Author Topic: Shortened 40m + 20m antenna project  (Read 3317 times)

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Online xrunner

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Shortened 40m + 20m antenna project
« on: March 20, 2016, 11:14:02 pm »
I've been needing a 40m antenna for a place where I don't have room for the total length of a full size 40m dipole (66 ft). I saw a design that MFJ had, and I thought I'd try to duplicate the design. The design is a combo 20m and 40m dipole.

After investigating some pics (see one attached), it looked like they had traps but there weren't any caps used. It must be that they were using the self-resonance of the coil to achieve the desired results. I counted the number of turns they used (34) and since I didn't have any wire that size I used some 30 AWG enameled wire that I had to make a prototype. I guessed at the size of the PVC pipe. I used the tracking generator of my spec-an to verify the dip, and the first pass yielded a nice dip at 13 MHz. I've since adjusted the turns to get the desired dip in the 14 MHz range.

I made up two prototype traps for testing, and hung them at the end of a 20m dipole I already had up. I then tested the effect of adding the traps at the end of the wire on the existing 20m SWR with my antenna analyzer and found no real effect on that band's resonance, which was very good. After adding a little extra wire after the traps (this is done also in the MFJ design) to make it work in the 40m band (7.0 - 7.3 MHz), I found that it indeed worked very well.

The windings took up about 15.5 ft of wire on each coil, and then I needed about a foot more after the trap to make it work in the 40m band. The original 20m dipole still functions as it did.

So looking at one leg-length (1/2) of the dipole:

We have 16.6 ft (original 20m) + 15.5 ft (trap) + 1 ft = 33.1 ft which is pretty close to one leg of a 40m dipole.

Now I will order some nice 16 AWG enameled wire and make a permanent good design and hang it up properly.

Oh yea, as a bonus, a 40m dipole will work on the 3rd harmonic on 15m, so it's really 3 bands in one.  :)
« Last Edit: March 21, 2016, 01:34:18 am by xrunner »
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Offline GreyWoolfe

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Re: Shortened 40m + 20m antenna project
« Reply #1 on: March 21, 2016, 01:22:03 am »
Nice job.  Did you make any on air contacts with it?
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Online xrunner

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Re: Shortened 40m + 20m antenna project
« Reply #2 on: March 21, 2016, 01:31:34 am »
Nice job.  Did you make any on air contacts with it?

Hello GreyWoolfe,

Why yes I did! Just in the last hour on 40m, I made contacts with YV5MBI (Venezuela for those of you non-hams) and a U.S. station using JT9 digital mode.

Pretty pleased right now!  :popcorn:
« Last Edit: March 21, 2016, 01:35:29 am by xrunner »
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Offline gfiber

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Re: Shortened 40m + 20m antenna project
« Reply #3 on: March 24, 2016, 01:29:03 pm »
Nice job...I have an Alpha Delta DX-EE up. Had to move it out of some tree limbs for now so it is not a great install. In western WA and regularly check into the noontime net on 7.268.50.

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Online xrunner

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Re: Shortened 40m + 20m antenna project
« Reply #4 on: March 24, 2016, 11:19:43 pm »
Nice job...I have an Alpha Delta DX-EE up. Had to move it out of some tree limbs for now so it is not a great install. In western WA and regularly check into the noontime net on 7.268.50.

Thanks, but I have more work to do. I got the 16 AWG magnet wire, and made what I thought would be a close replica of the original MFJ trap - but it did not resonate at the correct frequency, nor could I make it do so by adding many more or less turns than their original coil (~34). I suspect that the wire gauge is too large, thus upsetting the original design's inductance and capacitance. I tried to estimate what size they used but it was obviously wrong.  :(

I know 30 AWG wire works, but it's very small and I don't want to make a permanent coil with that small of wire, so, I ordered two more rolls of both 20 AWG and 24 AWG magnet wire. Hopefully one of them will work OK.
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