In advanced DX'ing, the user almost always turns, at some point, to a tuned loop antenna. Such an antenna vastly amplifies the incoming signal, and is fairly directional; moreover, it can be tuned for maximum performance.
Over the years, I've constructed a number of cardboard-box loops, and in doing so I've hit on some key construction techniques which can save you time.
This particular loop was constructed from four shipping boxes, each approximately 19" x 15"-- a fairly standard size. I've mounted them each on a 90-degree angle from the one before it, to make a strong core and a large box, approximately 34 inches on a side. Here are some pictures--you can see that I've bound the boxes together with duct tape.
As for the wiring, this, too, is easily accomplished. This procedure is for each corner, to make guide holes for your wires:
- first, measure in a fixed distance from the edge
- second, mark 15 ticks, spaced 5mm to 15mm (I used 10mm).
- now, using a paring or other small knife (in fact, a steak knife works quite well), cut an incision
at each tick mark; make it on a 45-degree angle and about 1 cm deep. Wiggle the knife slightly, to enlarge the hole.
Punch two holes near the first 'corner' you're about to wire. Punch two more near the final hole. You can see what I mean in the finished product:
Take about a foot of one end of your wire, and push it through and around so that it loops between the two holes and then enters the box. (You may have to lift a flap away for this.)
Now, counting from your first corner, wrap 13 turns of wire. Stick with the same hole at each corner, until you reach your starting corner; then move one hole in. Continue in this fashion, then loosely loop the wire through one hole and into the box.
Connect the two loose ends of wire. At this point, your loop should have some basic pickup. If it doesn't, try adding one turn to your wire.
Connect the ends of wire to a tuning capacitor. Try tuning around the band. Remember that for maximum performance, you'll have to re-tune the antenna at every stop.
I note that tuning capacitors are becoming a little scarce. I had the sense to buy up a few, surplus, some years ago; plus I've been cannibalizing old radios as they wear out.
When you are satisfied with the performance of your loop, fasten the far end of wire through the two holes, for a permanent fit.
Other improvements:
You can add some improvements to the loop. One, you'll probably be sitting it on the floor, so protect the underside of it from snagging and stretching. I've used several bands of duct tape, perpendicular to the wires. You might consider adding a cardboard bottom plate.
You can also mount the tuning capacitor, perhaps along the inner core of your loop.
Finally, one benefit: your receiver and even logsheet can rest comfortably on the inside shelf formed by a box. Quite handy--but remember it's in there before you go to move the loop!
Conclusions:
This sort of box design is big--you have no idea how big until you see it physically in front of you. It seems larger than an equivalent framed loop, but it occupies the same dimensions. It can be built in minutes, but is sturdy enough to last a few years. May it give you much entertainment.
-Bill
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