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From: Larry Moore[SMTP:km6iu@jps.net]
Sent: Sunday, June 28, 1998 4:25 PM
To: gary@noblepub.com
Subject: Dipole 17/12 meters
Noted your response/offer to Brad on a 17/12 m dipole design. Would be
very interested in it. Could you advise by return when you get a moment?
Larry km6iu
Larry (and other AntennaWare folks):
Multiband dipoles are readily constructed using the technique I dubbed the
"Coupled Resonator," also called the single open sleeve. This is the basic
technique used in the Force 12 multiband antennas and in WA3FET's Optimum
Wideband Antenna. HyGain uses a double open sleeve in some models.
It is a simple concept -- you build a dipole for the lowest band you want, then
add a nearby resonator for an additional frequency. The spacing versus diameter
ratio determines the feed impedance at the frequency of the added element. For
#12 wire dipoles on the HF high bands, this spacing will be around 2 inches.
For tubing elements, it is more like 6 inches.
This type of antenna is accurately modeled using either MININEC or NEC-2
antenna programs.
A rotatable dipole for 17 and 12 meters is a great project to show off the
technique. First, an ordinary center-fed dipole is constructed for 17 meters,
typically about 26-1/2 feet overall length using 1-inch aluminum tubing. Next,
a half-wave element for 12 meters (18.9 feet using 5/8-inch tubing) is built
and mounted so that it is about six inches on centers from the 17 meter driven
element. It is best to have this element insulated from the boom and clamps to
avoid unbalanced currents. I use PVC or ABS plastic pipe to insulate both
the driven and coupled-resonator elements. A few layers of electrical tape
usually is needed as a shim to make up the difference in tubing and pipe
dimensions.
The elements can be separated either vertically or horizontally, depending on
your installation requirements.
I used the dimensions below to model this antenna at 55 feet height in EZNEC:
Wire End 1 End 2 Dia # Segments
1 0,-13.250, 55.000 0,13.250.55.000 1.000 15
2 0.450,-9.450,55.000 0.450,9.450,55.000 0.625 11
Wire 1 is center fed. For speed I used the MININEC ground model.
This antenna is "broad as a barn" in bandwidth, as a tubing dipole should be.
I had a 30-17-12 meter version up for a while, and there are a couple 17-12
versions in regular use, and they all work as modeled. It's a good
low-windload way to add these bands to a "tribander + shorty forty" system.
73,
Gary Breed
K9AY
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