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N3FIX > TECH 03.01.19 21:36l 23 Lines 3592 Bytes #999 (0) @ WW
BID : 1839_N3FIX
Read: GUEST
Subj: A WARC band antenna you can build
Path: IW8PGT<IZ3LSV<IK6ZDE<F1OYP<AB0AF<KA3BVJ<N3DWB<N3FIX
Sent: 190103/1900Z 1839@N3FIX.#EPA.PA.USA.NOAM LinBPQ6.0.17
The WARC bands are three portions of the shortwave radio spectrum consisting of 30 meters (10.100–10.150 MHz), 17 meters (18.068–18.168 MHz) and 12 meters (24.890–24.990 MHz). They were named after the World Administrative Radio Conference, which in 1979 created a worldwide allocation of these bands for amateur use. The bands were opened for use in the early 1980s. They can sometimes be ignored by ham operators, but possess good opportunity for DX under the right conditions. My manual tuner works HARD to make my transceiver happy on these bands with my existing equipment for very poor efficiency, and very few contacts. The bands are nice and narrow bandwidth with little need to strive for obtaining wide bandwidth capability in a design. I normally run a commercial "fan" dipole which Alpha-Delta refers to as a "parallel" dipole for 80, 40, 20, 15, and 10 meters. What is needed is a simple wire solution. I was inspired by an article by W4DAN for this fan dipole design. The solution was simple, create a fan dipole for the WARC bands using first principles where the calculation is 468 / freq in Mhz = total length in feet for a half wave dipole.
To reduce interactions between elements, I spaced them 6 inches apart. This may be excessive, but works fine. The center is an old 1:1: balun I found at a hamfest. With the longest element of just over 23 feet in the center, the next longest on top and the shortest on the bottom. A Dacron rope makes up the difference between the 17m and 12m element to reach the last spacer.
30m just over 23 ft
17m just under 13 ft
12m just over 9.5 ft
When tuning this antenna, I put the center up on a thin bamboo pole supported by my son's wooden swing set, and supported either end so that I could reach up at the end and grab it. After getting the wire in hand I could walk hand-over-hand bending the bamboo pole down to reach the end of the element for tuning. This was rather more like an Inverted-Vee arrangement which will result in slightly shorter lengths and will be ideal for mounting where there are no high objects to tie to. At the recommendations of my Elmers, I started with the 30m elements and tuned them to under 1.5:1 SWR. Progression was to adjust 17m and then 12m elements. I noticed little interaction between those elements while tuning. I did jump back and forth once between 17m and 12m to get the tune just right. The bamboo pole worked well for a temporary tuning setup.
Using my VK5JST analyzer, we were able to get the tuning reasonably close. I discovered that a simple technique to keep me from cutting too much off the elements. It didn't seem to affect the tuning and I would encourage others to take advantage of this trick to enable you to re-tune an antenna in its final installation for perfect matching.
I folded the element back on itself and used a cable clamp to secure it. The loop can be adjusted easily by loosening the cable clamp and re-tightening. The element wire in the loop didn't seem to matter much, but I kept it to a minimum without too much wire involved in the loop simply for fine tuning.
Good high quality Thomas-Betts ty-wraps were used to keep the spreading spacers taut with the 30m elements, and with the spreader near the feed point to keep everything aligned. The ty-wraps can be adjusted slightly if needed but provide a good solid way of keeping the spreaders in place. This is what Alpha-Delta uses, so why not!
See photos at :
http://clyinstitute.blogspot.com/2017/11/amateur-radio-warc-band-fan-dipole.html
Catch you on the WARC bands!
73 de N3FIX
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