This quick and dirty field strength meter ive knocked up really isnt doing well! It would just about deflect with a 3W VHF handheld right by its antenna. So, I added a FET...
Very simple, its now little more than a crystal set with a 2N3819 FET and a 5k potentiometer. Powered from 3v, the pot allows me to swing the needle across the whole deflection range, yet when held within inches of a 4W lowband VHF transmitter - barely a movement!
Oh well, i'll look at it more tomorrow. Maybe its because im only using 3v instead of the customary 9v? Or, maybe its the meter movement isnt sensitive enough? But the multimeters minimum DC current range was 500uA, so im presuming its a 500uA movement?. Or perhaps its the fact that the original 'dc path' resistor, which is now the FET gate bias resistor, is just 10k...
Tomorrow, i'll swap that resistor for a 1M unit, and see how that works.
Oh well, couldnt wait. Went and added a 1M in series with the 10k, measured the meters FSD current (its near enough 0.5mA to call it a 500uA movement), even tried the FET the other way around (2N3819s are near enough reversible), and it still doesnt work! Oddly, close to the 4W Tx the movement deflection is actually towards the lower end of the scale!
Only thing left to try is the battery...
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