I have made a few modifications to my antenna analyser; adding 'Molex' connectors to the PCB to make it easier to dismantle.
And after using an external power lead for the FT290, to power up the MFJ, which has the opposite polarity on the socket, blowing up the analyser. I've added a diode in the DC track between the socket and the on off switch, 'after the horse has bolted'.
This picture shows the position of the variables that need to be adjusted to re-calibrate the analyser:
R17 adjusts SWR
R18 is the output signal level adjustment.
The resistance adjustment pot is R31.
R35 is the input level to the freq. Counter.
I understand that the MFJ's are factory calibrated at 4 MHz.
First I set the output with R18 to full scale on the resistance meter, no termination on the output.
Then terminated the analyser with a 50Ohm load and set the resistance meter to 50 using R31. After that I set the SWR meter to 1:1.
I checked the readings over the entire range of frequencies to make sure that I achieved best results for accuracy.
With some resistors I set the values of the resistance and SWR meters, i.e. 25 Ohm and 100 Ohm should give an SWR reading of 2:1 and 75 Ohm should show 1.5:1.
After these adjustments I have made the MFJ a much more reliable instrument giving better reading of resistance and SWR, especially on the higher ranges.
This Page is © 
2000