Shrunken 2 Element VHF Quad
Photos are at the end
This antenna takes up 1/8th of the air space of a regular sized 4-element quad antenna. And weighs a fraction of a full sized quad, making it great for handheld use at close-in sniffing and International T-hunts.
16-18 ga. copper wire (Lowe’s picture hanging wire 25′ Coil)
RG-58U Coax (Desired length)
RG-8 Shield only (18 inches for making the sleeve balun)
(1)-BNC-Male connector (To radio/attenuator)
(2)-1/4″ Orange fiberglass rod (Lowe’s driveway reflectors sticks, $1.99)
(1)-7/8″ Poplar wooden dowel (Lowe’s 36″ long, $3.19)
PVC pipe can be used to make the boom and mast if you prefer.
(2)-Ceramic Piston Capacitor 1-10pf ( Manufacturers are Johannson, Sprague, Arco, surplus caps work just fine)
(1)-Threaded coupler (sized to attach antenna to support mast)
(15″)-3/8″ Heat shrink
E6000 Adhesive (White-cures harder than clear for mounting caps)
Gorilla Super Glue (clear type)
60 grit sand paper (Dollar store, 20- 4″x6″ assorted pack $1.59)
Black Fine-tip Sharpee marker
25-40 Watt soldering Iron/Station, rosin core solder
Drills-5/16″ and 1/16″
Drill Motor (cordless or electric)
Dremal tool with sanding wheel or round sanding drum 5/8″ dia.
(A fine tooth file can be used instead of a Dremal tool)
Needle nose or pliers
Construction steps and assembly info:
Antenna Spreaders- (Photo 5)
Measure the fiberglass rods, and cut two pieces 15-1/2″ long.
Measure the fiberglass rods, and cut two pieces 15-1/4″ long.
Antenna Boom- (Photo 6, 7, 8)
Measure and cut the wooden dowel to 18″ long.
Measure 1/2″ in from one end of the dowel and drill a 5/16″ whole through the dowel. (Photo 9)
Drill another 5/16″ hole at the opposite end of the dowel in-line with the first hole.
Rotate the dowel 90 degrees and drill two holes, just missing the first set of holes. Use care when doing this step.
A drill press isn’t absolutely necessary, but it does make it a lot easier to drill straight holes. A vise or clamp is needed to hold the dowel in place while drilling.
After drilling the spreader holes, slide two of the spreaders into the holes on one end.
It should look like an (+) if your holes are straight. If they are at a slight angle, remove the fiberglass rod and re-drill the same hole
moving the drill bit back and forth ( a little at a time) in the direction you need to go, for the fiberglass rods to get in alignment with the rods at the other end of the dowel. When looking down the boom you should see two sets of spreaders, forming a (+) lined up with each other. Don’t worry about the rod being a little loose at this point. We will deal with the slop in the holes later when we start glueing the dowels to the mast.
Make sure you have the same length spreaders on the same end of the boom.
Insert a spreader, and measure both sides of the fiberglass rod, moving it in or out from the boom, until the distance from the boom to the tip of the rod is the same.
Hold the rod in place and with the Sharpee, place a mark on the fiberglass rod next to the boom on both sides of the boom. Do this measuring and marking to all four fiberglass rods.
These marks will assure you that the rods are in the correct spot as they are glued.
With two spreaders in position to be glued, twist the rod and pull it out about 1/4″ and while rotating the rod apply a small amount of the Gorilla Super glue, while rotating the rod, push it back to the mark next to the boom. If there is any excess glue around the joint where the rod and boom meet, wipe away any excess drippings.
You should see both of the marks on each side of the boom.
Do this to one end only ( two rods the same length), after about 30 minutes, the glue will setup to where you can do the other end of the boom.
Place the end you just glued on a piece of newspaper, with the boom sticking up vertical. The antenna will be leaning slightly to one side and resting on two of the fiberglass rods tips (make sure the rods didn’t move, and your marks are on each side of the boom.
When both rods have glue and are at the correct marking (distance measured from the ends of the rods), move any rod that you had to widen the hole for, so that it makes a (+) and tape it to the newspaper until the glue sets up.
When this glue sets up, it expands slightly due to a slight foaming action. If you wiped away any excess glue, when you re-inserted the rods, any amount that does expand as it is setting up, will make a nice small bead all the way around the rod to boom joint, creating a good strong joint.
Let the glue cure at both ends for several hours ( 5-6 hours) before doing any more work on the boom/spreaders.
The ends of the boom will be prepared later for the capacitors and the spreaders for the copper wire.
After the glue has cured fully, using the Dremal tool and sanding wheel, if you don’t have a Dremal Tool than use a file and sand down a flat spot 1/2″ long on seven of the fiberglass rods tips (Photo 10). On the #8 spreader tip sand a flat spot 1″ long, drill two holes 1/2″ apart, 1/4″ from the end. This will be the feed point for the Driven Element (Photo 11).
Drill a 1/16th” hole in the direction facing the next rod tip, allowing the wire to be pulled all the way around the four rod tips.(Photo 12)
Measure and cut two pieces of 18ga wire 44″ long. Using the 1/16th” drill as a form, wind a loop around the drill and twist a couple of turns back on itself (Photo13) and solder. This eyelet will keep your wire tight as you thread it from tip to tip. When you reach your eyelet, using the pliers or needle nose, pull the wire until all four sides are taught, wrap it onto itself, trim off the excess wire and solder.
On the #8 spreader tip that has the two holes, these are the start and finish points for your Driven Element (Photo 14).
The antenna feedline will have a sleeve or ‘Bazooka’ Balun 12.9″ in length. See the Antennas section on the T-Hunters website (WB6AMT.com) for construction details on building a ‘Bazooka Balun’. If anyone needs assistance on the Bazooka Balun, please call me. My info is on the Contact page.
Tuning the antenna:
With the feedline connected to the Driven Element, tuning the antenna is basically two steps.
Connect the antenna through an attenuator to a radio with a ‘S’ meter, or to a spectrum analyzer if available.
Turn on a Stationary low powered signal on 146.565 Mhz approxiamately 100′ away from the antenna.
Point the Driven Element at the transmitter (keeping the antenna as steady as possible) and tune the piston capacitor on the Driven Element for Maximum indication on the radio’s ‘S’ meter. If it peaks out the ‘S’ meter, insert some attenuation so that you can tune for maximum indication. Your hand will probably affect each adjustment so take your time and if necessary over adjust your tweaking of the capacitor so that when you pull your hand away it is close to where you want it. These capacitors take 11 full turns to go from one end to the other. This gives you a very precise and smooth adjustment.
Next turn the antenna around 180 degrees, with the Reflector end pointing at the low-power transmitter, adjust the piston capacitor for as deep a null as you can. Turn the antenna around and touch up your Maximum adjustment and then turn the antenna around again, to touch up the Deepest null you can get.
Once you have finished the tuning steps above, your antenna is ready to go.
This is a loaded type antenna, which means it’s going to have a very good front-to-back ratio with a sharp peak on the main lobe, and a deep null in the opposite direction.
This is an excellent antenna for doing close-in sniffing of the hidden transmitter with your HT and an attenuator in line, or connected to an amplified field strength meter when the signal is strong enough.
This antenna has been around for a long time and has proven itself to be a very efficient and durable close-in T-hunting antenna.
The antenna is a favorite with International Transmitter Hunters, who do most of their hunts running on foot in parks or wooded areas to track down as many as five transmitters per hunt. It has survived many encounters with bushes and trees, and fits in the trunk of most cars without having to be taken apart.
The data for this antenna is from a QST article in the April 1977 issue by Joe Moell N0OV.
I am available to help anyone with questions or needing assistance in building their antenna.
If a spreader is damaged……..
By having the glue bead on one side of the boom, it makes it easy to replace a spreader ( if damage occurs for some reason) by setting the boom on a solid surface with the joint right at the edge of your solid surface, the glue bead side should be pointing downward.
Strike the spreader with a hammer straight downward.
Once the fiberglass rod is loose, it can be replaced with a new rod. Follow the instructions above…measure, mark, glue, straighten, let cure, sand, drill , etc.
Here’s how mine turned out…..
Shrunken Quad Updates
More on the 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 fullsized
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 6meter version on every mobile hunt on that band. The 2meter version is still a favorite onfoot “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 multiturn 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.
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 mailorder
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.** A note about Bogdan. I got an email from Carolyn saying that Jim had a stroke last year. They no longer go to swap meets. She is going to check her storages and see what quantities of piston capacitors are available. I will obtain a quantity, so please check with me as I hope to have them available for this and other projects- Earl WB6AMT
Another source is Dan’s Small Parts and Kits, Box 3634, Missoula, MT 598063634
(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 RG58 coax feedline. The sleeve is connected to the RG58
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 .
(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 closein sniffing with a two element antenna.
7/12/2016 Shrunken Quad Updates
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 retweaked the trimmers, the quad worked normally.
For frequencies outside the twometer 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.
I rebuilt my original Shrunken quad using 1/4inch diameter fiberglass spreaders (covered with shrink sleeving) and 3/32inch bronze welding rod elements. It is now extremely rugged and has withstood lots of tramping in the brush. N0OV