Practical_Antenna_Handbook_0071639586

500 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
Ring Antenna
Another popular Jovian radio antenna is the ring radiator, two versions of which are
shown in Fig. 22.2. This antenna is made of a 5-ft-diameter loop of ½-in-diameter softdrawn
copper plumbing pipe found in any local hardware or plumbing supply store.
The single-ended version is shown in Fig. 22.2A. In this form the far end of the loop is
unterminated (i.e., open-circuited). The center conductor of the coaxial cable feedline is
connected to the near end of the ring radiator, while the coax shield is connected to the
chicken wire ground plane. The balanced version (Fig. 22.2B) has an RF transformer (T 1 )
at the feedpoint.
The ring radiator antenna should have a bandpass-filtered preamplifier—needed
because of the antenna’s low output levels. The preamp should be mounted as close as
possible to the antenna. The intent is to minimize the likelihood of strong terrestrial
signals in the adjacent bands desensing the preamp or receiver. Even a 5W CB transmitter
a few blocks away can drive the preamplifier into saturation, so it’s wise to eliminate
the undesired signals before they get into the preamplifier. In the case of the singleended
amplifier, a single-ended preamplifier is used. But for the balanced version (Fig.
22.2B) a differential preamplifier is appropriate.
Figure 22.2A Ring radiator: single-ended.