Practical_Antenna_Handbook_0071639586

596 P a r t V I I : T u n i n g , T r o u b l e s h o o t i n g , a n d D e s i g n A i d
Suppose the change calculated in Eq. (27.6) is –5 percent, i.e., the actual resonance
point is 5 percent lower than expected. Shorten the dipole by perhaps 3 or 4 percent
(split equally between the two sides) by folding the two ends back on the dipole and
remeasure.
It will be rare for a real antenna to have exactly 50-Â or 75-ÂW impedance, so some
adjustment of R and C to find the deepest null is in order. You may be surprised how far
off some dipoles and other forms of antennas can be if they are not in free space and
instead are close to the earth’s surface or other conducting objects of comparable dimensions.
Other Jobs for the Noise Bridge
In addition to its antenna-Ârelated “chores”, the noise bridge can find utility in a variety
of jobs around the radio shack. We can find the values of capacitors and inductors, determine
the characteristics of series-Â and parallel-Âtuned resonant circuits, and find the
velocity factor of coaxial cable, to name just a few.
Transmission Line Measurements
Some projects—such as the simple dipole tests and adjustment already described—
require a feedline that is either a quarter-Âwavelength or a half-Âwavelength at a specific
frequency. In other cases, a piece of coaxial cable of specified length is required for other
purposes—for instance, the dummy load used to troubleshoot depth sounders is nothing
more than a long piece of short-Âcircuited coax that returns the echo at a time interval that
corresponds to a specific depth. We can use the bridge to find these lengths as follows:
1. Connect a short circuit across the Antenna jack (J 1 ) and adjust R and X for the
best null at the frequency of interest. (Note: Both will be near zero.)
2. Remove the short circuit.
3. Connect the length of transmission line to the same jack. (It might be wise to
start with a length that is somewhat longer than the expected final length.)
4. For quarter-Âwavelength lines, cut short lengths from the line until the null is
very close to the desired frequency. (A l/4 line open-Âcircuited at the far end
will appear to be a short circuit at the near end.) For half-Âwavelength lines, do
the same thing, except that the line must be shorted at the far end for each trial
length. (A l/2 line will repeat the termination of the far end at the near end.)
As we saw in Chap. 4, we need to know the value of v F , the velocity factor, to calculate
the physical length of a transmission line corresponding to any given electrical length.
For example, a half-Âwavelength piece of coax has a physical length in feet of (492 × v F )/f
when f is given in megahertz. Unfortunately, the real value of v F is often a bit different
from the published value. The noise bridge can be used to find the actual value of v F for
any sample of coaxial cable as follows:
1. Select a convenient length of coax greater than 12 ft in length and install a PL-Â
259 RF connector (or other connector compatible with your instrument) on one
end. Short-Âcircuit the other end.
2. Accurately measure the physical length of the coax in feet; convert the
“remainder” inches to a decimal fraction of a foot by dividing by 12 (e.g., 32 ft