There’s
a variety of antennas that you can use for AM reception at the studio: many
engineers have used a whip antenna, usually mounted on a ground plane. I’ve seen automobile antennas used in this
way. Some favour a longwire antenna, but
I’ve always preferred the shielded loop.
It’s easy and inexpensive to make one, and although they’re not
particularly sensitive, their unique noise- and interference-cancelling properties
mean they can give surprisingly good performance in difficult situations. How come?
As a matter of fact, once you start to look more closely, many people
start to wonder how come they work at all!
This month I’ll try to explain their secrets.
The
first question many have runs thus: if the doggone antenna is shielded, how
does it pick up a signal at all? The
answer is surprisingly simple: our desired RF signal consists of
electromagnetic waves, which have an electric and a magnetic component. We shield the electrostatic component
only—and pick up the magnetic wave. Any
grounded conductor can be used as a shield against the electric wave—if we had
wished to shield the magnetic component, we’d have to use a magnetic material,
such as iron, steel, nickel or even mu-metal.
And sure enough, if we use a piece of steel electrical conduit for our
shield, we won’t get much of a signal.
Copper, on the other hand, makes an excellent electrostatic shield,
without affecting the magnetic field, so that’s what we’ll use today.
One
aspect of that shield that's bound to confuse is that there must be a break in
the loop--otherwise the windings inside will effectively couple to a shorted turn, and you'll get little or no signal coming out. The shield must be connected to ground or it
will be effectively invisible, and will provide no shielding action at
all. Depending on the details of
construction, it may be desirable to switch the ground connection to the shield
on and off, allowing the antenna to serve as a shielded or unshielded loop.
Since,
in its shielded form, the loop is picking up only half of the electromagnetic wave, that explains why its sensitivity is a bit low. The surprise is that the received noise is
usually attenuated even more, and that’s because most electrical noise is
electrostatic in nature. An added bonus
is that the rejection nodes of a well-constructed shielded loop are very
deep—perhaps –25dB! (Incidentally, this
explains why the shielded loop is so often used in radio direction
finders.) Often, when we’re faced with a
situation involving nighttime interference, we can benefit by forgetting about
peaking the desired signal, and instead concentrating on nulling out the
interfering ones.
If
you need more sensitivity, you can resonate the
loop by experimentally applying a small tuning capacitor--no more than 500 pF
or so--in series with the loop. You’ll
know when you reach the right value—the output level peaks up quite
sharply. One precaution with this
arrangement, though: it is quite easy to achieve a
loaded Q high enough to lop off the sidebands, which will result in a loss of
high-frequency modulation content, and distortion there too.
Received
signal strength is more or less in proportion to size: twice the size, twice
the signal. The optimum number of turns
to use is counterintuitive—more turns does not equal more signal. As a matter of fact, signal strength drops
off pretty quickly past the optimum number.
This is because we're typically trying to match into a receiver front
end that has a fairly low impedance--say 50 to 100
ohms. More than a handful of turns
results in a high impedance device, and leakage capacitance to the shield
starts to become significant, too. Flatter
results across the broadcast band can be achieved by using three turns or so in
the body of the loop, and connecting a balun—a balanced-to-unbalanced
transformer—at the output of the antenna.
This improves the impedance match and balance, because if you look at it
carefully, you’ll see that the loop itself is essentially a balanced
circuit. By inserting the balun, you’re
providing the right type of balanced load for this antenna. Ten turns or so on the ferrite toroid of your
choice, bifilar-wound, makes a very nice, compact, self-shielding balun.
So
there you have it: the shielded loop, unplugged! Next month, I'll show you how to roll your
own high-performance shielded loop antenna.