Practical_Antenna_Handbook_0071639586

352 p a r t V : h i g h - F r e q u e n c y A n t e n n a s f o r S p e c i a l i z e d U s e s
The Sports Fan’s Loop
Okay, sports fans, what do you do when the best game of the week is broadcast only on
a low-powered AM station and you live at the outer edge of their service area, where
the signal strength leaves much to be desired? You use the sports fan’s loop antenna,
that’s what! The author first learned of this antenna from a friend—a professional
broadcast engineer who worked at a religious radio station that had a pipsqueak signal
but lots of fans. It really works; in fact, one might say it’s a “miracle”.
The basic idea is to build a 16-turn, 60-cm 2 tuned loop and then place the AM
portable radio at the center with its loopstick aimed so that its null end is broadside
of the loop. When you do so, the nulls of both the loop and the loopstick are in the
same direction. The signal will be picked up by the loop and then coupled to the radio’s
loopstick antenna. Sixteen-conductor ribbon cable can be used for making the
loop. For an extra touch of class, place the antenna and radio assembly on a dining
room table lazy Susan to make rotation easier. A 365-pF tuning capacitor is used to
resonate the loop. If you listen to only one frequency, this capacitor can be a trimmer
type.
Shielded Loop Antennas
The loop antennas discussed thus far in this chapter have all been unshielded types.
Unshielded loops work well under most circumstances, but in some cases their pattern
is distorted by interaction with the ground and nearby structures (trees, buildings, etc.).
In the author’s tests, trips to a nearby field proved necessary to measure the depth of
the null because of interaction with the aluminum siding on his house. Figure 14.13
shows two situations. In Fig. 14.13A we see the pattern of the normal free-space loop,
i.e., a perfect figure eight pattern. When the loop interacts with the nearby environment,
however, the pattern distorts. In Fig. 14.13B we see some filling of the notch for a
moderately distorted pattern. Some interactions are so severe that the pattern is distorted
beyond all recognition.
Interaction can be reduced by shielding the loop, as in Fig. 14.14. Loop antennas
operate on the magnetic component of the electromagnetic wave, so the loop can be
Figure 14.13A Normal free-space loop.