Contents - Index - Previous - Next


Setting the input level


**SC** Setting levels if using a sound card

If you are performing measurements in conjunction with another remote station it will be very useful to establish communication on another frequency so that you both know what is happening and when.

When you click the 'Calibrate' button the reading level is shown both numerically and graphically.  If you have selected Triggered you will get two levels and two sweeps shown in different colours - one for the signal level and one for the trigger level.

a) At the transmitting station ensure that antenna polarisation is the same as the receiving station and transmit a plain carrier with beat note - (CW with key down, FM or AM with no mod).  It is important for calibration purposes that both stations are 'on beam' since the polarisation of a beam will change at off-bearing positions which will distort the polar diagram.

b) Set the receiver to SSB or CW, RF gain at maximum and AGC on.  Tune to receive the transmitted carrier.  The received audio note is not critical but it will be affected by the filters in your rig.  Try settings of between 500 - 1000 Hz. The audio frequency response of the system will not be flat, so ensure that both stations are sufficiently warmed up to eliminate frequency drift.

c) Align the beams at both stations for maximum signal strength.  A local map is useful if you are not certain of your relative locations.

d) At the transmitting station adjust the transmission power level for a received S meter reading in the range S9 to S9+30 with no noise on the signal.

e) At the receiving station run the PolarPlot program.  Click on 'Calibrate input' to activate the input level display and Input Volume control.

f) IMPORTANT! Reduce RF gain to a level at least sufficient to take the S Meter above its normal reading when RF gain is at Maximum. This will ensure that the AGC is inoperative and will give the best linearity of the receiver.  If you have the facility, switch off the receiver AGC which guarantees it will not be operating.

Adjust 'Input Volume' to set the reading in the 'input level' window into or just below the green range.  Once the slider has been 'selected' you can use Page-Up/Page-Down to vary by a large amount, and Up-Arrow/Down-Arrow to vary by a small amount.

You may need to go through the process of setting RF and AF levels several times in order to get good plots with no 'flat topping'.

g) If you are using the external Triggered facility, adjust the level of the external trigger audio such that it is greater than the defined threshold (default value is 100).

**WM** Setting levels if using a wattmeter

These instructions assume that the antenna under test is connected to the wattmeter and another antenna is connected to the signal source (probably your transceiver).

When you click the 'Calibrate' button the reading level is shown both numerically and graphically.  If you have selected Triggered you will get two levels and two sweeps shown in different colours - one for the signal level and one for the trigger level. A small window will also open showing the present wattmeter reading.

When initially performing this procedure with an unknown antenna take care that you do not overload the wattmeter. 

Start with the antenna under test pointing away from the transmit antenna and introduce some attenuation prior to the wattmeter.

a) Ensure that antenna polarisation is the same for both transmit and receive antennas and that the wattmeter is connected and powered on. Set the transmit power to the lowest level possible and transmit a plain carrier with beat note - (CW with key down, FM or AM with no mod).  

b) Check the reading from the wattmeter. If it is above its maximum level then reduce transmit power or increase attenuation prior to the wattmeter.

c) Gradually align the beams for maximum wattmeter reading ensuring you do not overload the wattmeter. It is important for calibration purposes that both stations end up 'on beam'.

d) Adjust the input attenuation and/or transmit power until you get an indicated power level of +10dBm when both beams are pointing directly at each other. Click the button in the dB Meter window labelled 'Cal as 0dB'. The figure for Relative dB down should now show as 0.0.

e) If you are using the external Triggered facility, adjust the level of the external trigger audio such that it is greater than the defined threshold (default value is 100).