The idea of using loop antennas to
minimize interference and to help dig weak stations from beneath
stronger ones on the same frequency may be old hat to many, but the
idea evolved for me. Like many other things in my younger days of
DX-ing, without a local mentor or access to a lot of printed
material, it was a learn as you go process.
The upside of all of that was while
learning, I also figured out how to get the effects of a loop without
spending a lot of money. Many today may look longingly at some of the
commercially built loops for sale or at the projects in publications
or on line and find the cost of the loops or the materials puts the
idea out of reach. However if one finds it acceptable to make a few
compromises that reduce performance only a little or provides a
product with something a little less polished in appearance, the cost
can be reduced to almost nothing.
My first inkling of the directional
affects of loops came when I was using one of my very earliest
receivers, my old trusty Watterson five-tube broadcast set. It had
the requisite loop on the back and I noticed that when it was placed
on the side of my desk to my left, some stations were not as strong
as when it was on the desk in front of me, or rotated 90 degrees.
At first this was an annoyance more
than anything, because it meant that to hear my favorite Chicago
station at night ( of course in the early sixties it was WLS!!) the
radio had to be to my left, making use of the desk for homework and
other things a little awkward. There was still no clue planted to
use the effect to find other stations. Then when I began using an
outside antenna connected to the radio, the effect was overridden and
more or less forgotten about.
It was when my younger sister got her
first portable transistor radio that the impact of a rotatable loop
began to be driven home. She received a high quality
eight-transistor radio for Christmas one year that was not only
highly sensitive, but selective as well. It had an RF stage and two
IF's and was small enough to be portable, but large enough to have a
five inch long ferrite loop in the top of its little cabinet.
Somehow I managed to get her to let me
use it to listen one night...probably with the promise of a couple
set of batteries since it went through them like water. Sitting at
the the desk, it could pick up stations that the Watterson without
the outside antenna could not separate. I was a little annoyed at
first that there was no way to connect my outside wire antenna to it
and the thought of opening it up to find a way to attach it to the
circuit was not an option with my sister watching very closely what I
was doing with her prized radio.
The effects of the internal loop on
the little radio were amazing to me. First of all, the little radio
was picking up by itself most of what I could hear on the Watterson
with the outside antenna connected. Rotating the radio did not
produce much of a peak in signals, but the nulls were amazingly deep
and I could almost make local stations disappear. It was easy to hear
KLOU form Lake Charles, Louisiana on 1580 khz despite the presence of
a local station, KBGO-AM on 1580. For the first time I found that I
could pick out stations on some of the crowded regional channels, in
some cases logging two stations and in one case three stations on the
same frequency.
I only had the little radio for an
hour or so before my sister demanded it back, but a lesson was
learned. At some point, a loop antenna would be an asset.
It was a few years later that the next
lesson was learned when I was a college student at the University of
Houston. My roommate had a Hammurland Super Pro receiver that he had
been working on but we didn't have an antenna to connect it to.
There was no way to string a wire outside the dorm as I had done
while at Texas A&M ( a whole 'nother story!!!) and he wanted to
listen to some broadcast DX. I remembered lessons in a radio-tv
correspondence course my dad had taken that described tuned loops,
but the details were a bit hazy. I remembered that they involved a
resonant tuned circuit for the loop and a link coil to provide a low
impedance connection to the receiver.
We had some insulated wire and
scrounged a broadcast band variable capacitor from an old radio and
went to work. But what to make the loop out of was the question. We
both spotted a yellow plastic trash can at the same time. Why not?
We started out with 15 turns of wire lengthwise over the top and
bottom of the trash can and attached the variable cap to make it a
parallel tuned circuit. The turns were spaced about a quarter inch
apart with the ends of the windings anchored by punching holes in the
plastic near the top of the can with an icepick. The coupling coil
was spaced over a couple inches and had two turns. We ran a short
piece of RG 58 coax we had from the link to the receiver.
The first try near the top of the band
resulted in signals, but not too strong. Rotating the capacitor did
not result in a peak. Obviously our wild guess on number of turns
was off. Checking at the bottom of the band resulted in a peak with
the capacitor almost completely open. Obviously too much inductance
was the problem. We started removing turns one at a time and when we
got down to eleven turns, we could get a peak at the top of the band
but not quite at the bottom. The capacitor was of the two gang
variety, so a clip lead to parallel the two gangs resulted in a
successful resonating at the bottom of the band, and the top could be
reached by unclipping the jumper.
The whole project took about thirty
minutes. A pretty good null could be obtained and the sensitivity
was pretty good. All the “ usual suspects” could be heard...the
Chicago stations WLS, WBBM. WGN, WMAQ all were in good...as was WSM
Nashville, WWL New Orleans, WLW and WCKY Cincinnati. As a bonus, it
appeared that a good bit of the flourescent light noise was reduced.
This was the beginning of my own love
affair with the improvised loop. While I have made some “ serious “
loops, including a wood framed three foot wide beauty my dad helped
me build years later, to loops inside plastic pipe in a round shape
with a neat tuning box at the bottom, there have been many times I
have put together impromptu tuned loops almost anywhere. Many times
while on engineering trips I have put one together in a hotel room to
do some DX-ing or to get rid of noise. Many times these were wound
around cardboard boxes of various sizes, a few times around trash
cans, and even once around a door ( opening and closing the door had
the effect of “rotating” the loop!!)
While some very fine, pretty and well
performing loops can be purchased if you want to spend as much as
$300, some pretty good performance can be had for next to nothing.
The size is not critical, though less than a foot across is about the
smallest I have gotten really good DX results with. I can't provide
you with exact numbers of turns for various sizes, that can be
determined experimentally, thought I would imagine there are formulas
that could be used to determine. However the time and effort to do
that are greater than just winding one and either adding or taking
off turns to get it to resonate.
These impromptu loops don't require
exotic materials other than a broadcast variable capacitor. I will
admit these are getting harder to come by in recent years, but
anyone playing with radios for any amount of time probably has an old
tube junker around that could be cannibalized. One could be ( Gasp!)
bought through a mail order parts house and kept for DX
“emergencies”. Some of these are probably good to have around
for various things in a well filled DX-ers junk box anyway. I
usually start with ten or eleven turns and just hunting for the
resonant spot. An old fashioned grid dip meter could be used to find
resonance very quickly. I have never tried one of the fancy new ham
antenna analyzers or a loop and am not sure what one of those would
really tell you anyway. Maybe someone who knows or who has tried it
might add a comment to this blog with their experience.
Impromptu loops are inexpensive and
can “grow” anywhere. They are particularly good when traveling
precludes carrying a lot of stuff on an airplane ( or doing so would
raise eyebrows in the security check!!!).
Coupling to the loop can be done
either with feedline or inductive coupling by placing a portable
receiver with an internal loop next to the larger loop's windings. I
actually did just that using the table top clock radio in hotel room
on a trip to the Bahamas once! The loop was wound on a plastic trash
can and set on top of a small table in the room and the loop and
radio moved around the table for rotation.
While the homemade impromptu loop
might not give quite the full performance of a commercially made
device or the more complex big ferrite core loops in some ways, its
practicality and availability for use outweigh those issues. After
all, the idea is “ DX With What You Have” and “ DX Anywhere,
Anytime”
By the way,. By adding more turns,
these loops do an excellent job on long wave frequencies and work
very well for NDB DX-ing, and with fewer turns do the same in the
160 meter amateur band and the lower tropical bands ( 120 and 90
meters) They don't seem to do much better than a random wire up
higher.
Have fun!
