Gary Breed wrote:
>
> ----------
> 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 u
> 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
I just did that exact exeriment. I tore up an old tribander and used
the aluminum
tubing to build a 12/17 meter rotatable dipole with open sleeved feed.
I used a pop-rivet gun to mate the aluminum together and that worked
like a charm!
My 17 meter element was about 1" od and reduced at the ends.
My 12 meter element was 7/8" od and reduced at the ends.
My spacing was exactly 7" and fed with 50 ohm coax. Mounted it at the
75 ft level on my tower and it works like gang busters. SWR is flat way
above
and below both bands. Next I will take it all the way to the top of the
tower and
mount it above my yagis. 73......
------------------------------------------
Steve Narducci
Amateur Radio Station - W9SN
All Ten Tec Equiptment
Toys for "Real" CW Men
------------------------------------------
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