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Toroid for trap dipole?




 
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Author Topic: Toroid for trap dipole?  (Read 865 times)
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WB3JOK
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« on: April 23, 2022, 05:29:37 PM »

Many years ago I built a 5-band trap dipole as shown in the '68 ARRL Antenna Handbook. I took it out of the closet and uncoiled it, and everything other than a few solder joints looked fine. So I laid it out on the grass, at the very edge of the yard.

Meanwhile a friend with a backhoe came over to spread some gravel... and for some reason he backed up so far that he ran over one trap.  Angry  53 feet of wire with the trap at 32 feet, and the tire hit the trap absolutely dead center perfect. I couldn't do that if I were TRYING to. Anyway the Air Dux coil is flat, and the vacuum cap is open to the atmosphere...

So - I looked up the price of a new length of Air Dux 2006 (6 tpi 12 ga, 2.5" diameter). I only need 9 turns (an inch and a half) but they are sold by the ten inch length and I almost had a heart attack when I saw the current prices (well into 3 digits)  Shocked

It occurred to me that a toroid coil could be used instead of air. The trap is resonant in the 40 meter band. Any reason I shouldn't use one? Power handling limitations maybe? If I go back to the specified doorknob cap instead of the relatively heavy glass vacuum cap, the weight will be about the same.
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K8DI
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« Reply #1 on: April 23, 2022, 05:51:36 PM »

I’ve had pretty poor luck making toroidal anything do anything close to what I want…  I’d see if anybody around here has a spare chunk, or just wind some wire on a piece of pvc…

My antenna, an 80-40-20 fan, has the 80 element shortened with a coil of 12 gauge stranded electrical wire on a hunk of pvc pipe on one side only (small lot/tree position issues). It’s lived for several years, no problems, 1.08:1 SWR at the middle of the 80m phone segment…

Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
w3jn
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« Reply #2 on: April 25, 2022, 02:44:53 PM »

Post a Wanted ad here, with a picture of what you need.  Many of us have odd bits of Airdux around and I'm sure you can get what you need for under 3 digits!
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WB3JOK
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« Reply #3 on: May 01, 2022, 11:05:42 PM »

As a test, I lowered the antenna and put jumpers across the traps. As a 106' doublet inverted-V, it works at least as well as the trap dipole did - although now it must be fed with ladder line due to the mismatch especially on 40m. I had already built a T-match and 100' of 400 ohm open-wire line, since this was my original plan... didn't want to buy 150+ ft. of LMR-400 or better anyway Wink

So forget the traps - it now comes down to using it at 106', or hanging a full 133' which will be a match on 80m. I made some contacts on it this evening (including Germany on 20m) so it must be working  Cool

Meanwhile, lightning protection is more of a problem with 100' of ladder line hanging from the top of the utility pole to my second story shack window, even with a ground rod at the base of the pole  Shocked If I dangle the line to ground level at the house (and bond it to the power ground around the side of the garage) then it'll be an obstacle in the back yard. For now, if t-storms are predicted, I unclip the feeder from the tuner and throw it out the window!  Roll Eyes
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