Monday, April 29, 2013

Antenna Topics: A limited space 40 meter dipole, post #184

KH6JRM's Amateur Radio Antenna Topics

A limited space 40 meter dipole.

The half-wave length dipole fed with 450-ohm feedline (the classic "doublet") is a basic antenna that will give you hours of enjoyment and plenty of DX if the flat top is at least 30 to 50 feet (9.14 to 15.24 meters) above ground.  Even at lower elevations, this simple, inexpensive antenna will provide strong local and regional coverage with a minimum of effort.

But, for those of us who face space restrictions (not enough back yard or oddly shaped house lots), getting a    horizontal flat top working can be a problem.  Like many amateur radio operators, my backyard is small and erecting two masts approximately 66 feet apart (20.12 meters) is not feasible.  Is it possible to shorten the horizontal length a bit and drop off the remaining portion of the dipole perpendicular to the flat top and still get decent performance?

According to Joel R. Hallas, W1ZR, you can modify the horizontal length and still get good DX.  Hallas writes "The Doctor Is In" column for QST Magazine.  On page 87 of the May 2013 edition, he answers a question that has "bugged" me for some time.  Jim, M0MAC, says he only has room for a span of 54 feet (16.46 meters) for his flat top dipole.  He wondered if it is "feasible to fold back wire at each end to make it fit."

In his reply, provides four EZNEC models which will provide a decent signal with support poles 54 feet apart (16.46 meters) and the flat top 25 feet (7.62 meters) above ground.  According to Hallas, an EZNEC modeled antenna "over typical earth" will be "only down 0.15 dB from full size."  The best example appears to be a flat top of 54 feet (16.46 meters) with 6.5 feet of antenna wire (1.98 meters) hanging down from each end.  The hanging wire can be secured to a wooden, pvc, or fiberglass mast.  That increases the total length of the dipole to 67 feet (20.42 meters).

Hallas adds that "while the bandwidth of each case covers the band with a 2:1 SWR, the full size dipole would have an even wider bandwidth if it were matched to 50 ohms."

I plan to build one of these modified dipoles and see what I can accomplish at my location.  Presently, my general purpose 40 through 10 meter antenna is an inverted vee fed with 450-ohm ladder line.  It does an excellent job, considering my proximity to neighbors and the lack of backyard space.  My inverted vee is susceptible to powerline noise, and I hope a modified flat top dipole suggested by Hallas will reduce the noise I pick up in the shack.  There is another benefit to dipoles and inverted vees--an extensive ground radial system is not required.  If you use a vertical antenna, you will need a radial or counterpoise system to supply the "missing half" of the antenna.

This looks like a good project for the weekend.


Hallas, Joel R., W1ZR.  "There's Always Room for Some Kind of Antenna."  QST, May 2013, p. 87.  Roy Lawaillen, W7EL, has various versions of this antenna modeling program.  I've found EZNEC valuable for my own antenna experiments.

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Aloha es 73 de Russ, KH6JRM
Along the beautiful Hamakua Coast of Hawaii Island.