Wednesday, June 26, 2013

Antenna Topics: Antenna Trimming Shortcut. Post #205

Most of us who use the general formula for a half-wavelength flat top dipole (468/f (Mhz)=L (ft), rarely give much thought on how accurate the dipole length really is.  We cut the dipole to its calculated length, cut the wire in half, attach center connector and insulators, attach a length of 50 or 75 - ohm coaxial cable, suspend the wire between two tall supports, and prepare for a day of radio fun.  In most cases, your dipole will work well, although a little trimming or pruning may be necessary to bring the wire antenna into resonance.

I'll admit to being lazy in this department.  As long as the SWR falls below 1.5 to 1, I don't bother trimming.  Even if the SWR hovers around 1.7 to 2.0 to 1, I always can use my trusty Drake MN-4 to bring the SWR below 1.5 to 1.

Suppose you wanted to make your antenna (in this case for 40 meters), as resonant as possible and do without a tuner altogether?  Instead of cutting and trimming, wouldn't it be nice just to cut the dipole length once and come up with a low SWR?

After doing a bit of research in several ARRL documents, I found an article by Joseph H. NeCamp (W4JBQ) in the 11th Edition of "Hints and Kinks for the Radio Amateur" that addresses that issue.  Following Mr. NeCamp's example, I duplicated his pruning example with a temporary 40 meter flat top dipole in my backyard.  I used two 33-foot (10.06 meters) MFJ fiberglass masts to support the antenna.

Like Mr. NeCamp, I cut a dipole resonant at 7.05 MHz using the general dipole formula (468/f (MHz)=L (ft).  The length of the dipole worked out to be 66.4 feet/20.24 meters or 33.2 feet/10.12 meters for each dipole element.  A check with a SWR bridge showed resonance around 7.15 MHz.  Obviously, my original calculated length was off by quite a bit.  I would have to add some more wire to each element to make the dipole resonant.  Here's how  W4JBO adjusted things:

"Multiplying the original 66.4 feet times the actual resonant frequency of 7.15 MHz yields 475, a new constant to use in the formula in this particular installation."  NeCamp continues by saying: "L=475/7.05=67.4 feet.  If the original short dipole is lengthened to 67.4 feet (20.54 meters), the resonant frequency will be very close to the desired 7.05 MHz."

I tried this pruning method and it works.  With each dipole element measuring 33.7 feet/10.27 meters. I had a SWR of 1.2 to 1 at the desired frequency without using the Drake MN-4.  Of course results will depend on your dipole's height above ground, the type of coax used as the feed line, and surrounding objects which could detune the antenna.  I was able to remove the antenna transmatch and enjoyed a casual afternoon of "rag chewing" on 40 meter cw.


NeCamp, Joseph H. (W4JBQ).  An Antenna - Pruning Shortcut.  Contained in "Hints and Kinks for the Radio Amateur", 11th Edition. Copyright 1982.  ARRL, Newington, CT, 06111. Page 5-9.

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Aloha es 73 de Russ (KH6JRM)

BK29jx15--along the beautiful Hamakua Coast of Hawaii Island.