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.
C h a p t e r 1 4 : r e c e i v i n g A n t e n n a s f o r H i g h F r e q u e n c y 341 Port A T 1 T 2 C 1 Port B ¯12 ft T 3 Receiver L 1 300 H R 1 1000 450 pF Figure 14.5 Beverage antenna with a steerable null. Snake or BOG Antenna The height above ground of a Beverage is the result of a compromise that invariably includes a need to locate the antenna high enough to avoid damage to or from humans and animals. As a result, the rejection of signals from undesired directions and signals with horizontal polarization is lessened. The Beverage on ground (BOG) or snake antenna attempts to minimize unwanted signal ingress by eliminating the vertical downlead at each end of the Beverage. A side benefit is the elimination of all supports! The primary disadvantage is a noticeable reduction in received signal level stemming from the very close coupling of the antenna to the ground beneath it. Typically, a preamplifier must be added to a BOG or snake in order to develop signal levels sufficient to override the receiver noise floor. Ideally the preamp should be located at the junction of the Beverage and its feedline, rather than at the receiver end of the feedline. Receiving Loops Even in their smallest implementations, the preceding antennas require a fair amount of real estate for their proper operation. In addition to the space taken up by the antenna itself, there is a “hidden” requirement of another l/2 or more distance between the receiving antenna and any transmitting antennas resonant on the same band(s). Further, it is wise to locate and orient Beverages so as to keep buildings containing computers, microprocessors, and other sources of RF noise behind the antenna—i.e., at the receiver, or unterminated, end. In general, a broadcast-band listener (BCL) or shortwave listener (SWL) contemplating the installation of Beverages for multiple compass headings will need a 10-acre or larger parcel. Two entirely different classes of antenna have been of great use to listeners with limited space for antennas. The first of these is the traditional small, multiturn loop that
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C h a p t e r 1 4 : r e c e i v i n g A n t e n n a s f o r H i g h F r e q u e n c y 341<br />
Port A<br />
T 1<br />
T 2<br />
C 1<br />
Port B<br />
¯12 ft<br />
T 3<br />
Receiver<br />
L 1<br />
300 H<br />
R 1<br />
1000 <br />
450 pF<br />
Figure 14.5 Beverage antenna with a steerable null.<br />
Snake or BOG <strong>Antenna</strong><br />
The height above ground of a Beverage is the result of a compromise that invariably<br />
includes a need to locate the antenna high enough to avoid damage to or from humans<br />
and animals. As a result, the rejection of signals from undesired directions and signals<br />
with horizontal polarization is lessened.<br />
The Beverage on ground (BOG) or snake antenna attempts to minimize unwanted<br />
signal ingress by eliminating the vertical downlead at each end of the Beverage. A side<br />
benefit is the elimination of all supports! The primary disadvantage is a noticeable reduction<br />
in received signal level stemming from the very close coupling of the antenna<br />
to the ground beneath it. Typically, a preamplifier must be added to a BOG or snake in<br />
order to develop signal levels sufficient to override the receiver noise floor. Ideally the<br />
preamp should be located at the junction of the Beverage and its feedline, rather than at<br />
the receiver end of the feedline.<br />
Receiving Loops<br />
Even in their smallest implementations, the preceding antennas require a fair amount<br />
of real estate for their proper operation. In addition to the space taken up by the antenna<br />
itself, there is a “hidden” requirement of another l/2 or more distance between the receiving<br />
antenna and any transmitting antennas resonant on the same band(s). Further,<br />
it is wise to locate and orient Beverages so as to keep buildings containing computers,<br />
microprocessors, and other sources of RF noise behind the antenna—i.e., at the receiver,<br />
or unterminated, end. In general, a broadcast-band listener (BCL) or shortwave listener<br />
(SWL) contemplating the installation of Beverages for multiple compass headings will<br />
need a 10-acre or larger parcel.<br />
Two entirely different classes of antenna have been of great use to listeners with<br />
limited space for antennas. The first of these is the traditional small, multiturn loop that
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