Practical_Antenna_Handbook_0071639586

C h a p t e r 2 7 : T e s t i n g a n d T r o u b l e s h o o t i n g 595
Continue to alternately adjust the two controls for the deepest possible null, as indicated
by the lowest reading of the S-Âmeter. (There is usually some interaction between
the two, hence the need for the iterative process.)
A perfectly resonant l/2 dipole in free space or at least a wavelength above ground
should have a reactance reading near 0 W and a resistance of 50 to 75 W. But “practical”
antennas will exhibit some reactance (the less the better) and a resistance that is somewhere
between 25 and 100 W. Remember, too, that the theoretical impedance of a monopole
is ½ that of a corresponding dipole, so don’t expect all types of antennas to exhibit
the same input impedance.
The impedance-Âmatching methods of Chaps. 4 and 24 can be used to transform the
actual resistive component to the 50-Â or 75-ÂW characteristic impedance of the transmission
line and eliminate the reactance. Some helpful hints when using a noise bridge:
• If the resistance reading indicates zero or maximum, suspect a short or open
circuit somewhere between the bridge and the antenna. But unless you know the
exact electrical length of your transmission line (in wavelengths), you can’t be
sure which it is because odd multiples of l/4 will transform one into the other.
• A reactance reading on the X L side of zero indicates a net inductive reactance at
the point on the transmission line where the noise bridge has been inserted,
while a reading on the X C side of zero indicates a net capacitive reactance.
Again, without knowing the exact length of the transmission line, there’s a limit
to what you can do with that information.
As we saw in Chaps. 4 and 26, the impedance of an arbitrary load is repeated every
half-Âwavelength along the line, going back toward the source or transmitter. If we can
make the transmission line between the antenna and the noise bridge an exact multiple of
l/2 at the frequency of interest, we can obtain the input impedance of the antenna directly
from the front panel markings on the bridge. (Remember to account for the velocity
factor of the transmission line when determining its electrical length.) Otherwise, we will
need a Smith chart or other means to translate our readings into something useful.
Suppose, for instance, that we have a 40-Âm dipole with a full wavelength of transmission
line between it and a place where we can insert the noise bridge. Under those
circumstances we can treat the noise bridge settings as being roughly the same as the
antenna feedpoint resistance and reactance. In particular, if the bridge indicates a net
inductive reactance at the design frequency, we can conclude that the dipole is somewhat
longer than the resonant length. Similarly, a reading on the X C side of center implies
a length somewhat shorter than resonance.
If we wish to directly connect a transmitter to this feedline without an intermediate
ATU in the line, we may choose to adjust the length of the dipole. To determine the correct
length, we must find the actual resonant frequency f RES . To do this, set the reactance
control to zero, and then slowly tune the receiver in the proper direction—lower in frequency
if we think the antenna is too long or higher if we think it’s too short—until the
null is found. Call this frequency f EXP . On a high-ÂQ antenna, the null is easy to miss if
you tune too fast. Don’t be surprised if that null is out of band by quite a bit. The percentage
of change is given by
( fRES
− fEXP)
× 100
% Change =
f
EXP
(27.6)