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Re: [IRCA] IRCA Loop update
- Subject: Re: [IRCA] IRCA Loop update
- From: "Steve Ratzlaff" <steveratz@xxxxxxxx>
- Date: Sat, 12 Nov 2005 11:44:56 -0800
Yesterday I built a 5-foot diameter square copper pipe loop, using 1/2 inch
copper pipe (two 10 foot sections), and an FT-140A-J high-permeability
toroid. Fortunately I took the toroid with me to the plumbing supply store,
thinking I would buy 3/4 inch pipe. But this toroid just barely slipped over
the 3/4 inch pipe with no room left to wind the secondary winding, so I had
to go with the smaller 1/2 inch pipe. (Regarding the elbows--they are
anything but a snug fit on the pipe ends, they just fit loosely and easily
pull out with the slightest effort. I don't believe you could make a
portable loop with loose pipes and elbows fitted together, without some
additional securing method. You could wrap aluminum foil around the joints
and tape them together--I think that would work, for a portable loop.)
It takes a LOT of heat to solder the elbows to the pipe ends. I started out
with the standard propane torch, but my cylinder used up within 5 minutes--I
hadn't used the torch in years. This was late in the day and no chance to
drive to a store for a replacement (50 mile round trip). Then I remembered
my oxyacetylene torch that also hadn't been used in years. But both bottles
had gas, so that worked fine for soldering the connections, and I even
remembered to slip the toroid transformer over one section!
My thinking on the toroid transformer was the pipe resistance was in the
range of 0.1 ohms; a match to 50 ohms coax would be the square root of
50/0.1 or 500, which rounded off gave a convenient 22 turns for the
secondary winding on the toroid. I used #24 gauge enamel wire, with a BNC
connector attached. I hung the loop outside between two trees (where my TA
EWE ended) and used that coax cable for the loop cable.
(A 5 foot square loop is actually a pretty big loop, once I got the thing
constructed and was carrying it outside.)
I have an antenna switch indoors next to the radios, so it's simple to
switch between the various antennas to check the signal levels.
It was immediately obvious that the loop's output was very small compared to
my other wire antennas--very, very small, on the order of 5-7 S units lower,
on the MW BCB, with more output at the high end. Using my R75, with Preamp 1
about +10dB gain and Preamp 2 about +20dB gain, use of Preamp 2 brought
signals up quite a bit. Down at LF, I only have two beacons that give strong
S9 to S9+20 on the longwires, with the loop using Preamp 2, they were
audible but anything but strong. I could just barely hear 60kc WWVB (which
is S9+40 on the wires). But the loop could be used on MW, even with no
preamp.
So, on LF I would not want to use this if I had any other LF antenna, but at
MW it would certainly work, with so much stronger signals.
I got strong QRM buzz on the loop, at the low end of the BCB that was barely
heard on the longwires--apparently the noise was coming from the house. The
loop was about 100 feet from the house.
I tried an external preamp, a +21dB ARR broadband preamp, but that didn't
work any better than Preamp 2 of the R75.
This morning for my TP reception, I tried switching to the loop a number of
times; there was never a trace of any TP heard on the loop.
So, for my initial experiment with a 5 foot copper loop, frankly, I am quite
unimpressed--at least compared to what I can hear with my wire antennas. I
bought enough extra pipe and elbows to make a 10 foot diameter loop, but
based on this first test, I'm not sure I'll build it. It will be a very huge
affair and won't be easy to suspend and rotate.
But if you're thinking about building a copper loop and trying it out, by
all means do so, you may have much better results than I had, especially if
you don't have decent outdoor wire antennas as a (perhaps unfair) comparison
to test against.
Steve
NE Oregon
----- Original Message -----
From: "Russ Edmunds" <wb2bjh@xxxxxxxxx>
To: "Mailing list for the International Radio Club of America"
<irca@xxxxxxxxxxxxxxxx>
Sent: Saturday, November 12, 2005 10:43 AM
Subject: Re: [IRCA] IRCA Loop update
>
>
> --- Bob Foxworth <rfoxwor1@xxxxxxxxxxxxxxx> wrote:
>
>>
>> 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)
>>
>
>
> *** Let's think copper elbows with an inner diameter suitable for the
> pipe to fit snugly into. This could be snapped together and taken apart
> without a lot of difficulty. Seems to me as long as the fits are snug
> enough to not break the conductivity you'd have a knock-down portable
> model which would fit in a fishing pole ( or pool ) carrier.
>
> Russ Edmunds
> Blue Bell, PA ( 360' ASL )
> [15 mi NNW of Philadelphia]
> 40:08:45N; 75:16:04W, Grid FN20ID
> <wb2bjh@xxxxxxxxx>
> FM: Yamaha T-80 & APS9B @15'
> AM: Hammarlund HQ-150 & 4' FET air core loop
>
>
>
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