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300 ohm feed doublet antenna with coil

KD2GOE

KD2GOE

W9WDX Amateur Radio Club Member
May 30, 2013
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I made a doublet antenna fed with 300 ohm twin lead i wanted to hit 80 meters. i am tuning it with my MFJ-949B i can tune 10-15-17-20-40&80 easy.
But it started out as an 80 meter antenna feed with 50ohm coax, i did not have the room to make it full length so i made it with coils 3/4 the way down and tuned it to 80 meters my taking winding off and cutting the ends. Then i decided to switch it up and slapped 300ohm twin lead on it and it seems to work good...
at the risk of messing it up and having to remake it and wasting more wire i was wondering if i would have a nicer tune if i moved the coil to where 40 meters ends 32' 6"? i guess what would be consented a trap but i have no tools to tune one...
 

the linear LOADING coil placement should not affect the band it is wound for (75/80), it only makes the wire "look" longer.
Loading coils can be installed anywhere along each leg of the shortened dipole. The further out on the leg the coil goes, you gain a higher radiation resistance (good), but requires a larger coil and higher resistive loss (bad).

Therefore, as you move the coil toward the end, there is a balancing of competing efficiencies.

that being said, in the real world, I don't think you will see a difference on 75/80, however,........ Placement of the loading coil will be important if you plan multi-band operation using a tuner.

The loading coils will act as RF chokes at the higher freqs and will isolate the remaining antenna wire at each end of the dipole (doublet). This could make your antenna very reactive on some bands, hard to tune

If it ain't broke,...... don't fix it

YMMV good luck
 
I'd leave it alone. If you want to upgrade build a real balanced tuner and a doublet with no coils.

If you want to try to improve what you have get an rf ammeter and measure current on each leg of the twin lead before you do anything else. If you change something and the current increases you've made an improvement.
 
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KD2GOE said:
ok not going to touch it...
Click to expand...

Well, after thinking about it, you didn't say exactly how it performed on the higher bands, soooooo. I might consider moving the coil inward and see what happens to 40 (and the lower part of 15) meters.

Don't CUT the wire between the center feed point and the coil, just fold the excess back on itself. that way, you can "undo" what you have done.

of course, you will have to add the same amount of wire to the other side of the coil to keep the same overall length.
 
I can say it performs less then my I-Max 2000 on 10,12 and 15 and the receive is nicer then my 10,20,40 meter Fan dipole. the noise flow is down and receive from other stations on average is 1 to 2 s-units stronger with the 300 ohm feed antenna... i may even take down my Fan dipole...
My way of thinking was if i move the coil down to 40 meters it may isolate 40 to 10 meters.. "Less work on the tuner so to speak"
 
I've had better luck with a vertical CB antenna on 10 thru 15. If you have the window line already in the shack you have the opportunity for big improvements.

You can build a junk box tuner for cheap that will work so much better than the mfj. If you haven't heard of it google the K1JJ tuner. You can build one out of pvc, scrap wire and a variable cap.

If you like it and want more power you can build one with beefier parts. At that point you've outgrown the 300 ohm twin lead. As long as it's balanced it doesn't radiate so you don't need a case...just don't touch it while it's hot. I use a 68 ft doublet on 75 meters with a couple hundred watts AM. A high end store bought tuner would have a melt down. The ladder line and antenna have to be made of manly wire.

160 meters is uncharted territory. It loads up there but I've only hit it with 50 watts. I don't expect it to work very well but have done no real testing. It's got to be slightly better than a dummy load under my work bench.
 
543_Dallas said:
I've had better luck with a vertical CB antenna on 10 thru 15. If you have the window line already in the shack you have the opportunity for big improvements./QUOTE]
Click to expand...

o yeah i don't tune 10 though 15 on it. that what the imax 2k is for..
i just want to try to maximize my 20 and 40 meters wile being able to tune in 80...

the MFJ tuner i have is the OLD one with the big tunning caps in it.. my friend got a new one and you can see the tuning caps are 1/2 the size with the plates right on top of each other.. good way for a spark to jump..
 
In your antenna's design situation, I'd put the coil at the correct length for 40 meters. For 20, consider running an additional wire on each leg cut exactly for 20m. Take a look at the Alpha Delta DX-CC antenna - it is designed exactly like this, although it also includes elements for 10m.
 
i moved the coil to 32' and Lost 20 meters and 80 meters cant get them under 3:1... more playing around to do...
 
The downgraded caps in the new models sucks. My dislike is the tiny balun they throw in at the last minute. Feeding a balanced antenna was an afterthought. Non resonant doublets are like voodoo to the modern ham.

Sorry if I missed it but how long of a dipole can you fit in the space you have? You may be better off without the coils and just using a normal doublet. It would be easier to predict what the tuner will see on what band. If you have tuning issues you can adjust the feedline. Just make the antenna as long as possible.

Something like a g5rv or the shorter zs6bkw will work decent on multiple bands if you lose the coass and take the twin lead straight to the tuner.
 
Simply moving a coil along the length of wire will not change bands that the antenna is resonant on. The coils need to be TRAPS in order to isolate one section of the antenna from the rest. If you do use TRAPS and not just loading coils then those traps MUST be tuned to the proper band and be inserted in the proper place. Unless you are running a trapped dipole antenna get rid of any and all loading coils and run a simple doublet connected to the balanced line. Messing around with untuned coils or moving traps to where they should not be is a waste of time, effort, and efficiency.
 
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Another thing to add is that coils by and of themselves are NOT traps. They MUST be tuned to resonance with a capacitor in order to make a trap. This cap is in parallel with the coil. At resonance they create a very high impedance that effectively isolates or cuts off the rest of the antenna beyond the trap. Below the cut-off frequency the traps act as loading coils for the next LOWER band.
 
Captain Kilowatt said:
Another thing to add is that coils by and of themselves are NOT traps. They MUST be tuned to resonance with a capacitor in order to make a trap. This cap is in parallel with the coil. At resonance they create a very high impedance that effectively isolates or cuts off the rest of the antenna beyond the trap. Below the cut-off frequency the traps act as loading coils for the next LOWER band.
Click to expand...

I do understand coils are not traps.. i did mention i have no way to make or tune a trap....
I was thinking if i shift the coil i might help the radashion of 40 and 20 i guess i was wrong....
I may just cut it for 40 ill let you guys know the exact distance i have to work with...
 

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