Practical_Antenna_Handbook_0071639586

354 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
Figure 14.15 Q-multiplier.
shielded against E-field signals and electrostatic interactions. In order to retain the ability
to pick up the magnetic field, a gap is left in the shield at one point.
There are several ways to shield a loop. You can, for example, wrap the loop in
Âadhesive-backed copper-foil tape. Alternatively, you can wrap the loop in aluminum
foil and hold it together with tape. Another method is to insert the loop inside a copper
or aluminum tubing frame. Or . . . the list seems endless.
Sharpening the Loop
Many years ago, the Q-multiplier was a popular add-on accessory for a communications
receiver. These devices could be found in certain receivers (the National NC-125 from
the 1950s comes to mind) or offered as an outboard receiver accessory by Heathkit, and
many construction projects could be found in magazines and amateur radio books. The
Q-multiplier improves receiver performance by increasing the sensitivity and reducing
the bandwidth of certain receiver stages.
The Q-multiplier of Fig. 14.15 is an active electronic circuit placed at the antenna
input of a receiver. It is essentially an Armstrong oscillator that does not quite oscillate.
These circuits have a tuned circuit (L 1 /C 1 ) at the input of an amplifier stage and a feed-