Re: [IRCA] IRCA Loop update

["Bob Foxworth" <rfoxwor1@xxxxxxxxxxxxxxx>]

I am seeing where there are two different physical
aspects of this antenna, which is no big eureka-
type discovery, but I haven't seen it mentioned
in so many words. One is the length of the loop
section, which would be 40 feet for use of four
10-foot pipe sections. The other is the diameter
of the section, for which Craig is using
one inch dia. pipe.

The diminishing point of return, for making the loop
smaller, then has these two different parts, length
and cross-sectional area..

I am having a hard time believing that the miniscule
amount of current flow in the skin of the copper
pipe, due to received signals, is resistively affected
by reducing the diameter of the pipe. So, what would
be the effect of going to 1/2 inch dia pipe, then 1/4
inch dia pipe? You must reach a point where the
mechanical instability gets so great that the antenna
becomes physically unusable, but is the RF
performance still good enough at these diameters?

What would happen if the antenna was made from
semi-rigid copper strap mounted to an X-frame?
(besides probably costing more..) Or to a box frame
with X-shaped supports? Could the antenna be
made from 4 straight sections designed to be
attached at the corners, then made transportable
in some sort of oversized fishing-pole-type
carry bag? (That's probably over-kill, but an
interesting thought anyhow. You'd need a
really rigid corner attachment mechanism)

What about using a length of 1-5/8 inch heliax
arranged in a circle and using the corrugated shield
as the pickup? Too heavy? How would use of a circle
instead of a square affect the nulling ability?

- Bob                                                               sent
1126 est




----- Original Message ----- 
From: "Craig Healy" <bubba@xxxxxxxxxxxxxxxxxxxx>
To: "Mailing list for the International Radio Club of America"
<irca@xxxxxxxxxxxxxxxx>
Sent: Saturday, November 12, 2005 09:47
Subject: Re: [IRCA] IRCA Loop update


> > Well, the outer diameter of a conductor is really what gives it the
> capacity
> > to handle current, due to the "skin effect."   I wonder if the
collective
> > outer diameter surface area of Craig's cooper pipe loop is
sufficient to
> > overcome the collective outer diameter surface area of all of the
wires in
> a
> > wire loop.  Mathematically I can see how it could be the case.
Maybe the
> RF
> > capture success is directly related to the surface area of the
antenna.
>
> It may be.  There was no math or thought given to the design of this
puppy.
> I saw the one on the Stormwise site and thought it looked interesting.
> That, and the fact that I had a bag of ferrite toroids and the
plumbing
> supply is just up the street.
>
> > The ability to conduct current, and the ability to snag microvolts
from
> the
> > ether, are undoubtedly related in some way.  To take maximum
advantage of
> > surface area, maybe we could DX with a big brass sphere behind a
curved,
> > rotatable "nulling" shield.
>
> That has been tried using a barrel and some screen.  No go.  It takes
a loop
> to act as a current probe.
>
> However, a shielded loop is possible.  In FM broadcast a common
transmission
> line is a 3 1/8" coax in both rigid and flex.  I'm trying to round up
a 12'
> length of the flex stuff.  It can be bent in a loop and the center
> conductors put through the toroid and soldered together.  Then the
outer
> shield can be grounded on one end.  It'll be a unwieldy thing, but
will
> pretty much be a shielded version of the last 1" loop I built.  It may
drop
> the noise pickup down a bit, though it really is good now.
>
> I continue to be amazed at the wideband ability of this thing.  I put
the
> two foot version in my truck yesterday.  Just bungee corded it to the
spare
> tire.  I have a 1988 K5 full size Chevy Blazer.  This has a short
pickup bed
> with a fiberglass cap over it.  RF can get in.  The loop picks up no
> ignition noise at all.  CBA-1070 was perfectly readable at 3pm
yesterday
> afternoon.  Normally weaker stations like 1510 Boston and 580
Worcester were
> easily listenable.  On FM it could get maybe 80% of what I can get
with the
> roof mounted mag mount whip.  That was really surprising.  Even SW in
the
> 49m band was good.  Nothing heard on LW, but it was all external line
noise
> with no ignition racket or alternator whine.
>
> There was a directional nulling effect noted, but it was not a real
problem.
> It may turn out to be an asset at times.
>
> Priced some Amidon cores.  The FT-240-77 core is $9 each and the
larger
> FT-290-77 is $18.  I will buy some of each for comparison.  Once I get
the
> larger core I will make a loop out of 1 1/2" tube to see if that makes
any
> difference.  There is probably a point of diminishing returns.
>
> Craig Healy
> Providence, RI
>
>
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