[Elecraft] (OT redoux) MAG LOOP ANTENNAS

[Doug Turnbull]

Fred,
    Well put and useful.    I can though testify that the AlexLoop can work
amazingly well from inside a frame house and even a stone house as I have
tried this.   It also works wonderfully from a hotel balcony; though I
understand these are less common in W land for unfortunate reasons.   Keep
the magnetic loop off furniture with metal springs inside.

   The AlexLoop or Magnetic Loop is my antenna of choice for travel as it is
so unobtrusive and yet amazingly can work.    This sounds like snake oil so
perhaps I am over enthusiastic.  I have used the AlexLoop in a part time
effort for a WPX contest.   Operation was from Connecticut with the antenna
inside my sisters house using KC1AV as my call.   I worked 270 some Qs with
a good 30% of these being with Europe.    My power was 5 or 10 Watts -
memory fails me on this point.

     I have at times found the AlexLoop out perform a Buddipole at full 1/4
wave length for twenty meters with two radials.   How this could be well I
can not answer.   I am suspicious.    At night it is surely easier to retune
the antenna when changing bands than having to go outside and change the
length of the antenna and its radials.  The AlexLoop capacitor is within
arm's reach.

     When possible for portable operation though I prefer an end fed dipole
but this only works when staying with understanding friends or family.

                     73 Doug EI2CN

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Fred
Jensen
Sent: 22 January 2017 22:50
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] (OT redoux) MAG LOOP ANTENNAS

I'm not sure about a mag loop inside a room, I've never tried mine that 
way.  When used vertically outside, they tend to be insensitive to 
surroundings [trees, ground, you sitting under it] because the 
permeability of most such surroundings is 1.  This may not be true if 
you're set up on the track of a railroad yard ... or if there's a lot of 
ferromagnetic material in your walls.

If you try it, be aware of several things:

1.  Small magnetic loops involve some very high voltages and circulating 
currents, even at surprisingly low powers.  AA5TB has an on-line 
calculator and there is a similar one at 
http://www.66pacific.com/calculators/small_tx_loop_calc.aspx

2.  The equivalent radiation resistance of small loops is 
extraordinarily low -- in the milliohms range.  Your power will be split 
between the radiation resistance and the RF resistance of the 
conductors, which is higher than the DC resistance because of the skin 
effect.  You really need to achieve the lowest possible RF resistance 
[high conductor surface area].

3.  Mag loops are resonant transformers or autotransformers.  To radiate 
at all, they MUST be exactly resonant at your TX frequency.  "Getting 
close and letting the ATU match it" will create a fairly effective dummy 
load.  Bypass the ATU and tune the loop for exact resonance.

4.  The usable bandwidth will be very low [i.e. you need to re-resonate 
the loop when you QSY, even a little].  As the circumference of the loop 
becomes smaller in relation to the wavelength [i.e. on lower frequencies 
for a fixed size loop], the BW can become smaller than the occupied BW 
of a SSB signal.

Commercial mag loops [e.g. my Alexloop] are usable from 40 up. Tuning it 
on 40 is really touchy.  It's easier on 20 and up.  They tend to run in 
the $400 range.  You can build one much cheaper, but remember #2 above.

73,

Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County

On 1/22/2017 6:16 AM, Robert 'RC' Conley wrote:
> Guys;
> I'm anticipating a move to a seniors villa and I know I will be unable to
> to take my antenna farm and most of the ham shack. I've decided on my KX2
> with ATU (grab-n go). A MAG LOOP should allow indoor operations at QRP
> power levels.Thanks for your thoughts.
> 73
> KC5WA
>

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