Practical_Antenna_Handbook_0071639586

340 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
to switch a multipole double-throw relay in the enclosure can be multiplexed on the
coaxial cable.
Assume first that the feedline attached to transformer T 1 is terminated in Z 0 . Signals
coming from the right side of the page build in voltage with respect to ground
along both Beverage wires equally. At the left end of the wires, this common mode
voltage appears across the entire primary winding of T 1 , including its centertap, and
drives the top end of the primary winding of T 2 , whose secondary feeds the active
coaxial cable to the receiver. Signals coming from the left side of the page build up
equally on both Beverage wires from left to right, appearing on the entire primary of
T 3 , including its centertap, which drives one end of a secondary winding. The signal
across the secondary of transformer T 3 now drives the two Beverage wires in pushpull
or differential mode, and that signal propagates from right to left along the wires
until it reaches the primary of T 1 , where it is coupled to the secondary and dissipated
in that winding’s Z 0 termination. To receive signals from the opposite direction, the
receiver is connected to the secondary of T 1 , and the secondary of T 2 is terminated
in Z 0 .
The exact number of turns required on each transformer is dependent upon the
specifics of each individual Beverage installation, including wire diameter and length,
spacing between the two wires, height above ground, and ground conductivity. If the
termination and cable switching is done near T 1 and T 2 , care must be taken to isolate the
ground end of the termination from the coaxial cable ground (through a separate pole
on the relay), or the ground connecting the two must be virtually perfect.
Phased Beverages
Additional directivity and signal amplitude can be obtained by phasing two or more
Beverages. At least two different methods of phasing are currently in use:
• Two identical Beverages, parallel to each other, spaced l/4 or more apart, with
no offset relative to the desired receiving direction. (That is, they can be thought
of as two opposing long sides of a rectangle.) The outputs of the two wires are
combined in phase before reaching the receiver.
• Two identical Beverages, parallel to each other and closely spaced (i.e., within a
few feet), but offset somewhat in the desired receiving direction. (Similarly,
picture the long sides of a very skinny parallelogram.)
A Beverage erected with two wires—parallel to each other, at the same height,
spaced about 12 in apart (Fig. 14.5), with a length that is a multiple of a half-
Âwavelength—is capable of null steering. That is, the rear null in the pattern can be steered
over a range of 40 to 60 degrees. This feature allows strong, off-axis signals to be reduced
in amplitude so that weaker signals in the main lobe of the pattern can be
received. There are at least two varieties of the steerable wave Beverage (SWB).
If null steering behavior is desired, then a phase control circuit (PCC) will be required—consisting
of a potentiometer, an inductance, and a variable capacitor all connected
in series. Varying both the “pot” and the capacitor will steer the null. The
direction of reception and the direction of the null can be selected by using a switch to
swap the receiver and the PCC between port A and port B.