Saturday, June 20, 2009
Another SWR meter (part II)
...Forget the LM3914 for swr display, will be used as a VU-meter, couldn't get the simple circuit to work as it should, except when de swr was high (full scale on the display), intermediate values... nothing, zero, nada! I was doing something wrong for sure also the rf circuit was not the same on the original drawing.
Anyhow the RF part on this swr meter is working fine, in fact will replace probably another one that was also running fine...
This is a standard design, nothing new except I changed the number of turns on the transformer, normally the schematics give 10, I placed 16; on the antenna analyzer I have flat swr until about 50Mhz and 1.5 on 170Mhz, I am sure it will not work nice at those frequencies.
Calibration is easy, place on any frequency on "AM" or FM push the PTT and adjust the forward reading to the power you have (by rotating the FWD pot), if you have a 100w transceiver then adjust to 40 on the scale (that's probably your "AM" power), reverse the connections on the in/output antenna and transceiver press the PTT and adjust the pot of the "REV" part so the meter also gives 40. It's set, easy hum?
Diodes used were not the ones in the schematic, it was the BAT85 diode. I am sure it will work with the 1N4007.... very limited but will work...
On to the schematic: