Back in late December I posted some questions about loop
antennas. I got many interesting answers and opinions, and
later Tom W8JI posted some more information about loops on
Several asked me to report back on my experiences. For
the benefit of the archives, here is a summary of my
practical work to date.
The original intent of this project was to construct a
loop antenna that I could use for DFing a local QRN source
on 160m, using a battery-powered SWL receiver (Yaesu
FRG-100) as a detector.
The FRG-100 is a fairly light MF/HF receiver that runs
off 12 Vdc and has an illuminated S-meter (needle, not bar
graph), and a reasonably robust design. The S-meter needle
will be useful in detecting a null in signal strength while
DFing. While this may not be the best performing, nor most
compact, solution for a detector, it has the advantage that
I already own it ;-)
The ARRL RFI book and ARRL Antenna Book both suggested
that a loop for 160m not exceed about 5 ft in diameter.
From a practical standpoint, a 5 ft diameter loop is about
as bit as I can stick in the back of my pickup truck while
driving around the neighborhood taking DF and signal
I started out by building a "shielded loop": 2 turns of
RG-6. The shield was opened for about 2-3cm at the top of
the loop, directly opposite the feedpoint at the bottom. An
octagon of 3/4" plastic electrical conduit contained the
coax. The feedpoint was in a 4" x 4" x 2" electrical
utility box at the bottom. The feedline connects to the
utility box with an F-connector. The feedline center
conductor connects to the center conductor of the first RG6
loop. At the opposite end of that turn, a toggle switch
connects the center conductor to the shield of the feedline
(i.e., one turn loop) or to the center conductor of the 2nd
turn of coax (continuing in the same direction around). The
center conductor end of the second turn attached to the
feedline shield via the metal utility box.
All the parts came from Home Despot except the
F-connector and switch, which are available from Radio Schlock.
This is the core of the basic design in the books.
This simple design was tested with an Orion receiver on
160m for general performance. (I didn't use the FRG-100 at
this stage in order to eliminate one more variable from the
testing scenarios. I know how my QRN sounds on the Orion.)
The loop was pretty deaf... but, on local AM stations, the
loops showed very sharp nulls off the ends -- easily >40 dB.
The nulls were symmetric: rotating the loop 180Â produced
a null of the same depth. So the system seems well balanced.
But on 160m I could not hear the local QRN, which runs
S-7 to 8 on the NE beverage antenna. In fact, I couldn't
hear any real change in the noise floor when I rotated the
loop. To cut a long detective story short, I discovered
that the noise I was hearing on the Orion was actually
leakage from the unselected beverage antenna. The Orion
antenna switch matrix has about 40 dB of isolation (~7
S-units)... just enough to leak an S-8 noisy signal from one
port to another and make it appear to be just twitching the
S-meter above the receiver's noise floor - hi!
Lesson #1: disconnect ALL the other antennas from the
The next step was to find a variable capacitor to attach
from the feedline center conductor to the feedline shield.
A capacitor decade switchbox filled in for the task of
figuring out what level of capacitance was suitable. (The
books suggested 400 pF, but point out it varies with the
characteristics of the coax.) My switchbox is pretty
coarse, in 100 pF steps. But the signals on 1.8 MHz peaked
up with about 1400 pF using one turn. Using two turns, the
required capacitance was about 1/2 the one-turn value --
much in line with expectations set by reading W8JI's
website. (The second turn does not make the loop more
sensitive; it just changes the inductance to be resonated by
Signal strengths were still very low, however... not
strong enough for good DFing.
W8JI's website points out that the use of shielded turns
is really immaterial to the behavior of a loop. At the
bottom of the page Tom shows several unshielded designs. So
I decided to try a simpler design and make a comparison.
Design #2: a single turn of ordinary hookup wire (still
in my 5 ft diameter plastic octagon) with the resonating
This design has much better sensitivity. The single
turn resonates with 800 pF at 1.8 MHz, decreasing to 600 pF
at 2.0 MHz. The sensitivity to adjustment is fairly sharp;
a change of 100 pF on the switchbox will drop signals by
several S-units. My absolute signal strength measurements
may be lower than could be obtained by a continuously
variable capacitor. (I now have one on order from Antique
Attaching a DX Engineering pre-amp brings the local QRN
up to S-4 (compared to S-7/8 on the beverage). If I add the
Orion pre-amp, the signals come up another 2 S-units. At
this point we have a workable antenna.
Last night I did some general listening tests to compare
the ability of the loop to discern signals vs the beverages.
While this may not be so pertinent to DFing QRN, it could
be of interest to those looking for low band receiving
antennas in small spaces.
W1AW, a high angle signal here on the southern
Loop with DX Eng preamp: S-7
4-square: S9+30 dB
west beverage: S9+20 dB
south beverage: general noise floor at the time = S3.
Signal = S6.
loop with DX Eng pre-amp: general noise floor = S2.
Signal = S3/4.
European signals: In general the signal-to-noise ratio
was lower on the loop by a couple of S-units, compared to
the NE beverage (single wire about 420 ft long). But I was
pleasantly surprised at how often signals could be
discovered by tuning around the band while listening to the
loop. (And, in accordance with Lesson #1, all the other
antennas were disconnected from the radio during these
tests.) While conditions seemed generally good last night,
the performance of this simple single-turn loop is better
than what I would have expected for such a small antenna at
ground level. (But it wasn't hearing 6O0N well enough for
copy; he was reliably copyable on the NE beverage but just
above the local noise level.)
1. Check the loop for symmetry of pattern. I plan to
do this with some local AM stations, and to use an AEA
analyzer as a weak signal generator a few hundred feet away
2. Add a series capacitor (to match the impedance to
the feedline) and low-C isolation toroid balun, as suggested
on Tom's website. (See the last figure on his page on
loops.) As presently built, the loop is balanced but the
coax is not. I may also slip on a string of ferrite beads
to eliminate shield currents on the feedline if it appears
pattern symmetry is being disturbed. I don't expect either
of these steps to materially increase the signal to noise ratio.
I'll provide further results after these steps. I hope
this is helpful to others.
-- Eric K3NA
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