Practical_Antenna_Handbook_0071639586

338 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
age. With average ground characteristics, R TOTAL (the sum of the termination resistance
plus the ground path between the meter and the grounded end of the termination) will
probably range from 1.5 R TERM to 4R TERM , depending on the length of the Beverage and
the characteristics of the ground beneath it. The primary value of this test is simply to
prove continuity through the Beverage wire and termination resistor into the ground.
This test should be performed when the installation is new, and the value of R TOTAL immediately
recorded for future reference.
If you have the necessary test equipment, consider “sweeping” the impedance of
the Beverage at its feedpoint over the frequency range of interest while you adjust the
value of the terminating resistor for flattest response over the useful range. (Temporarily
use a noninductive potentiometer in place of the permanent terminating resistor.)
Make sure you terminate or match the feedpoint end of the Beverage according to the
instructions with the test equipment. This is an excellent use of a vector network analyzer
(VNA) of the type discussed in Chap. 27.
An inexpensive Beverage feedline is 75-Ω RG-6 coaxial cable purchased by the 500-
or 1000-ft spool from a building supplies store.
When using either 50-Ω or 75-Ω coaxial cable for the feedline, a 9:1 step-down transformer
at the Beverage feedpoint is a smart addition. (Do not put the transformer at the
receiver end of the feedline!) Figure 14.3 shows how to build an appropriate transformer
using a ferrite core and a trifilar winding. Type 31 or Type 43 materials should
work well, but many mixes will work nearly as well. Ideally, the ground for the shield
of the coaxial feedline at the transformer needs to be kept isolated from the grounded
end (winding terminal a 2 in Fig. 14.3) of the transformer winding that connects to the
Beverage, which would require a second, completely isolated coaxial cable output
winding identical to winding a 1 -a 2 on the same core and a second “ground” terminal,
but the author has three “very satisfactory” short Beverages (each 250 to 300 ft) with no
ground isolation at all that provide excellent signal-to-noise enhancement on 160 and
80 m most nights.
After connecting the feedline to the step-down transformer at the unterminated end
of the Beverage, perform the same dc continuity test with an ohmmeter located at the
receiver end of the feedline, thus checking the integrity of that portion of the signal path
as well. Of course, you should see a very low value of R—no more than a few ohms, at
most. The exact value will depend on the length of your feedline and the resistance of
its conductors.
At this point, if a VNA or other suitable piece of test apparatus
is available, a sweep of the entire antenna system
To beverage c 1 from the receiver end of the feedline to the termination resistance
at the far end of the Beverage wire can be an extremely
c 2 valuable record should troubleshooting be necessary in the
future.
b Much has been learned and disseminated about the Beverage
in the past decade. A book nearly as large as this one
1
b 2 could be written about the antenna and all its subtleties and
“picky details” that should be attended to. Nonetheless, “imperfectly”
constructed Beverages can make a whole new layer
a 1
To receiver
a 2
Figure 14.3 Trifilar-wound 9:1 step-down Beverage transformer.