIR4.0P NATOPS
Sent: Thursday, May 28, 2015 11:10 AM
To: smcara at mailman.qth.net
Subject: Re: [SMCARA] How to trim a dipole

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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