[Racal] 2 + 2 = 5?

[Alan Tasker]

The information that in some of the HF rigs, a voltage on the RF output 
line is used by the ATU brought to mind that the Racal TRA-967 VHF rig has 
a voltage on its RF output BNC connector, which I had thought was strange. 
I looked into it further, and found that the voltage was dependent upon 
frequency setting as follows (measured with no load on the BNC connector).

30-43,975 MHz, 0 Volts
44-59.975 MHz, ~half battery voltage
60 and up, battery voltage

I wondered if this was used for tuning, so I powered up the funny Racal 
backpack set (the one that has a TRA-967 plus a PA box and an ATU box), and 
listened carefully. There are definite sounds of relays clicking inside the 
ATU as the frequency is changed. Measurements show that the 967 voltage 
output does pass through the PA box unscathed, so we now know why this 
voltage exists.