My Magnetic Loop Antenna
By Alexander Krist, KR1ST
INTRODUCTION
I
built this antenna when I lived in an apartment complex that
does not allow antennas and where I was without any real estate
to facilitate one.
Recently I also built a receive-only magnetic loop, just for short wave
listening. This loop covers a frequency range of about 5 MHz to 22 MHz
and is built from readily available parts. You can find my How-To
article on this loop on “My Magnetic Loop For Short-Wave Listening”
(SWL) now in Archive VIII, Article #52. That article includes a sound
sample showing the dramatic difference between the magnetic loop
and the built in whip on a portable short wave receiver.
The magnetic loop is small, reasonably efficient even close to the
ground, has a low take off angle and it has a deep null on each side
which lets you null out interference. The antenna also displays
directivity characteristics similar to that of a dipole when mounted
vertically. The only disadvantage I've experienced so far is that you
have to retune quite often because of it's narrow bandwidth, especially
on the lower bands. On the other hand it also means that the antenna
acts somewhat as a pre-selector that might just prevent your receiver
front end from overloading because of strong (broadcast) stations.
I built mine from 10 feet (3 meters) of 1/2 inch (12.5mm) coiled
copper pipe that I purchased at a local hardware store. The support
structure is made of 3/4-inch schedule 40 PVC pipe. The base is made
from a new (hey, they're only $10 these days) torchiere light. When
you get one make sure the 3/4-inch support pipe and a 1/2 inch
threaded PVC cap will fit inside the tube of the torchiere. The reason
for this is that a PVC cap makes an excellent bearing. Simply drop the
cap in the tube before you put the antenna with the 3/4-inch PVS
support pipe in the base. You'll see that the antenna rotates very
smoothly this way.
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The capacitor is an old Hammarlund split stator capacitor that I picked
up at a hamfest. It has a range of 6-32pF. Since the two sections are
put in series, the range is actually 3-16pF. This gives the loop a tuning
range from 15 through 10 meters. To use the loop also on 17 and 20
meters I made two fixed capacitors from RG58 cable. These capacitors
can be put in parallel with the tuning capacitor with an alligator clip
whenever I want to work on 17 or 20 meters. If you decide to use the
same technique make sure that the braid is at least one-half inch
removed at the end of the short pieces of coax cable. If you don't do
this then it will arc at the coax ends whenever you key up at, say 100
Watts.
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Later, I added a "tuning stick". This makes tuning a bit smoother. I
always first tune the antenna for maximum noise on the receiver. The
problem was that with my hand so close to the tuning capacitor, I
detuned the antenna too much which meant that I had to retune quite
often. Now with the Tuning Stick I can get very close to 1:1 SWR when
I only tune for maximum noise.
The construction is very simple as you can see below. I drilled a hole
in the tuning knob and put a bamboo skewer (grocery store) in the
hole with some epoxy.
antenneX Issue No. 111 – July 2006 – All Rights Reserved Page 3
I'm a QRP operator and never use more than 5 Watts (SSB). With the
antenna in the living room and at this power level I've been able to
work Europe, Asiatic Russia, the Caribbean, South America, North
America, and West Africa. I have been able to work over 80 countries
with only 5 Watt SSB and this antenna. –30-
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