BACKGROUND It
all started with finding a nice, wooden embroidery hoop at a local
flea market, about 22 1/2" in diameter. Close to two feet across.
It is about an inch and a half wide. I got to thinking about making
it into a loop antenna, and since it was only three dollars, I went
ahead and bought it, figuring the idea would hit me at some point.
Well... After guessing, thinking, over the last few weeks, I just
went into my shop and started building what my mind had started to
see. What
has resulted is a very stable, well-balanced and powerful loop antenna,
light-weight and able to achieve deep nulls, and is so far really
impressing me! It's in a fairly preliminary, 'prototype' stage, and
improvements and additions are forthcoming, but for now read on...
MAKING THE "HOOP LOOP" I
knew I wanted it mounted to a stable base/lazy susan for rotation,
and that was easy. The big outer hoop mounts to a 9" square,
1" thick piece of plywood. Easy. Then, I wanted to make the inside
hoop swivel to get an altazimuth feature like the big Kiwa model,
which would surely assist in deep nulling of unwanted signals, noise
and so on. Very carefully, I measured where my two points would be
to hinge it with the best balance (more on balance later) and drilled
the holes for the nylon screws
I had in the junk drawer. It worked fine, and it swivels beautifully.
After
drilling the holes and inserting the screws to test the ease
of angling, I then removed the screws and began the tedious
work of affixing the wire to the outside surface of the inner
hoop. I realized I had to wind the loop in two sections, so
that the screws for the altazimuth hinges could be accomodated.
Measuring and testing, I found that I could wind 6 turns of
#22 insulated wire on one side of the inner hoop, wound with
no spacing between turns, THEN leaving about 1/8" space
for the screw holes, and then another 6 turns to complete the
12 turn loop.
I
thought about simply pulling the 7th turn away from the first
'set' of 6 turns, at the bottom of the hoop, and then continuing
the rest of the winding, leaving two equally sized 'sets' of
windings, but continuous, leaving a space for the screws on
the sides of the hoop... but this seemed kind of too quick and
dirty for my mood. It will work though, but my method is kind
of hard to describe.
I
drilled four holes in the bottom of the inner hoop, and placed
my starting end of wire into the farthest hole. Then, when I
got to six turns (so long as you keep good tension on the wire
as you wind it, it won't fall off the hoop. You could try to
use tape or something to hold it, but it was pretty easy for
me without it. Just keep it taut!), I cut the wire and inserted
that end into the next hole. Pulling it through the hoop, I
anchored that end to a small eyehook for temporary tightness.
Then
I started another end of wire, following the same direction
as the first set of 6 windings, and anchored the end to keep
it tight, and finished the last 6 turns. Then, put the end of
the winding into the last of the four holes I had drilled, and
anchored that too. When that was done, I had two sets of 6 turns
each, each set wound without spacing, but the two sets spaced
from each other about an 1/8" apart to accomodate the screws
for swiveling.
It's
a lot easier to see it and do it than to describe it. See the
images. (They're homemade MSPaint images, sorry if they're not
clear enough).
Anyway,
the two anchored wires from the center of the hoop (where the
space is made for the screws) are soldered to each other, keeping
them tight. Now you've got a continuous loop. The other ends
of the loop go to a 365pf variable capacitor which rests on
the inner hoop, inside the hoop. Again, see image to get the
concept.
Now, the loop will rotate side to side AND top to bottom.
The next
thing was to get it to lead out to my receiver (R75) which does not
have a ferrite antenna built in. I spent some time on this question,
then as a quick experiment I took a small ferrite rod, 3" I think,
and wrapped it with about 20 turns of #18 insulated wire. The two
ends of this winding I soldered to a length of small coax cable (RG174),
one end to the center conductor of the coax, the other end to the
shield/braid. The other end of the coax hooks into the + (center conductor)
and -- (shield) terminals of the HiZ antenna input on the R75. I left
enough cable to allow free movement and rotation of the loop, no less
than a foot of cable. Too much cable might de-tune or upset the balance
of the loop, but experts may be able to answer this question.
At any
rate, the ferrite rod bundle is placed on the inner hoop next to the
365pf cap, facing perpendicular to the plane of the loop (just as
you would place a portable radio in relation to a loop)... the ferrite
rod is attached with some double-sided foam tape, but a more permanent
mount can be created. This is, after all, a prototype! :-)
In order
to get good balance, I had to mess with how wide I had the outer hoop
opened up (recall, this is an embroidery hoop, you know the type,
you have to open it up to place the fabric into it, and tighten it
again to secure the work) and where the holes were drilled for the
swiveling screws. I also had to add some counter weight to the top
of the inner hoop to balance the weight of the 365 cap and ferrite
bundle on the bottom of the inner hoop. I used some very sticky double
sided foam tape, and attached a spent AA battery. Tried the swiveling,
but still bottom-heavy. Then I added another battery and so on until
I had five AA batteries up there, and now when I angle the inner hoop,
it stays put it any position! Lucky me.
The obvious
thing to do now is to find a decent weight that looks nicer and is
a little more accurately weighted for really good balance for angling
and staying put. Also, I want to add a varactor-tuned system, to allow
me to remove the small 365 air-core variable AND the ferrite pick-up
loop, leaving one wire (probably RG174) coming from the inner hoop,
to a small control box (tuning knob, circuit and battery!) at my fingertips.
This would decrease the bottom weight of the inner hoop, and demand
less counterweight accordingly. The coax to the receiver would come
from the control box, not the loop, so no pick-up loop or ferrite
is required, and therefore less stress and drag on the loop.
Also,
it seems much better in null balance than any other loop I've used.
Perhaps it's the two sets of windings rather than a single, spaced
set of turns. All I know is, at night time, I can find the same dB
of nulling at 180 degrees of either rotation. It's not skewed at all!
No, it's no Kiwa, has no Q-multiplication or amplifier, or heavy base
for stability like the Kiwa, but the electronics can be added to the
control box if I wanted to do so. It's light enough to be placed on
a simple wooden base, and rested atop a plastic lazy susan, without
stressing anything at all. It's very smooth in rotation and angling.
I'll probably give the whole thing a good paint job when I'm done,
to look a bit nicer (what are we if not civilized?) but even as it
is, it's REALLY working well.
Nulls
are deeper, sharper, than any other homebrew loop I have used, and
the size of the loop FAR outperforms the S-A-T and the Terk loops,
and in terms of nulling AND strong-signal handling, works better than
the 4' box loop I used to think was my best loop (it was until now!)...
RESULTS
ARE QUITE IMPRESSIVE Between
10:30 pm and 1 am, I was able to null two very strong locals, to complete
silence, enabling me to copy two stations I had not been able to hear
before. Ever.
Also, one of my age-old DX targets has been KNX1070, Los Angeles.
I used to listen to them all the time when I lived there, but have
only been able to hear them in SW Missouri once or twice over the
last decade, even with an MFJ phasing unit and two very long wires.
This loop was able to get rid of offending sports and gospel stations
(well, 'offending' isn't really the word, I like sports and gospel
music... I should say 'unwanted' stations!) on the same frequency,
and while there was still some noise, I logged KNX1070 for the first
time in years! All attempts at achieving the same level of clarity
with the phased antennas could not compare to the loop. In fact, I
could not hear the signal on either of the longwire antennas, and
phasing only resulted in very weak copy with a lot of hash and two
hets whining away in the background. In average conditions, this loop
heard something that no other antenna *I* use could hear. Very impressive
for me!
Daytime
use has proven to me how deep and lasting the nulls this loop can
acheive really are. Across the band, I nulled into oblivion all major
stations in my area (560, 940, 990, 1060, 1220, 1260, 1510 and 1610,
three of which transmit at full power less than ten miles from my
location). It also picks up a number of daytime broadcasters within
a five hundred mile range in a clean, noiseless manner which is superior
to my longwires, phased or not, during the day. The signals are there
on the phased array, and louder than the loop, but far noisier and
when properly nulled are still much noisier than the loop gives me.
In short, the Altazimuth Hoop-Loop project has been WELL worth the
effort, and only cost $3 since I had everything else in the junk boxes.
If I had to get it all new, it might be a $20 project (the wire, about
70 feet of #22, the tuning cap is about $10 from the Xtal Set Society,
small and smooth-tuning, and the hoop, well, if you can find one,
who knows? Cheap-cheap!)...
WHAT'S IN A NAME? I
thought about calling this loop the KIWOOD Loop, but thought Craig
S. at Kiwa wouldn't like that much. His loop is state-of-the-art,
of course. The Termite's Feast? Washington's Teeth Special? The Lumber-Loop?
Whatever
the name, this is a very worthy passive, altazimuth round loop, which
covers the whole MW band and affords the user with excellent nulling
and directionality. I like surprising myself! You may be surprised
too!
