4-6
ACCURACY OF BRIDGE MEASUREMENTS
To assure accurate measurements, the rf signal applied to the bridge must be properly adjusted each
time a calibrated instrument is used. Essentially, this adjustment is a repetition of the previously described
reversed-bridge procedure. The following steps are to be performed:
1. Connect the line to the input terminals of the bridge and connect the transmitter to the output
terminals.
2. Adjust the transmitter coupling until full-scale deflection is obtained. From this point on, the
coupling must be left untouched.
3. Reconnect the bridge in the usual way and proceed with the measurement.
POWER OUTPUT VERSUS IMPEDANCE MATCHING
For maximum transfer of the power out of an rf source, with minimum heating from reflected power,
the total output impedance sensed by the rf source must be equal to the internal impedance of that source.
A perfect impedance match between transmitter and load would exist if the swr were 1 to 1. As discussed
in NEETS, module 10, test equipment designed to measure the instantaneous voltage of a standing wave
will give you a voltage standing-wave ratio (vswr). Test equipment designed to measure the instantaneous
current component of a standing wave will give you the current standing-wave ratio (iswr). Regardless of
the type of test equipment selected, both ratios will be the same.
Q-2.
What vswr is a perfect match between a transmitter and its load?
SWR METERS
The Hewlett-Packard Model 415E swr meter, shown in figure 4-3, is a commonly used swr meter. It
is extremely accurate, sensitive, lightweight, easy to use, and portable. It is essentially a high-gain, tuned
audio amplifier with a square-law meter that is calibrated to read swr directly. The meter is designed to be
operated at a mean center frequency of 1,000 hertz.
