[Elecraft] Magnetic Loops

[David Y.]

Hi All,

A friend of mine here locally once did quite a bit of modeling for loops. 
He determined that setting up a loop vertically was easier than he expected. 
Horizontal loops are difficult here in the desert.  Anyway, his modeling 
disclosed that the horizontal height of a vertical loop (full wave) only 
needed to be about 30 degrees to start being quite effective.  In other 
words, a 40 meter vertical loop only needed to be about 11 or 12 feet high 
on each end, with the rest of the loop run horizontally.  This is not all 
that difficult to do with readily available supports.  Of course, the bottom 
of the loop should be off the ground as much as possible, so the total 
vertical height should be something more than just 11 or 12 feet.  Vertical 
loops are quite directional, but they have a lower take-off angle for DXing.

Smaller magnetic loops may have similar tendencies.  I know it is important 
to get them up off the ground as much as possible, and I have always had 
much better results using them vertically as opposed to horizontally.  I 
built a couple of these things from magazine articles, and they do work. 
It's hard though, to imagine the efficiency being all that great. 
Nevertheless, they are fun to play with, and not too difficult to make 
yourself.  Bill Jones, KD7S, had a very good article in QST some years back 
which really gives you a lot of important info about doing various things to 
make the antenna as efficient as possible.  His "trombone" capacitor was 
very interesting, but I haven't tried that yet.

I would think that apartment dwellers, hopefully having a balcony, might 
have some real fun tinkering with these antennas.  If you are high enough 
off the ground you might get decent results using the loop horizontally.  A 
low RPM motor to tune the capacitor makes life easier too.  I bought one 
from Edmund Scientific, and it worked very well.  The tuning will be 
extremely sharp however.  I was never able to go more than a few khz without 
having to retune.

Dave W7AQK

----- Original Message ----- 
From: "Julian, G4ILO" <julian.g4ilo at gmail.com>
To: <elecraft at mailman.qth.net>
Sent: Tuesday, August 04, 2009 1:35 AM
Subject: Re: [Elecraft] Magnetic Loops


>
>
>
> Gary Hinson wrote:
>>
>> By the way, I don't know if it's the same with magnetic loops but the
>> fullwave loops radiate mostly "broadside" to the plane of the loop.
>> If the loop is vertically oriented (e.g hanging between two trees),
>> the polarization of the signal is determined by whether the loop is
>> fed at a horizontal or vertical section of wire.  If the entire loop
>> is laid 'flat on its back', most of the radiation heats the ground,
>> some heats the clouds but I'm uncertain what polarization the signal
>> would be - probably horizontal I guess since all the wire is parallel
>> to the ground but I may be wrong.
>>
>>
>
> With magnetic loops most of the radiation is in the plane of the loop, and
> it's vertical polarization if the loop is vertical. But there is not much
> directivity. There are nulls axially through the centre of the loop but 
> they
> are extremely sharp. The polarization also changes to the horizontal as 
> you
> move round to the side too.
>
> According to MFJ if you can get it up high enough you can mount their
> magnetic loops horizontal and get omnidirectional horizontal polarization
> with nulls straight up and down.
>
> It's interesting switching between the magnetic loop and a dipole, 
> sometimes
> the dipole has the stronger signal and sometimes the loop. A receive
> magnetic loop would make an inexpensive and easy to make antenna for
> diversity reception for those who have the 2nd RX in their K3.
>
> -----
> Julian, G4ILO. K2 #392  K3 #222.
> * G4ILO's Shack - http://www.g4ilo.com
> * KComm - http://www.g4ilo.com/kcomm.html
> * KTune - http://www.g4ilo.com/ktune.html
>
> -- 
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