29 November 2011

UV-3R VHF second harmonic

Definition: a second harmonic is a spurious emission of a transmitter located at twice the fundamental frequency (or first harmonic or "dial"). In most cases the second harmonic is not produced on purpose.

UV-3R, especially the single-line display, exhibits too little suppression of the second harmonic of the VHF transmitter. This occurs when the RTX is used on the VHF HAM band (144-146/148 MHz). Suppression gets better and within required limits when the VHF transmitter is used above 150 MHz.

This means un-modified UV-3R is not certified for VHF amateur radio operations. Transmitting, say, at 145.500 MHz, will result in a parallel transmission on 291.000 MHz, which is probably allocated to a Defence Ministry. In the Files area of the Yahoo! group some LPF modifications have been measured and described in detail. Unfortunately they are not for everyone.

So, what options do I have?
  • use unmodified UV-3R on VHF only if strictly needed, being aware you might be disturbing communications around 290 MHz. On UHF the little "R" is spurious emissions compliant.
  • insert a VHF band/low-pass filter between UV-3R and the antenna
  • have someone apply the LPF modification
Why is it so? No one has figured out why the VHF signal is so dirty. It might be a driver/final wrong bias current. UV-3R Mark II is apparently performing better, but still above the required suppression level.