For a typical setup the test generator is connected to the input port, the antenna or device to be tested is connected to the test port, and a signal level monitor is connected to the output port. When the test port is terminated with 50 ohms, no energy is reflected and no signal will appear at the output port. If the load at the test port is not 50 ohms then some of the test signal will reflect and appear at the output port. For example, if the test port is completely open, the test signal reflects completely and the output will equal the input. If the test port is shorted, the test signal will reflect inverted and the output will be an inverted version of the input. Other loads will reflect a portion of the test signal.
To measure the return loss of a device or antenna,
first open or short the test port and observe the signal level at the output
port. Then connect the device to be tested and observe the drop in signal level
at the test port. The amount of drop is the return loss. This test may be done
in the presence of other signals if a tuned meter or spectrum analyzer is used
to monitor the test signal. Adjust the test signal frequency until it is sufficiently
far from the other signals to easily measure and verify that the other signals
present are not too large for the circuit to handle (below 16 dBm).
The transistors will run hot so use a low-capacitance heat sink. Increase the 10 ohm resistors in the emitter circuits to reduce the transistor heating but the large signal handling ability will be reduced. The isolator is a 50 ohm device due to the resistors near 50 ohms. (The 56 ohms are a bit higher since they are shunted by some circuit resistance.) Other impedance like 75 ohms may be achieved by changing these resistors to, say, 75 and 82 ohms. Ordinary small-signal transistors like the 2N3904 operating at much lower current make a useful device for testing antennas and low power devices at lower frequencies - perhaps to 50 MHz. Try 2, 82 ohms instead of the 2, 10 ohms in the emitters. Remember that the signal handling capability will be quite low so use signals at or below 0 dBm (1 mW).
Some points for the experimenter to ponder:
Original document from http://www.wenzel.com/documents/circtran.htm
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