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Author Topic: Loading Coil Design Questions  (Read 327 times)
KE5HJO
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Posts: 207




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« on: June 21, 2007, 10:21:06 AM »

I have a 20M 1/4 vertical antenna that I made from a telescopic 20-Ft fiberglass pole.  It works great on 20 but I would also like to use it for 40 Meters.  I downloaded one of the many loading coil programs that are found on Ham sites around the internet and designed me a nice coil that would fit my needs.

I installed the coil but the SWR remained above 6 all across the 40 M band.  I trimed the coil, lengthened the coil, tried various other things none of which worked.

My questions:

(1)  Can loading coils be made of insulated wire wrapped around the base of the fiberglass antenna pole?

(2)  If the answer to #1 is "yes", then does the loading coil effect the antenna wire inside the fiberglass pole since it is located coaxially around it?

FYI - I also built a coil out of bare aluminum wire which was also wrapped around the base of the poll.  The SWR never dropped below 3.  Dazed, tire, and confused - I gave up and went to bed.

Any help / tips would be greatly appreciated.

Mike
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AA4PB
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Posts: 12700




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« Reply #1 on: June 21, 2007, 10:32:48 AM »

You shouldn't run the antenna wire down thru the center of the coil. The antenna wire should end at the top of the coil and connect to the top of the coil. Running anything conductive thru the coil will have a negative effect on the coil (change the inductance, add shunt capacitance, etc).
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WA3SKN
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Posts: 5443




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« Reply #2 on: June 21, 2007, 10:33:46 AM »

Answer to question 1... Yes.
Answer to question 2... Maybe.
You will want the coil to be about as wide as it is long, and space-wound. If you use bare wire, you can short turns.
A grid-dip meter will get you into ballpark resonant frequency, then you tap the coil for 50 ohm input.
There is trial-and-error with this... but it will work.

-Mike.
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K8AG
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Posts: 349




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« Reply #3 on: June 21, 2007, 10:45:32 AM »

The whole idea of the loading coil is to make the antenna "rfwise longer".  If the coil is a resonator, that is something different.  A resonator for say 40M would form a current stop on 40M on an antenna designed for say 40 and 80M operation.  It would add a certain inductance to the 80M operation.

The loading coil goes in series with the antenna.


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 Feedpoint
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Ground/counterpoise


A coil can also go other places rather than the base.  It may go near the center of the element.

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 Feedpoint
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Ground/counterpoise

In this case the coil would be different as the current/voltage is different that near the base.

Good luck

73, JP, K8AG






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K8GU
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Posts: 716


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« Reply #4 on: June 21, 2007, 11:15:54 AM »

My answers...

1. Sure.  As the others have said, it might not be the best coil; but, it will work.

2. Probably a bad idea.

Any time you find a calculator on the Internet, it has limitations.  Unless it's well-documented, you don't know what these limitations are.  I wouldn't trust it as far as you can throw it.

A quick-and-dirty way to determine a base-loading coil value is to measure the input impedance of your antenna without it.  Then, resonate that impedance with an appropriate coil.  (This is a different approach to the grid-dipping idea presented above.)  I would use an L-network to get it back to 50 ohms rather than the autotransformer, though.

I don't know how you're measuring the match; but, this is going to be far more difficult without an antenna analyzer or something similar.

You also might try top-loading with a capacitance hat.  It's almost always more efficient than base-loading.
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KE5HJO
Member

Posts: 207




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« Reply #5 on: June 21, 2007, 12:26:40 PM »

Thanks guys.  I think I'll skip the coil for now and use the height of the pole to build an Inverted Vee Dipole for use on 40 Meters.

Thanks!

Mike
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WA3SKN
Member

Posts: 5443




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« Reply #6 on: June 21, 2007, 12:41:50 PM »

My above post was based on your trying to add a home-made coil to a working antenna. My preference to multiband this vertical would be an "auto-tuner" mounted at the base, and working against ground.
But, you never mentioned your ground system... which could have a major effect on your readings.
Just what type of a ground radial//counterpoise system are you using with this?

-Mike
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SSB
Member

Posts: 1




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« Reply #7 on: June 21, 2007, 01:27:53 PM »

Using a loading coil on a short antenna is a 2 step process.  The purpose of a loading coil is to compensate (cancel) the capacitive reactance at the feed point.  When the reactance is zero, the feed R is NOT 50 ohms, its much lower if you have a good ground.  A 20 meter long antenna used on 40m has a Resistance of around 2 to 15 ohms. You must transform the low feed R to 50 ohms using an L network or something.  You need to read an Antenna book now or you will waste alot of your time doing it by trial.

Alex......
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N3OX
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Posts: 8853


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« Reply #8 on: June 21, 2007, 03:29:28 PM »

SSB says, correctly, "You must transform the low feed R to 50 ohms using an L network or something. You need to read an Antenna book now or you will waste alot of your time doing it by trial. "

Here's a cheat sheet for building such a coil, from a prior post I made in the last "need vertical help" thread:

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Take a coil of about 14 turns #10 wire, three inches in diameter and about 3 inches long connected between the antenna element and *ground*.

Connect the coax shield to ground too and tap the coax center conductor onto a coil turn a couple of turns up from the bottom. You can use an alligator clip for testing, and solder it once you get a match.

If you operate this antenna over a good ground system, it'll be within 1dB or so of having a full size vertical on 40m... you'll only be able to cover about 80kHz of the band, but you can also tap the antenna element down a turn or two from the top of the coil to raise the resonant frequency of the antenna.

- - - - - -

73,
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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