Magnetic Loop Antenna
There is nothing original about my 20
meter magnetic loop antenna. 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, but the latter is easier to work with, and 3/4 inch fittings
seem to be more commonly available than larger ones.
Supplies: Copper pipe can be obtained at Home
Depot or Lowes
or nearly any hardware or plumbing
supply. That is the
easy part. Bear in mind that constructing an antenna is not
the same thing as plumbing. Do not even think
about explaining your project to the store assistant who asks if you
help. The photo (left) shows cut pieces for the main loop.
My recollection is that each segment was 23-1/2 inches,
making the loop roughly 16 feet in circumference. The exact
length is not critical. Some of the cut pieces in the photo
45-degree elbows attached. That is what they are called. 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. I am hesitant to suggest a
particular parts list for this project, as the prospective builder
would want to use whatever is on hand or readily available from the
neighborhood hardware supply. In any case, no such thing as
the exact desired part exists.
it together: The first thing I did was to layout
and join all the
pieces on the garage floor, which is a reasonably flat surface,
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. Each piece
of pipe was then secured in a vice while I soldered on the elbow. Don’t touch
the pipe 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. I suppose it
couldn’t have been anything but flat with
just one end soldered. Concern for flatness comes into play when two or
more pieces are soldered together.
small loop: The driven element in a magnetic loop
antenna is a smaller loop about 1/5 the circumference of the main loop.
used 1/4 inch copper tubing because I had some left over from a
previous project that also had nothing to do with plumbing. The photo shows the small loop after
secured to the mounting frame, a six foot length of 1 x 4.
terminals that fit snugly over the 1/4 inch tubing were soldered to the
ends, and connected to an SO-239 socket.
This was to be a QRP antenna, handling
no more than about 15 watts, so I purchased a butterfly variable
MFJ-23 that had a high enough
voltage rating for the purpose. In the photo the capacitor
is shown bottom side up (mounting tabs at the ends). The attentive
reader might have surmised that this one component substantially increases the
cost of the project, in fact more than doubling it!
strategies for tuning are possible, though perhaps not as easy to
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
assembly and initial measurements: In the end I soldered all elbows
except one. The antenna can be broken into halves (approximately). Also
to facilitate breakdown I used a couple of connectors for the capacitor
hookup. The capacitor can be unplugged, as
shown in the photo (right—this photo does not show a fixed capacitor
that was added later). For initial testing the assembly was
clamped in the vice. I don’t have a good photo of this setup,
as the background is cluttered. As a first step I hooked up a receiver
and tuned for maximum signal in the CW part of the 20-meter band
(around 14.06 MHz). Then I
connected the antenna analyzer and fine-tuned it. These preliminary
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).
the Air testing: My wife Becky (N4EFS) likes to work
digital modes and wanted to try out the loop. We live in an imitation
‘Charleston’ style house in an HOA-restricted neighborhood.
Our 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.
Becky’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
Europe or South America, however, even after turning the loop in what
we thought to be appropriate directions. Her subjective assessment was that the
antenna received as well or better than the vertical, but did not
transmit as well.
The next morning Becky 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 most informative.
The spotter detected the loop all over North America and
spot in England. At the same power, frequency and time of day
(mid-morning) the vertical was spotted in several European countries
South America as well as North America.
My junk box (one of them) contained a plastic rod of the same diameter as
the butterfly capacitor’s tuning shaft so I extended the
shaft, joining the two plastic pieces with heat shrink tubing. I also
had a Vernier knob from a previous project, and used that to
further 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 don’t know anything about antenna design—it is not my field. If in
doubt, trust the ARRL Antenna Book or an equivalent
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