20 foot 80 meter dipole coils

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Bob Waterfield:
I am attempting to help a fellow ham who is blind put an antenna on his apartment balcony. He has a slinky antenna for 40 meters but not the room to add to it for 80 meters.  The balcony is 7.5' tall X 4' wide X 23' long.  There is a metal railing that is 3.5' tall with concrete floor and ceiling.  I was thinking of making a dipole that has a 5' length a coil and another 5' length of 1/2" copper pipe for each of the two elements 20'+ coils total length.  I am good with VHF antennas but HF is another animal for me.  What size wire, and how many turns should I use if I wind them around 1" PVC for the coils?  He has a tuner is his HF rig.  Can I feed it straight with RG8 or do I need a balun of some sort?

Bob

Dan:
You'll need something in the ballpark of 200 turns of #18 enameled wire close wound on an 8" long 1" form.

Power handling will be limited because the coils will dissipate almost all of the applied power.

Later on I'll try to figure out something better.

The best plan is to use some capacitance hats at the very ends of the copper pipe and have coils between the pipe and the hats.  I'll figure out some reasonable coils and get back if no one beats me to it.

20 feet is very short.

73
Dan

Bob Waterfield:
I know 20' is very short, but it is all the area I have to play with.  By hat, do you mean some sort of radials?
Similar to what a discone has on the top?

Bob

Lee A Crocker:
You'll spend a couple hundred bux and hate it

73  W9OY

Dan:

"By hat, do you mean some sort of radials?
Similar to what a discone has on the top? "

Yeah, but they could even have a perimeter wire around the outside.

Here's an extremely fancy one:

http://people.zeelandnet.nl/wgeeraert/baken/foto02.jpg

Adding some 1/4 inch rods (or whatever it takes to self-support) with a wire around the outside of them will help the antenna's efficiency A LOT.

A fairly efficient for its size, but also very narrow-band antenna (no getting around that if you want it to be efficient), could have 20 feet of copper pipe with eight 3 foot long spokes on each end with a wire around the outside (I know it's kind of big for a balcony, but it'll fit and this is 80m).

Then you put a big coil near each hat.  You need approximately 30uH.  40 turns on a 3 inch form of #10 wire spaced 1 wire diameter apart might do the trick.  (again, form is about 8 inches long).  That's actually probably enough to resonate at the bottom of the band, but you'll want to adjust the thing to resonate somewhere in the band (more on that later), and a too-big coil will let you solder or clip a tap onto the coil to tune it.

In other circumstances, if the end user could go out and adjust the loading coils, you can put a small shunt coil of about 1uH to bring the impedance up to 50 ohms at resonance.  10 turns on a 1 inch form about 2.5 inch long (again, #10 will make a nice coil that in this case will self-support with no problems).

But maybe you don't even want to bother with that. It seems that just a few kHz off resonance  the shunt coil sort of shorts out the antenna (it *doesn't* at resonance, it matches it up to a nice 50 ohms but the 2:1 SWR bandwidth is only 15kHz or something stupid)

I think maybe what you want to do is adjust the loading coils for resonance at your friend's favorite frequency, if he's got one, and let the tuner try to cope away from there (but it might be tough)

You might have to intentionally add a bit of loss for the sake of operational convenience.  Many shortened antennas do that.

If you want to get fancy you could have different coil taps for different parts of the band, and that would help a lot with efficiency.  If I were building this beast I'd probably motorize it, but that makes your build complicated and expensive.  You need good decoupling chokes at the feedpoint for the motor drives, etc, etc.

However, if your friend is otherwise able to go adjust something out at the antenna aside from not being able to see it, you could potentially make a bunch of preset manual coil taps for his favorite frequencies.  Anderson PowerPoles are pretty easy for me to mate without looking at them and your friend could just count taps to know which one he was after.

It's simple and rugged and would greatly increase the efficiency since I think even with the rig tuner, it's going to be hard to efficiently cover more than about 60kHz of the band and it'll get hard for the tuner to tune.

But with eight coil taps on each coil and the tuner inside, I think you could have a halfway decent performer on 80m.  My EZNEC estimate of the efficiency of this antenna is about 40% compared to a full size dipole if you change coil taps.

73
Dan





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