Practical_Antenna_Handbook_0071639586

592 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
R 2
68
L 1
balance
C 3
140 pF
J 2
Figure 27.2C Circuit with balance control added.
Note The instrument shown in Fig. 27.2 depends for its operation on an external source of RF,
applied at J 1 . The power-Âhandling capabilities of this device are limited by the ratings of the
resistors and the current through the meter. Never connect this device directly to a
transmitter. Instead, use a calibrated low-Âlevel RF signal generator or a known attenuator of
adequate power rating between the source and the instrument.
RF Noise Bridges
The RF noise bridge was once associated only with engineering laboratories but turns
out to have applications in general communications servicing as well. It is one of the
most useful low-Âcost and often overlooked test instruments in our arsenal of devices for
troubleshooting antennas.
Several companies have produced low-Âcost noise bridges: Omega-ÂT, Palomar Engineers,
the Heath Company, and, most recently, MFJ. The Omega-ÂT and the Palomar Engineers
models are shown in Fig. 27.3. The Omega-ÂT device is a small cube with a single
dial and a pair of BNC coax connectors (antenna and receiver). The dial is calibrated in
ohms and measures only the resistive component of impedance. The Palomar Engineers
unit does everything the Omega-ÂT does, and, in addition, it allows you to make a rough
Figure 27.3 Commercially available amateur noise bridges.