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KJ4NBM
12-08-2010, 10:59 PM
Ok here's my question,

I have a homemade GP vertical (groundmounted) I'm using for HF. It is a 1/4 wave resonant length for 20 meters-about 16'.

I'm wondering what the dimensions of a base loading coil would need to be to operate the 1/4 wave 20meter on 40meters. What about 80? Anybody know of an online coil calculator that might help?

KF6ABU
12-08-2010, 11:30 PM
any chance you can just add a capacity hat to it? think of guy wires comming off the top of the antenna. you could add 4 4ftish wires of the tips for 40m performance.

WB2WIK
12-08-2010, 11:33 PM
any chance you can just add a capacity hat to it? think of guy wires comming off the top of the antenna. you could add 4 4ftish wires of the tips for 40m performance.

I don't know if 4' top hot wires are really long enough for 40m, but there's a free calculator program that can be used to find out:

http://www.smeter.net/antennas/short-top-hat-loaded.php

Certainly loading via a "top capacitance hat" is cheaper and lower loss than any sort of base loading coil would be.

KJ4NBM
12-08-2010, 11:43 PM
What about an LC tuning circuit at the base? That's another idea I've considered based on reading in some older ARRL antenna books.

WB2WIK
12-08-2010, 11:48 PM
To load a short antenna, you normally wouldn't need any "C" at the base, just "L." The antenna will have a lot of capacitive reactance, and you need series inductance to tune that out.

KF6ABU
12-09-2010, 02:31 AM
What about an LC tuning circuit at the base? That's another idea I've considered based on reading in some older ARRL antenna books. are you wanting to use it on multi bands often?

K4EEZ
12-09-2010, 03:02 AM
look what this guy did

http://www.southgatearc.org/articles/g0ftd/shopping_trolley_wally.htm

add 2 servos cables, switches and you'l be all set to go.

KJ4NBM
12-09-2010, 04:04 AM
are you wanting to use it on multi bands often?

Yes, I would would like to be able to work 40 and 80. The vertical section is made of two pieces of telescopic aluminum tubing. I can shorten it for 17 and 15 if I want.

I would like to leave it at resonant length for 20 and tune it to the other bands. An automatic remote AT would probably be best but I don't have the funds for that at the moment.

W8JI
12-09-2010, 12:37 PM
You better think twice. It is a tough project to make a 15 foot antenna work on 80 meters efficiently. A 43 foot vertical, to radiate just 100 watts of equivalent field strength intensity, has 1000 volts peak across the base insulator or loading coil.

Voltage roughly goes up by about the square of the length reduction, and a 15 foot vertical would actually have about 10,000 volts peak across the loading coil insertion point (and insulators) on 80 to radiate 100 watts of useful power.


You need a taller antenna, a 15 foot tall mast above the loading coil will be marginal on 40 and terrible on 80. Some auto tuners will handle it on 40 with a tuner at the antenna, none will handle 80 meters unless you add a lot of loss someplace to eat up power as heat.

W5DXP
12-09-2010, 01:58 PM
I'm wondering what the dimensions of a base loading coil would need to be to operate the 1/4 wave 20meter on 40meters.

EZNEC says you need a base-loading coil of ~7.2 uH on 7.2 MHz to match a 16 foot vertical monopole made from 1 inch aluminum tubing. Such an antenna might be about 33% efficient on 40m. The Hamwaves inductance calculator can help you design a loading coil. To convert inches to mm, multiply by 25.4.

http://hamwaves.com/antennas/inductance.html

KJ4NBM
12-09-2010, 03:01 PM
Hmmm. Maybe its time to consider a dipole or random wire for 40 and 80.

K0RGR
12-09-2010, 03:47 PM
Hmmm. Maybe its time to consider a dipole or random wire for 40 and 80.

This is often the conclusion many of us reach for an efficient antenna on the lower bands. The trade off there is antenna height required to be an effective low angle radiator for DX purposes. If you can't put up an antenna higher than 15 feet, that's a problem. Under normal conditions, a dipole that low would be very effective out to about 300 miles and poor beyond that. If your goal is to check into your state traffic nets on 80 meters in the afternoon, a 15 foot high dipole is probably a good choice.

I think most of the people I work on 80 are using dipoles at 40 feet. Such a device is very effective for regional work out to a few hundred miles. Beyond that, it's not as effective as a well installed vertical with plenty of radials.

But, yes, a 15 foot vertical is pretty short for 80 meters. The HyGain AV-18VS is an 18 foot vertical with a loading coil that works on 80. You can read the eham reviews here: http://www.eham.net/reviews/detail/2281 I suspect the glowing reports are from those who have never used a full sized vertical on 80. But, hey, my 8' loaded whip works as a mobile antenna on 80 - very poorly, but much better than nothing!

WB2WIK
12-09-2010, 03:52 PM
Hmmm. Maybe its time to consider a dipole or random wire for 40 and 80.

You might be able to use the 20m vertical GP you already have by adding a 20m trap to the top and extending a horizontal wire from that out to a tree limb or some other support as high above ground as possible (using an end insulator and lead rope, of course). This gives you a GP on 20m and an inverted-L on 40m. That works, and can work pretty well.

Since your vertical is elevated and presumably has 20m-sized radials now, you'd likely want to add some longer radials also to reduce losses on 40m.

KJ4NBM
12-09-2010, 05:26 PM
You might be able to use the 20m vertical GP you already have by adding a 20m trap to the top and extending a horizontal wire from that out to a tree limb or some other support as high above ground as possible (using an end insulator and lead rope, of course). This gives you a GP on 20m and an inverted-L on 40m. That works, and can work pretty well.

Since your vertical is elevated and presumably has 20m-sized radials now, you'd likely want to add some longer radials also to reduce losses on 40m.

That's an interesting idea. Although the vertcal is ground mounted (about 1' above ground) with 32 roughly 20meter sized radials.

WB2WIK
12-09-2010, 06:41 PM
That's an interesting idea. Although the vertcal is ground mounted (about 1' above ground) with 32 roughly 20meter sized radials.

This will still work. You probably have enough radials, although adding more could never hurt.

Have a tree or something that's 20 feet away and tall enough?

It's a cheap experiment that will likely work fine.

You can make a trap, or buy one.

M0WAN
12-09-2010, 10:53 PM
That is a better idea.

The problem with base loading a vertical is that although you can cancel out the capacitive reactance with the coil, the radiation resistance will be pitifully low, causing high currents in an already lossy earth system and a poor match to the feeder, seriously impacting the efficiency.

This is why an earlier post mentioned LC matching at the base - at least that way you also get a good match between antenna and feeder.

But if you can top-load with a horizontal wire, radiation resistance will be higher and therefore efficiency. Also remember that it is the high current portion of an antenna that radiates the most energy, and that is (roughly) the lower 1/8 wave of a quarter wave antenna, so you want that bit to exist as a real bit of the antenna, not a coil of wire.

KJ4NBM
12-10-2010, 03:37 PM
Thanks all for the input here. I realized there's a couple of articles by W5DXP over at eham related to loading coils that are interesting ("Degrees occupied by a coil" and "80meter base loaded mobile antenna"). However I'm really considering an 80/40 trap dipole in an inverted V now.

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