By Werner Funkenhauser and Pete Haas

This 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.)  

• Drill a hole in the center of the frame's stiff cardboard to mount a 365 pF variable capacitor (see below for sources).
• Drill 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.
• Thread #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.
• Secure 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.)

• Trying 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.
• First 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).

Orientation: 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.

Positioning: 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 a snap.

Variable 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 from 
http://www.midnightscience.com/ and 
http://www.tubesandmore.com/13450.html

Band coverage: 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.  

3.27.1998 A variation and some notes

Here is a slightly modified S-A-T work-alike by KC2CAU.

There is 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.
Photographs showing the loop among Joseph's other monitoring and ham equipment.

Generally, 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 flat surface.

Tonight, 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.

I've got 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.

A Commercial Source for Variable Capacitors

Daniel Grunberg re-posted the article and also the information below in rec.radio.shortwave. In his post, Daniel wrote:

I've 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.

Mounting screws for 24TR218, Mouser Stock No. 48SS003, US $0.04/ea.,  two screws required

Now here's 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.

Please 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 
http://www.nyx.net/~dgrunber/

Mouser's URL is: 
http://www.mouser.com/
They list variable capacitors on the bottom of Page 224 in Catalog #592 (November 1997).

1998.04.01 UPDATE!
PURCHASING MANUAL 593 (good to 1998.04.30) LISTS THEM AS Part # 24TR222 @1.52
MOUNTING SCREWS  Part # 48SS003 @ .04 (2 required).

GOOD DEAL!

Here's how to get a shaft on these little things.  

Subject: Re: Variable caps
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.
Dave

The easiest 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.

I've used 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. 8-(

In either 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.
Regards,
Werner Funkenhauser


(This article first appeared in Monitoring Times, reprinted and modified by Werner Funkenhausen..)
More Funkenhausen on loop antennas at his
WHAMLOG web site.