Short 40 meter dipole

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Scott Dordick:
Is it posible to start with a half wave dipole then, shorten it and just make some kind of coil at each end with the excess wire length? Thanks Again, Scott

Steve Katz:
All things are possible if you have the time and energy!

You can definitely end-load dipole elements, it's been done for decades; unfortunately, there's no real "formula" for this, because the end-loading inductance required will be based not only on coil diameter and number of turns, but also wire gauge, insulation used and proximity effects to virtually everything, since the tips of dipole elements are very high impedance points that react with everything around them.

So, if you have time and an MFJ Antenna Analyzer (preferable) or a small transmitter and SWR bridge (will work, more clumsy to use), start coiling the wire and making measurements until the antenna "dips" at the frequency of interest on 40 meters.

Alternative approaches which work well, and possibly better than end loading, include linear loading.  That subject was covered by a great article in a recent issue of QST detailing linear loading HF dipoles.  Overall length can be reduced about 50% with only a very small decrease in performance.  The recent QST article showed the use of 450 Ohm "ladder line" with an additional linear loading element (wire) strung through it to provide resonance with shortening.

WB2WIK/6

Dale Hunt:
The easiest thing to do would be to start with a half-
wave dipole and bend the extra wire at the ends to run
out in different directions (or even hang down towards
the ground.)  Presuming you can get at least the center
half of the dipole to run in a straight line, the ends
can run at various angles and the antenna should still
work.

The loading coils are not simply a way to use up the
"missing wire" on an antenna.  There are charts in the
ARRL Antenna Book to calculate the amount of loading
inductance required to resonate different length
dipoles based on where in the dipole the coil is
placed.  Simply winding up the "left over wire" rarely
will give the right amount of loading inductance.

If you have to shorten the antenna more than can be
done by just bending the ends to run in other
directions, here is one way you can build a shortened
antenna:

First, put up as long of a dipole as you can fit in
your space (including bending the ends where possible).
Then connect a coil across the center of the dipole.
I use a dip meter to find the right inductance for the
coil to make the antenna resonate at the desired
frequency, and the Antenna Book charts will give a
reasonable starting value.  Once the antenna appears
close to resonance, I wind a few turns of wire around
the outside of the coil and connect it to the coax.
Adjust the main coil to change the resonant frequency,
and vary the number of turns on the coax coupling coil
to get the minimum SWR.

A center loaded dipole is not quite as efficient as
one with the loading coils out towards the ends, but
this combination is relativley easy to adjust.  (Short
loaded dipoles have a low feedpoint impedance, so the
SWR at resonance may be higher than desired.  This
approach allows you to adjust both the resonant
frequency and the impedance match at the antenna.  As
an added dipole, the coupling coil reduces the need for
a balun.)

Good luck! - Dale WB6BYU

Bob Evans:
You might want to check this one out: http://www.cebik.com/aledz.html
I've built one and it works very well on 40 thru 10 meters. Mine is up about 40 feet. It is fed with 150 feet of ladder line. I use an MFJ-969 and a Kenwood TS-570S/G. I've worked a fair amount of DX with it.
73, Bob N6YMA

Jim Harris:
I used a Van Gorden SD-40 (40 meter shortened) dipole for several months.  On 40 meters and 17 meters it worked better than a Carolina Windom.  IMHO  Of course, the windom works better on other bands.  The SD-40 has a built in balun and price was right for me.

Enjoy!!

73

Jim, AB0UK

 

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