More on the Shrunken Quad

Outline of Shrunken Quad Complete construction plans for the Shrunken Quad by KØOV are in "TRANSMITTER HUNTING---Radio Direction Finding Simplified" pages 173 to 176. Volume (airspace) of this antenna on the two meter band (146 MHz) is only one eighth that of a full-sized four element quad, making it ideal for "sniffing" out hidden transmitters on foot. Unlike a yagi, it has no sharp points, so it is safer for the user.

Q: Does the Shrunken Quad really work? I can't get mine to tune up.

A: I have had excellent results with the Shrunken Quad and many other hams have too. I use my 6-meter version on every mobile hunt on that band. The 2-meter version is still a favorite on-foot "sniffing" antenna.

A high-Q antenna like this can be tricky to tune up. Carefully follow the steps in the book.

The biggest pitfall for most builders is the choice of capacitors. Ordinary air variable and ceramic trimmers are unsuitable because they go from minimum to maximum capacitance in just one half turn. This is far too coarse for this high-Q adjustment. You need a multi-turn trimmer, usually called a "piston" capacitor because the rotor screws in and out of the cylindrical stator like a piston.

However, not just any old piston trimmer will do. The easiest ones to find are the kind that look like an open ceramic tube about a half inch long with plating on inside and outside. A screw with rather coarse thread goes in and out. Builders have discovered that these don't work either, probably because the adjustment is still too coarse.

The correct ones to use are about 1/4 inch in diameter and 1/2 inch long, completely enclosed in ceramic and brass. They have a constant-torque drive with very fine thread. It takes 11 turns of the tuning tool to cover the the full capacitance range on the ones I use. The piston and bellows are internal and the whole thing is sealed, so moisture/water won't upset the tuning.

Photo of piston capacitors

Q: Where can I get the right piston trimmer capacitors?

A: The ones in the photo are made by Johanson Manufacturing Corporation of Boonton, NJ.

Suitable capacitors are also made by Sprague Goodman of Westbury, NY. You can buy them by mail-order from Mouser Electronics of Mansfield, TX. Mouser has no minimum order requirement.

Surplus piston trimmers are available from Bogdan Electronic Research & Development (Jim & Carolyn), PO Box 62, Lakewood, CA 90712. They sell them at southern California swap meets and also by mail.

Another source is Dan's Small Parts and Kits, Box 3634, Missoula, MT 59806-3634 (406) 258 2782. Be sure to select the Johanson trimmer, not one of the long tubular ones from other manufacturers.

Q: I don't understand the balun. Do I need it?

A: Yes. The balun on the Shrunken Quad is of the "bazooka" or "sleeve" type, which consists of an extra length of braid, an electrical quarter wavelength long, placed over the antenna end of the RG-58 coax feedline. The sleeve is connected to the RG-58 shield only at the end closest to the receiver input. The antenna end of the sleeve is left open.

Here's how to build the balun for two meters: Make a quarter-inch cut in the feedline jacket 12.9 inches from the antenna end, exposing the braid there. (Don't remove the entire foot of jacket---just a little quarter-inch circle of it.) Now take some braid from old coax, smooth it down over the outside jacket on that last 12.9 inches and connect it to the shield at the point where you cut the jacket. Leave the other end open. Put tape or shrink sleeving over the added braid to prevent shorts.

For more information on sleeve baluns, see the "Linear Baluns" section in the "Transmission Lines" chapter of a recent ARRL Handbook. Note that classic bazooka designs require the sleeve to be spaced away from the coax shield to form a sort of special transmission line. That would be the optimum way to make such a balun for precision work, but the method in the book is entirely adequate for close-in sniffing with a two-element antenna.

Detail of bazooka balun

Q: How critical are the dimensions?

A: Don't worry about getting element lengths precisely correct. A quarter or half inch difference probably won't matter. This is not like a conventional quad, where element resonant frequency is determined almost entirely by circumference. In the Shrunken Quad, resonance is determined by both the element circumference and the value of the loading capacitance. There is enough range in the trimmer capacitors to make up for minor variations in element circumference. In fact, there is so much range that some builders have accidentally mistuned their quads such that the reflector became a director, so the quad worked backwards! When they re-tweaked the trimmers, the quad worked normally.

For frequencies outside the two-meter ham radio band, scale the element circumference and spacing dimensions given in the book by the ratio of the frequencies. For instance, for the 223 MHz band, the dimensions would be 146/223 or two-thirds of the two-meter dimensions. Scale the bazooka balun length also.

Ruggedized 2m Shrunken Quad

I rebuilt my original Shrunken quad using 1/4-inch diameter fiberglass spreaders (covered with shrink sleeving) and 3/32-inch bronze welding rod elements. It is now extremely rugged and has withstood lots of tramping in the brush.

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This page updated 28 January 2004