Plotting a polar diagram involves measuring and recording a received signal strength at known intervals.
**SC** To do this you require a constant level signal source and a receiver. The audio output of the receiver is connected to the line-in socket of a sound card in a computer.
**SC** Reception of a plain unmodulated carrier as a beat note in SSB or CW mode gives a good correlation between RF input voltage and AF output voltage as long as the AGC circuits are inoperative and there is no overloading. Modulated tones in AM or FM cannot be used, particularly FM which is inherently designed to maintain audio level regardless of the RF input level.
**SC** The program relies upon the linearity of the complete system for accuracy of plot and the measurement of gain. Whilst the linearity of the average sound card is generally quite good, the linearity of the receiver depends on how it is operated and the presence or absence of any AGC.
**SC** To measure the transmitting antenna's polar diagram the receiving antenna remains stationary and the transmitting antenna rotates. To measure the receiving antenna's polar diagram the transmitting antenna remains stationary and the receiving station's antenna rotates.
**WM** The antenna under test is the one that is rotated. The signal is unmodulated FM, AM or CW. The wattmeter used to collect signal strength readings is connected to a PC running PolarPlot. Either:
the wattmeter is connected to the antenna under test and another antenna, preferably a beam, provides the signal source; or:
the wattmeter is connected to a sense antenna, preferably a beam, and the antenna under test transmits the source signal.