Low
power 20-meter
Magnetic Loop Antenna
Design:
There is nothing original about this 20
meter magnetic loop antenna project. I used the calculator at http://www.66pacific.com/calculators/small_tx_loop_calc.aspx
to compute dimensions. According to this convenient on-line
calculator, 1-inch copper pipe is slightly more efficient than
3/4 inch. However, the latter is easier to work with, and 3/4 inch
fittings are more commonly available than larger diameter ones.
Acquiring
Supplies: For
this type of project, it is generally possible to use materials that
are either on hand or readily available from a nearby hardware
supply. There is no exact parts list. Copper pipe can be purchased from Home
Depot or Lowes
or any
plumbing
supply source. That is the
easy part. Of course, constructing an antenna is not
the same as plumbing. It is not advisable to explain the project to the
store assistant who offers to help with locating bits and pieces.
The photo (left) shows cut pieces for the main loop. Each segment was 23-1/2 inches,
making the loop roughly 16 feet in circumference. The exact
length is not critical. Some cut pieces in the photo
already have
45-degree elbows attached. To me they are
135-degree elbows, but in plumbing they are called 45-degrees.
That goes along with some other anti-intuitive things about
plumbing that I’ll skip for now.
Putting
it together: The first step was to layout
and join all the
pieces on the garage floor, which is a reasonably flat surface,
although there
was barely room between the grill and the sawhorse to lay the thing
out. My original plan was to solder most of the joints, leaving a
couple of them unsoldered for easy disassembly. I had envisioned the
possibility of using this antenna in portable operation.
Disillusionment did not strike home until later.
Before soldering elbows I fine-sanded
the pipe ends. Low resistance connections are particularly important in magnetic loop antennas. Each piece
of pipe was then secured in a vice (but not overtightened) for soldering the elbow. It is a very bad idea to touch
the pipe shortly after soldering it. Soldering or ‘sweating’ copper joints
requires a blow torch. It isn’t the same as soldering a resistor or
capacitor to a circuit board, but not difficult to do. Many Internet
pages explain or demonstrate how to do this. After soldering one elbow
on each piece, and waiting for it to cool down, I reattached the piece
to the big loop to make sure it was still flat. In retrospect it
couldn’t have been anything but flat with
just one end of each joint soldered. Concern for flatness comes into play when two or
more pieces are soldered together.
The
small loop: The driven element in a magnetic loop
antenna is a smaller loop about 1/5 the circumference of the main loop.
I
used 1/4 inch copper tubing because there was some left over from a
previous project that also had nothing to do with plumbing. The photo shows the small loop after
it was
secured to the mounting frame, a six foot length of 1 x 4. Ring
terminals that fit snugly over the 1/4 inch tubing were soldered to the
ends, and connected to an SO-239 socket.
Tuning:
This was to be a QRP antenna, handling
no more than about 15 watts, so I purchased a butterfly variable
capacitor
MFJ-23 that has a high enough
voltage rating for the purpose. In the photo the capacitor
is shown bottom side up (mounting tabs at the ends). This one component substantially increases the
cost of the project, in fact more than doubles it!
Alternative
strategies for tuning are possible, though perhaps not as easy to
implement.
The butterfly capacitor is a bit clunky
and I was not able to scrounge an enclosure of
suitable size, so with the help of a friend who has a band saw, and
knows how to cut straight lines, I made a birdhouse-like enclosure of
Plexiglas.
Final
assembly and initial measurements: In the end all elbows
except one were soldered. The antenna can be broken into halves (approximately). Also
to facilitate breakdown a couple of connectors were inserted in the capacitor
hookup. The capacitor can be unplugged, as
shown in the photo. This photo (right) does not show a fixed capacitor
that was added later. For initial testing the wooden mounting frame was
clamped in the vice. I don’t have a good photo of the test setup,
as the background is cluttered.
As a first step the antenna was connected to a receiver
and tuned for maximum signal in the CW part of the 20-meter band
(around 14.06 MHz). Then it was disconnected from the receiver and
connected to an antenna analyzer and fine-tuning. These initial
tests revealed that tuning was terribly non-linear and too
sensitive at the frequency of interest (nearly open capacitor). This
made me think to reduce the maximum capacitance by adding a capacitor
in series with the butterfly (68 pF, 3 KV). With
this configuration tuning was less touchy, and the range of tunable frequencies
included both the 30-meter and 20-meter bands (roughly 10 to 15 MHz).
On
the Air testing: [December 2016] — My
wife N4EFS enjoys working digital modes and volunteered to
try the loop on the air. We live in an HOA-restricted
neighborhood. Our imitation
‘Charleston’ style house has an upstairs porch or balcony, facing
west. Actually it faces the neighbor’s house, but the direction is west.
Using my patented Harbor Freight antenna rotator (photo) it is
possible, of course, to aim the loop in any direction.
N4EFS’s first JT-65 contact on the loop was
with Canada’s Yukon Territory (mid-afternoon
Eastern US time). That contact was followed by several others ranging
from the east to west coasts of Canada and the US. She
couldn’t get
Europe or South America, however, even after turning the loop in what
we thought to be appropriate directions. Her subjective impression was that the
antenna received as well or better than the vertical (less noise), but did not
transmit as well.
The next morning she worked Australia
on the loop, again at 15 watts, but this time using JT-9.
This result was satisfying, although we do seem to have a
pipeline to Australia on both 20 and 40 meters with the vertical
antenna as well. Soon
after completing this contact she thought of a more objective test,
namely to check PSK Reporter after test
transmissions using each antenna. This test was informative. The spotter detected the loop all over North America and
displayed one
spot in England. At the same power, frequency and time of day
(mid-morning) the vertical was spotted in several European countries
and
South America as well as North America.
Miscellaneous: The junk box contained a plastic rod of the same diameter as
the butterfly capacitor’s tuning shaft so it was possible to extend the
shaft by joining the two plastic pieces with heat shrink tubing. I also added a Vernier gear-reduction knob to
reduce the sensitivity of tuning.
Far as I know nothing about the project
is critical. The web calculator can be used in a trial-and-error
fashion to match available materials to practical dimensions for the
frequency of interest. The loop has no right-side up or upside-down
orientation, if Google images can be believed. However, I know very little about antenna design—it is not my field. If in
doubt, trust the ARRL Antenna Book or an equivalent
authority!
Project descriptions on this page are intended for entertainment only.
The author makes no claim as to the accuracy or completeness of the
information presented. In no event will the author be liable for any
damages, lost effort, inability to carry out a similar project, or to
reproduce a claimed result, or anything else relating to a decision to
use the information on this page.
