On Tue, 1 Mar 2011 02:25:00 +0000, ka7niq
wrote:

This has been a good thread, I have little room for an antenna, a mag
loop may be just the ticket for my small Tampa QTH ?

Hi OM,

Well, as you can imagine (barring the numerous errors and moral
judgments), it all depends upon the band you want to operate - with
the 40M and higher frequencies quite well served.

A lot of myth surrounds what are called "magnetic loops" and this
thread has corralled some of them - including from Norbert as his page
which forces the argument that fairly agrees that below 40M
performance dives. However, through sloppy bookkeeping, the Ciro
Mazzoni line is not one I would walk away from for stated
"inefficiencies."

The principle consideration is the ratio between radiation resistance
(power that is expressed into making contacts) and Ohmic loss (bulk
metal conductivity power that is expressed into making heat). Wimpie's
choice of 20mm diameter stock (how that arrived in the mix is a
mystery) compares poorly with the Ciro Mazzoni 50mm tubing for its
smallest design.

The 80M design from the vendor uses 75mm stock for good reason and
this should be a selection guide for your application. Their second
80M design uses 140mm stock! Pushing this further with conductance
now nailed down, you want a large loop because the radiation
resistance varies by the fourth power of dimension. That is to say,
if you double the loop radius, you obtain 16 times the radiation
resistance. Small changes in loop radius can quickly escalate or
emasculate efficiency. Radiation resistance is the beneficial
characteristic of how we manage to couple a signal out into space and
which is typically thought of as being 50 Ohms (although this is
rarely the actual value that more often varies between 35 and 70 Ohms
for simple wire antennas of conventional length).

As you can see from these resistance figures, the difference between a
radiation resistance in the thousandths of an Ohm, and typical values
in the tens of Ohms is a hallmark for caution. When paired with metal
resistance in the Ohms (something that ordinarily only comes with
using wire-wrap wire for long runs), you want to boost radiation
resistance as high as possible. When paired with metal resistance
that is in the thousandths of Ohms, there is every chance you are
looking at 50% efficiency for 1 meter diameter loops.

Bigger radius comes with its own problem, however. It limits the high
band of operation as these designs are optimized for being a small
portion of wavelength. Observe the various design options from Ciro
Mazzoni, and you will observe they are specified over only two octaves
for any particular design. That should give you a clue if you want to
homebrew your own, because you will encounter the same limitations of
coverage regardless of construction method.

So, this returns us to the first statement above: it all depends on
which band(s) you want to work. It further depends upon your pain
threshold for poor efficiency if you choose to push beyond the
coverage limits. Professionals describe this in terms of a
cost/benefit ratio. If we restrict discussion to non-professional
qualitative expressions of benefit: super, great, fantastic, maximum
and peg escalating dollar amounts to each with corresponding
breathless emphasis - then there are many deals for sale on those
terms for the gullible.

73's
Richard Clark, KB7QHC