The Folding Loop Antenna

Folding loop antenna - deployed.

Folding loop antenna - folded.

In my study of AM Box Loop Antennas, I have quickly realized that no matter how good the reception improvement may be using a loop - the loop antenna itself is large and cumbersome. This limits its usefulness to home installations. The large loops do not lend themselves to portability. How can that be changed?

As a teenager in Midland, TX, I brought the best radios available to the pool where I spent many a lazy summer afternoon. No matter what I tried, good radio stations such as KLIF Dallas was just too weak. Even WFAA, during its short career as a top 40 station, was so weak that I had to turn the volume control all the way up just to hear the signal. I wish I had known about loop antennas at the time, but more than that - I wish I had invented a "folding loop", one that would be easy to carry in, quick to set up, yet not compromise the performance of a large loop in any way.

Now, 25 years later, I am faced with a similar situation. My daughter loves to listen to "Radio Disney" - KMKI, a 5000 Watt AM station on 620 kHz from the Dallas area. She wants to hear it when she visits grandad in Lubbock, TX 330 miles away. And she wants to hear it at a pool in the neighborhood where he lives. To her, all that matters is the music. It must be clear. What we carry to the pool must be a convenient size, light weight, easy to set up, and quick to adjust. It must work the first time, there is no time for tinkering - and it must be reliable. A perfect application for a "folding loop".

I know that one company makes a "pocket loop" - no doubt taking advantage of the spring characteristics of their loop antenna structure to allow it to be twisted into a smaller shape - similar to some automotive shades. Good idea, but I bet the things get tangled after a while - like a slinky. Their unit costs about $150, making it a lot more serious problem once it does get tangled!

So - with twisting coils not an option, what else could I try? Some 27 inch shelves discarded from a home entertainment system provided the answer.

To construct the folding loop, it is necessary to procure the following material:

• Five 27 inch lengths of 1 by 8 lumber (the actual size of 1 by 8 lumber is 11/16 by 5 1/2 inches).
• Ten small hinges and their mounting screws. I found some brass furniture hinges at Home Depot that were perfect. They must be small to avoid interfering with the loop tuning.
• Four small 5 1/2 inch long blocks of wood. 1 by 1 lumber scraps are perfect.
• Eight 1 inch wood screws.
• 150 feet of #20 guage solid wire. Probably the cheapest source is thermostat or doorbell wire, which comes as insulated twisted pair. Simply measure the length of both of the wires, and then take one conductor of the two for the loop. The other conductor can then be used as the secondary wire, or for other purposes. Another source of wire is discarded telephone or network cable – simply splice sections together to make the desired length. These types of wire, however, will give poor results – particularly at the high end of the band. Multi-stranded antenna wire is better.
• A 9.6 to 365 pF tuning capacitor – the large set of plates in a tuning capacitor salvaged from a junk radio.

Click on the pictures to see them full size ---

• Connect two of the pieces of 1 by 8 lumber together at the edge with two hinges.
  
  
  
  
• Continue to connect pieces of 1 by 8 lumber with hinges at the edges until 4 pieces have been joined.
• Connect the fifth piece of 1 by 8 lumber to the side of the first piece with two hinges.
  
  
  
  
• Attach the tuning capacitor to the exposed edge of the fifth piece of lumber.
  
  
  
  
• Fold the 4 pieces into a square shape, and place two additional hinges to connect the end pieces together and complete the frame.
  
  
  
  
• Fold the tuning capacitor piece of lumber in towards the center of the loop, making sure the edge is inside the top and bottom pieces of the frame. This will support the frame in a square shape - the shape used by the loop when it is in use. When not in use, the tuning capacitor piece is folded back, which allows the loop to collapse for storage.
  
  
  
  
• With the loop in its "in use" shape, as a square frame - wind thirteen turns of wire on the frame. The exact spacing is not terribly important as it has little impact on the inductance of the loop, but the windings should be spread out equidistant from each other.
  
  
  
  
• Prepare 4 pieces of 1 by 1 wood, cut to 5 1/2 inches long. With a saw blame, notch each piece 13 times. A good way to measure the location of these notches is to put it against the turns on the loop, and marking with a pencil. Drill 2 holes on either side of winding notches.
  
  
  
  
• Screw the pieces of wood over the windings near each hinge joint, making sure the wires slide freely through the notches. It is not necessary to have a piece of wood on each side of the joint, but it would not hurt anything. The purpose of these pieces of wood is to support the wire when the loop is collapsed. When the loop is collapsed, the wire goes slack.
  
  
  
  
• Connect one end of the winding to the ground terminal of the tuning capacitor, the other end of the winding to the other terminal of the tuning capacitor.
  
  
• A single turn "sense" winding can be added to the outside of the loop, if an additional notch is added for it.

The end result is a odd looking contraption when folded, 54 inches long, about 11 inches wide half of its length, and 5 1/2 inches wide for the other half. It has droopy wire hanging on it, and has a tuning capacitor sticking out. It is long enough to be slightly inconvenient, and heavy enough to be slightly annoying - but the weight can be reduced by changing the thickness of the wood, 1/4 inch plywood, perhaps.

You are sure to get a few stares walking into a public area such as a pool with this contraption, but as long as you re-assure anxious lifeguards that you have no intention of taking it into the water as a "surf board", you should be fine!

If the receiver has a ferrite bar antenna, there are two methods of coupling. One is to wrap a few turns of wire around the ferrite bar at the end opposite the existing windings, and couple those windings to the coax. The other is to simply get the radio close to the loop.