the author of the website I posted has a magnetic loop for 80M that is 12
feet square, made from 2 inch aluminum tubing and is vertically mounted with
the bottom 6 inches off the ground.
he says on the website that it works equally as well as an 80M dipole.
What I like about the magnetic loops is that their performance seems to
equal something similar to a dipole, but you can set one up in your living
room or on your balcony very easily. Especially for the higher bands...a
magnetic loop for 10M would only be 2 feet square or less...around 85 to 90
percent of the quarter wave of the band you wish to work the most on.
A guy could easily have several loops for 6M 10M 12M 15, 17 and 20M all set
up on the balcony.
In this article he uses a coaxial stub as a capacitor, but you can also get
a gennings variable cap rated for 10KV that you can motorize and remotely
tune the antenna for allot of different bands and frequencies.
You get polarization if the loop is vertically polarized as well...you get
lobes off the sides in a figure 8 pattern with a nul at the center of the
square...so you can use it as a directional antenna or to null out QRM or
QRN.
Of course, with 100W into the antenna, the capacitor has something like 3KV
on it, so that piece has to be well above or beyond human contact.
Definitely an interesting project to think about.
These antennas are pretty hard to tune up with out an antenna analyzer as
you have to tune the feed point, then tune the capacitor...difficult to do
as oftent he antenna's feed point resonance will be way way out of the ham
band...after tuning the feedpoint, you tune the stub or cap to bring it up
to resonance on the band you wish to use.
Essentially, you use an antenna analyzer after hooking up the feedline by
the method recommended...then, where ever you see a dip in the SWR, you move
the center conducter up or down the side until you achieve a flat match
regardless of the frequency.
Then, you start cutting your coaxial stub until your very close to the
frequency you want, and have a flat match still. With a coaxial stub, it's
something like 1 inch of coax for every 200KHZ or so.
In his example, he starts out with 3FT of coax, and a feedpoint match
frequency of 10MHZ...he cuts the coax down until he's at about 14.00MHZ.
Then he just slides the outer brade of the coax back another half inch to
get into the phone portion of 20M.
It doesn't seem to matter what the feedpoint resonant frequency is, as long
as you can get it down to 1 to 1 somewhere in the HF spectrum.
apparently as long as that point is lower in frequency to the frequency you
wish the antenna to perform on, then your ok.
I don't yet completely understand the theory behind this style of antenna,
but I certainly understand what needs to be done to make them work
properly...even if that is a bit counter intuative to other kinds of antenna
theory.
73
Colin, V A6BKX
----- Original Message -----
From: "Lou Kolb" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, February 24, 2011 4:40 PM
Subject: Re: magnetic loop antennas?
> Colin,
>
> I've never used one but worked a guy using one a couple months ago on 40
> meters. I'm in PA and he was somewhere in New England but he was 20 over
> S9
> and I was amazed when he told me what he was running. He had not only a
> competitive signal, but an outstanding one. As you say, he couldn't
> really
> move from that frequency without readjusting, but the thing worked
> remarkably well. As you might imagine, we spent most of the QSO
> discussing
> the merrits and drawbacks of that antenna. Sounds like a fun and
> worthwhile
> project. 73, Lou WA3MIX
> ----- Original Message -----
> From: "colin McDonald" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Thursday, February 24, 2011 6:11 PM
> Subject: magnetic loop antennas?
>
>
>>I have a couple questions for the list regarding magnetic loop antennas.
>> Firstly, has anyone used them on HF? I'm speaking of the small
>> transmitting loops that are made in a square and use a capacitor on one
>> side
>> to tune to resonance.
>> And, if so, what sort of results has anyone found with them?
>> Also, has anyone on list built one using a coaxial stub capacitor?
>> Check out this website:
>> HTTP://www.standpipe.com/w2bri/
>>
>> he's got a very straight forward, step by step description of how to
>> build
>> a
>> magnetic loop antenna from copper plumming supplies, and using a piece of
>> coax as a capacitive stub.
>> The antenna looks exceedingly easy to build, and his claims, as well as
>> others, say the 3.5 foot square loop will perform as well as a half wave
>> dipole on whatever band you tune it for...of course, as long as the
>> loop's
>> circumphrence is something less than a quarter wave on that band...for
>> instance, this loop is 14 feet all around and therefore is less than a
>> quarter wave on 20M. I've heard allot of great things about the magnetic
>> loop type antenna...of course it's only draw back is that it has an
>> extremely narrow band width...something like 20 to 30KHZ.
>>
>> anyway, check out that website and tell me what you guys think.
>>
>> 73
>> Colin, V A6BKX
>
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