Coupled Loop Antenna
Funkenhauser and Pete Haas
inductively coupled booster requires no power, and has no amplifier
to introduce noise of its own. It is easy to construct from common
material. This article will assume the use of an 11" x 14" plastic
picture frame, however masonite of the same dimensions with a
nail driven in each corner could be used as a less expensive substitute.
(Or glue a frame of half-inch pine with a piece of thin
wood panel of appropriate size glued to it.)
Put a label
around the capacitor shaft; attach a pointer knob to make approximate
calibration markings. (The last step in construction is to glue
a flat piece of thin panel to the frame as a foot, so that the antenna
can be stood upright.)
a hole in the center of the frame's stiff cardboard to mount
a 365 pF variable capacitor (see below for sources).
two angled 1/16 inch holes in one corner of the frame, one on
either side. This is where the antenna coil begins and ends.
#20 wire through one hole, leave enough to attach to the capacitor
- Wind 17
turns of the same wire on the plastic frame--you don't have
to be particularly neat here. If you have an inductance bridge,
you're shooting for 225 microhenries.
the winding with electrical tape on each side and solder the
ends of the wire to the tuning capacitor. (Solder one end
of the wire to the rotor and the other to the stator.)
How much signal increase can you expect? I made tests on the AM
section of a hi-fi receiver with a movable antenna coil on the back
of the set. Here in the Akron (Ohio) area I tuned in a weak Toronto
signal. With the booster, it jumped several S-units. The improvement
is most dramatic on weaker radio signals. Give it a try--it's small
compared to a longwire antenna, easy to build, and you'll be amazed
at how much signal increase is possible.
it Out: The booster was designed to work with AM radios that
have an internal ferrite bar antenna although it would work
as well with a tube type set that has an air coil of Litz wire
at the back of the set. The windings of the radio's internal
antenna coil and the booster's coil should parallel each other
for maximum signal transfer.
tune in a weak signal on your radio, and rotate the radio for
best reception. Then bring the booster 4" to 6" away from the
radio (you may need to experiment--see below-wf).
In practice, I have found that locating the antenna with respect
to the internal loop at other angles sometimes helps minimize
QRM. So, be prepared to experiment a bit, and be prepared to locate
the loop closer than Pete's 4-6 inches.
Looking at the radio/loop layout from above, it should look like
an upside down T with the radio as the crossarm and the loop pointing
away from the radio at a right angle. For best signal the radio
should be broadside to the received station and the loop's
long side should be pointing towards the station. To make it easire
to orient the radio/loop toward the maximum signal (or away from
QRM) buy or make a lazy susan turntable large enough to mount
the equipment as described above. The lazy susan makes aiming
Capacitors: A miniature mica variable capacitor would be the
kind to use in this project. They are difficult to find today
but Radio Shack sells a crystal radio kit for about $7.00. One
of the components is a 365 pF variable capacitor. Or, salvage
the dual section variable capacitor from an All-American-Five
AC/DC radio. Solder the two tabs of the two insulated sections
(stators) together and the resulting parallel circuit will give
approximately 410 pf (270 from the RF section and 140 from the
oscillator section). Daniel Grunberg
has found a source for new variable capacitors. See his article
at the bottom of this page. 365 pf capacitors are also available
If you find that your new antenna doesn't quite tune to the
top of the band, remove one turn of wire and try again until it
does. A simpler solution if you're using a dual section capacitor,
is to connect the dual sections together as described above, but
through an SPST switch. With only one section switched in you
can tune the upper part of the band and include the Extended Band,
and even above, with both sections switched in, you can tune the
lower portion of the band. Click here to view a GIF
image which describes this concept.
A variation and some notes
is a slightly modified S-A-T work-alike by KC2CAU.
an excellent two-part article on AM DXing, written by
Joseph Cooper VE3FMQ,
which appeared in the March and April '98 editions of Popular Communications.
Joseph's article presents a thorough approach to AM DXing.
A very useful resource to both the veteran and the beginning AM
DXers, it covers DX techniques, receiver selection, AM DX resources,
among many other topics. In the article, he describes the construction
of an efficient, inductively coupled receiving loop for AM, and
how best to use it. Also included are instructions on how
to build a Lazy Susan stand, to permit both radio and antenna
to be aimed in precisely the most optimum direction for best signal
capture or maximum null of QRM.
showing the loop among Joseph's other monitoring and ham equipment.
I have found that an inductive loop doesn't greatly enhances
my SR II's already good sensitivity, but it does help in some
instances. However, this type of antenna really shines with receivers
that have poor AM performance because of miniscule ferrite bar
antennas that are used in otherwise very good receivers. For highest
signal transfer, the loop's parallel wires should be in the same
direction as the winding of the radio's ferrite loop. This usually
dictates a placement of the loop and the radio at 90 degrees
to each other. In most radios, the ferrite loop is placed along
a line running the length of the radio. Best signal transfer between
the loop and the radio's ferrite loop may require you to experiment
with finding that "sweet spot". Also, experimenting with
variations in angle between the the loop and the radio's internal
ferrite bar may sometimes help in nulling QRM even after optimum
placement of the loop against the radio.
I have built
a few of the picture frame antennas. At the moment, I'm using
a variation of the same antenna, a 10" by 16" frame made from
3" strips of sub-floor, glued. The loop is 17 turns spaced at
1/8" intervals tuned with an old non-descript capacitor which
I suspect is around the right value <BG> -- it allows me
to tune the entire band, including a good part of the expanded
band. Constructed from this strip, the antenna sits well on a
I am able to get a so-so signal from WPHG-1620 with the SR II
barefoot, but by placing the loop up against the back of
the GE, WPGH is heard measurably stronger. The whole assembly
looks like an upside down T with the GE (the top of the T) aimed
broadside to WPHG. The loop forms the vertical part of the
upside-down T and is aimed right at WPHG. The entire affair is
mounted on a lazy susan for ease of rotation.
a 10 transistor promotion radio from the '70s which hardly pulls
in locals. The ferrite loop inside it is one of those cute "flat
bar" types only 3 inches long. However, with the loop I'm
able to hear WPHG pretty well with the same quality (poor) as
on the GE.
Commercial Source for Variable Capacitors
re-posted the article and also the information below in rec.radio.shortwave.
In his post, Daniel wrote:
found a source for the antenna's variable capacitor. Mouser
Electronics, (800) 346-6873, offers the following items in their
"Purchasing Manual 588":
Poly Film RF Tuner Capacitor, AM Tuner 2 Sections, Mouser Stock
No. 24TR218, US $2.98/ea., each section has tuning range of 5
to 266 pF.
screws for 24TR218, Mouser Stock No. 48SS003, US $0.04/ea.,
two screws required
the great part, "Purchasing Manual 588" says that Mouser has
no minimum order in USA, Canada, and Mexico. On prepaid orders,
"Purchasing Manual 588" says you total the order and add 10%
(US$2.00 minimum). Sounds to me like a capacitor and two screws
can be had, delivered, for $5.06.
be assured that I don't work for Mouser. I only know what I
read in the Purchasing Manual. <g>
Daniel A. Grunberg Kensington, Maryland USA
My home page's URL is
They list variable capacitors on the bottom of Page 224 in Catalog
#592 (November 1997).
PURCHASING MANUAL 593 (good to 1998.04.30) LISTS THEM AS Part
# 24TR222 @1.52
MOUNTING SCREWS Part # 48SS003 @ .04 (2 required).
to get a shaft on these little things.
Date: 29 Mar 1998
From: Werner Funkenhauser
To: David Moisan
David Moisan wrote:
In other words, when a fixed coil is used in circuit with a capacitor,
the resonant frequency rises as the capacitor value decreases.
In variable capacitors, when the plates are meshed (closed) you
have maximum capacitance, which decreases as the plates are unmeshed.
Looks like I'll want a grid-dip meter or noise bridge when the
MIT flea market starts up again this April. :)
Speaking of variable capacitors, how do people get knobs for the
plastic ones? I'm referring to the ones that have a stubby
shaft with flats and a a threaded hole for the screw the knob
is secured with. Those are a pain.
method I know is to trim 3/8 dowel to fit into the hole, then
cut to length.
Make sure things line up properly, fill the srew hole and dress
the trimmed end with epoxy. Jam tings in place.
Make sure everything is straight and let it set.
After things are really firm, slap more epoxy on the joint to
fill the gap. If you want to make it look neat, let the second
epoxy dope set a bit, then slip a short sleeve made of a fat drinking
straw over the joint, just far enough to cover the joint.
matchsticks, epoxied into the screw hole, then slipped a straw
sleeve of the required length, filled with epoxy over the matchstick.
Lost a couple that way, when the epoxy leaked down to the case.
case, it would be wise to make a tight-fitting washer of several
pieces of kitchen wax paper to snuggly slip over the capacitor's
shaft. Or you could probably find a rubber O ring to slip over
the shaft. This is just so the epoxy has no chance of bonding
things together down there. After the epoxy has set, cut these
with an Xcacto knife.
(This article first appeared in Monitoring
Times, reprinted and modified by Werner Funkenhausen..)
More Funkenhausen on loop antennas at his