EMRFD Message Archive 2743

Message Date From Subject
2743 2009-02-26 04:57:49 Vojtěch Bubník rf power measurement?
Hi gang.

Inspired by the Elecraft K1 and K2 transceivers, I am trying to add a RF output
detector to my ATS-3b transceiver to measure output power when the load is
known. K1 and K2 have the RF detector wired in the same way. There is a 1.5K/226
Ohm voltage divider connected directly to the antenna, which sources one 1N5711
Shottky diode with reverse voltage of 70V. The diode is loaded with 1nF
capacitor a and 100k resistor in parallel. The detector voltage is measured by
PIC controller (18C452 in case of K2), which has 10 bit A/D, referenced with
VCC (6V in case of K1, 5V in case of K2).

The voltage divider of the RF detector in K1 and K2 consumes about 6% of the RF
energy, which is 300mW at 5W output power. This is about the limit of 1/4W
resistor. Also if one transmits without an antenna, the RF voltage will jump
much higher. K1 has only passive PA protection using 48V Zener diode parallel to
the final transistor. I am concerned that the RF voltage divider may burn as
much as 700mW with antenna disconnected. Am I right? Well, maybe the collector
wave shape will have much lower RMS that I expect? I guess I need to make a
smoky experiment. This will be no solder smoke, but resistor smoke.

Is there anything special about the values in the divider of K1/K2? Why 226 Ohm?
When using controllers, one usually wants to have the numerical value converted
to be multiple of 10ths, but I was not able to match it with the A/D resolution
and A/D reference voltage of K1/K2 controllers. Also what should be the minimal
current in the divider for the RF detector to work reasonably? I suppose it has
something to do with the impedance of the detector, but I am clueless.

Thanks, Vojtech OK1IAK