Ironically, one of the easiest to make, this loop seems to be the most efficient ... so far.
It's 2 feet on a side. The cross pieces are, therefore; 24" x 1.414 = you do the math (
*see below).
Details of the
intersection, a "tee"
drilled through or a
"cross" will do. I used
1/2" PVC pipe. The 4
ends are "tees" with
elbows for feet. Use a
small piece of pipe to
join the "tees" to the
"elbows."
Details of the windings. There are
7 or 8 turns which resonate with
the 15 - 365 pF air variable
capacitor. Note that I've added a
triple section cap for lowfer (LF)
reception. This loop covers 2 MHz
down into the beacon band and LF
band. Note the pickup winding and
"F" connector
This circuit is as basic as it gets. I
won't put a diagram here, because
there are a number of good sites
on how to build a box loop for AM
broadcast.
Simply put: It's a resonant loop of
spaced windings (spaced to keep
stray capacitance low). I'm
guessing, but I figure there is 56
feet of wire. The coil is wired in
parallel with a variable capacitor
(usually a 15 - 365 pF). This parallel
circuit should resonate
somewhere from 0.5 to 1.7 Mhz
(500 - 1700 KHz).
Depending on the components,
you may have to add or remove
wire to put the resonant limits
within the band of your choice. I
had to add more cap to cover the
loop's intended purpose. But,
experimentation is easy and
educational. The nice thing about
PVC over the more popular wood
frame is the spring it has, thus
keeping the windings fairly tight
(note the slight bow to the
structure which keeps tension on
the wires).
Use an ordinary portable AM radio
placed near the loop to fine tune
the windings. Check the peaking at
the low and high ends of the b'cast
band ... it's quite amazing actually.
The antenna can be turned and
tilted to peak a station or null and
interfering one.
The pick-up loop is soldered to an
"F" fitting. Make this a one-turn
loop and space it as shown or
experiment a bit before you make
it permanent (I found that about
1.5" was good for this loop). With
the pick-up loop, you can feed the
low impedance input of a
communications receiver.
With a portable radio, that doesn't
have an external antenna
connection, just orient it near (or
inside) the antenna, turn the loop
and radio for best reception of
your selected station and don't
use the pick-up winding
connection.

Happy listening,
Bob, N1KPR

Feel free to copy info, but please give credit as
necessary. July 31, 2005  
AM (MF) Broadcast Band
Box Loop Antenna
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* Okay, okay...here's the answer from above:
Cut the cross pieces 33.94 inches (call it 40"), plus 1/2" on each end for insertion into the tees, for an overall length of 41 inches.
Construction hints: I once built an 18" loop on a wooden box frame made of 1" x 8" shelving lumber. I finished the wood with a
stain that matched our family room furniture and set my various portable radios inside, on the bottom rail. The center of the
bottom rail was bolted to an old "lazy Susan" turntable. I eventually gave the thing away to a friend who wasn't very handy with
tools and lived in a condo with the (all too) usual outdoor antenna restrictions. i kind'a miss that thing ... maybe I'll make another
one ... some day.