Tri-Band Dipole for 40m, 20m & 15m.
A simple and effective multi-band dipole for portable use.
(click on thumbnail to enlarge picture.)
- Quick installation by one person
- Good performance due to height
THE TRICK WITH 15M:
It has long been known that a 40m antenna will also work on 15m. It will have a decent SWR "somewhere" within the 15m band. The problem is, if you place your 40m resonance in the middle of the band, the 15m resonance is too high.
The 40m antenna needs to be about 15cm (6") longer on each side to bring the resonance down to about 21.150 MHz. This is accomplished "quick and dirty" simply by soldering an alligator clip onto 15cm of wire and when 15m operation is desired, simply lower the ends of the dipole, clip the short stubs of wire onto the ends of the dipole, and raise the ends again. The stubs hang down, vertically. (Banana plugs will work too - or Anderson Power Poles.)
HEIGHT IS IMPORTANT:
In recent years, high quality, lightweight telescoping fiberglass poles have enabled significant improvements in portable antenna performance.
In order to take advantage of the improvement in height, you have to design your wire antenna from the ground up, to be used with lightweight poles. This mandates making everything as lightweight as possible. By choosing your material carefully, you can build a high quality, reliable lightweight antenna.
You can never place the feedpoint of your dipole all the way to the top of the pole. The pole is too thin at that point. HOW HIGH you can place the feedpoint on the pole will depend on how lightweight you make your antenna.
BEWARE OF THE ARRL:
The ARRL does a lot of good work and offers some outstanding publications at OM-friendly prices.
Unfortunately in their handbook and antenna book, in the chapter about dipole antennas, they are still using the same old pictures that they were using 60+ years ago when I was first licensed. The components shown are for building antennas for use with steel or aluminum masts, not lightweight telescoping fiberglass poles.
"Built like a tank" is not compatible for use with telescoping fiberglass poles.
If you want to build a dipole for use with lightweight fiberglass poles,
DO NOT BUILD IT LIKE SHOWN IN THE ARRL PUBLICATIONS.
GOOD QUALITY LIGHTWEIGHT COMPONENTS:
The following is a list of components which I personally use. You may substitute but try to keep the quality high and the weight low.
- The Spiderbeam 12m Heavy Duty lightweight, telescoping, fiberglass mast: (SOURCE: Spiderbeam)
- Lightweight insulators. Center Insulator made of plexiglas (home brew) and lightweight end insulators. (SOURCE: Spiderbeam)
- CQ-532 stranded copperweld wire, insulated, AWG-18. Even better, CQ-534 stranded copperweld wire, insulated, AWG-26. (SOURCE: Spiderbeam or THE WIREMAN). And finally, for a good combination of lightweight and strength, PolyStealth-22 (source: Davis RF).
- Good quality 5mm or 6mm coax. In USA I used LMR-240 (SOURCE: CABLE X-PERTS) and in Germany I use AIRBORNE-5 (SOURCE: Spiderbeam) - however for up to 100w, even short runs of RG-174 are acceptable. I personally used RG-174 on my motorbike trips, due to its lightweight and small size.
- For portable guying and supporting the wires, I use PVDF Monofil Line, or 1mm Kevlar rope. (SOURCE: Spiderbeam)
- For permanent installations, I use 2mm Kevlar rope. (SOURCE: Spiderbeam).
SEE ALSO: MODERN ANTENNAS for more information about building antennas which work well with lightweight masts.
LINKS to SOURCES:
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