[Elecraft] Mag Loop

[Morgan Bailey]

Having played with a Loop, I made one octagonal about 64 inches across, and
mounted it vertical or perpendicular to the ground, I observed sharp nulls
through the center of the loop. Additionally, I took a 4 or 7 watt
flurescent bulb and while transmitting (low Power) ran the bulb from top to
bottom and found that about ( reference feed point at the bottom, 6 o'clock
position, im calling it zero degrees and tuning cap at the top, im calling
180 degrees or 12 o'clock position) 60 degrees about at 4pm point the bulb
started to glow and was brightest at about 100 degrees 2:30 position and
dropped back to dim to extinguished at 180 degrees. The loop was made for
20 and 40 meter operation. It is physically too large for 15 meter
operation.  When I made a tuning cap to resonate it on 80 meters knowing
the efficiency would be maybe 3 percent, the only place the bulb lit was
near the top with no glow from zero degrees 6 o'clock to 150 degrees or
about 1-130 o'clock positions. All NVIS on 80 meters with a small loop.
This was reinforced by its behavior on 20 and 40 meters. 40 had about a 5
or 10 degree higher angle of radiation over the 20 which was more evenly
current distribution on the loop, with the main (brightest) lobe lower in
the loop.

So when we look at loops and directionality, the frequency of a multi band
loop does change the radiation pattern. The bigger the loop for the band
the more evenly distributed current was distributed along the loop. Up
until the physical size of the loop does not follow the loop formula and
gets too big to be considered a magnetic radiator. On 40 it radiated
brightly at the 3 to 4 oclock positions but on 20meters, the current was
more evenly spaced from about 430 to 1 oclock positions. On 20 it was
brightest at about 245 positions.

Maybe this is not scientific but my believe is that, this
is demonstrating the angle of the lobes coming off the loop. It seems that
it radiated well in the 20 to 30 degree take off angles on 20 and 40 meters
with 40 meters being a bit higher more like 25 to 45 degree take off
angles. Having this loop in my dining room on the floor of my house over a
3 foot crawlspace, I was able to work Europe repeatedly during the
afternoon hours. I did this more than one time months apart. It was not a
propagation fluke. Rig was 75 watts CW from a Yeasu FT 450d. I use it as my
test rig.

I built the loop to prove that they do not work....yet again I was proven
wrong.

My loop was made out of 1.5 inch by .25 inch thick Radiused edged aluminum
bar. I got this from onlinemetals.com. I used radiused bar because RF does
not flow around corners well. And, structurally, it was self supporting
with a small bracket for a stand. And, it is just easier to work with flat
stuff rather than round stock. By using the calculation on the web it
performed identical to a 1.5inch copper tube. It was cheaper and much
easier to work with. Additionally, I made it so it could be disassembled,
yes I know the problem with resistance, don't go there, after all it is a
compromise antenna and I was building it for portability and sturdiness.
This loop really performed beyond my expectation.

Was it better than a full sized vertical or a trap vertical with
radials...Oh hell no, but I did make good contacts and even checked into
many 40 meter and 20 meter nets from having it inside my house on the floor
of my living room. I positioned the plane of the loop east west and was
easily able to work both coasts and into Europe. Most of my contacts on it
were on CW. See the attachment of it in my living room.

When I rotated the loop 90 degrees on the axis the signals dropped from 2
to 6 S units, mainly depending on the distance from my location. The closer
in the less drop. I attributed this to the NVIS component of the radiation
(the 1 o'clock radiating area).

[ATTACHMENT TOO LARGE FOR FORUM STANDARDS, IF YOU WANT A PIC I WILL GLADLY
FORWARD IT.]

Well this is my experience with my loop and here is a picture of the loop
in its final configuration (see attachment) and no I don't want to be
flamed about the construction but it works and with some fine emery paper
and no-al-ox on the joints. It has a very sharp resonance and narrow band
width of maybe 5kc 2to1. After that, it ,the SWR climbs like a wall > 25 to
1  But even though the SWR may be high off the sharp resonance transmit
frequency, it still was a good rx antenna broad banded 100khz with very low
noise. I can not say it was immune to but, it did not pick up electrical
noise like the full sized vertical and the noise floor was lower than the
typical vertical antenna. I have a full sized 40 and 20 meter vertical with
60 radials to compare it to and it does not get the same sig reports as
that vertical but I did not expect it to. On occasion, with a preamp for rx
(dx engineering one) copy was better on some stations over my outside
vertical---only because of the reduced noise floor. It was a fun project
and I really enjoyed the construction.

They are not a miracle antenna but any antenna is better than no antenna
and for many this may be the only option to be able to operate. Hell when I
get moved into a retirement home, I will use it for a clothes rack and
transmit on it when no one is looking.  Since digital, PSK**, would be an
ideal use for a loop as you don't need much power for the computer to do
the heavy lifting. QRP signals are still good enough for computer ears, but
not necessarily for ours.. LOL. In the end, it is another example of "QRP
Works."

Good luck with your Loops

Vy 73,

Morgan Bailey NJ8M