This is very useful test instrument that every radio amateur should have in the shack and can be built for next to nothing. Let's look at this very handy instrument and its uses.
A passive RF field strength meter is a simple, un-powered device used to detect and compare relative radio frequency signal levels. It uses an antenna to capture RF energy, germanium diodes to rectify the AC signal into DC, and a sensitive analog micro ammeter to display the relative output level without requiring a battery.
Core Components
- Antenna: A telescopic whip or tuned dipole to pick up local RF radiation.
- Germanium Diodes: Low forward-voltage components (like 1N60 or OA47) used for RF rectification.
- Analog Meter Movement: A highly sensitive zero-center or standard 50µA / 100µA panel meter.
- Capacitor & Potentiometer: Optional bypass capacitors to filter RF and a variable resistor/potentiometer to attenuate strong signals.
Common Uses
- Checking transmitter output and relative power.
- Adjusting and comparing directional antenna radiation patterns.
- Testing the efficiency of baluns, tuners, and antenna setups.
I decided to build two of these instruments. In this issue we will be looking at the passive RF Field Strength Meter version.
Diagram
Bill of Materials
2 x Germanium Doides (IN34, OA70, OA90, IN60, BAT43 or 85)
1 x 470 pF (471) Cap
1 x 10 K Potentiometer
1 x 50 uA Meter
2 x DC Power Posts (Red + Black)
1 x Piece of Veroboard (PCB)
1 x 500 mm Stiff Copper Wire (Antenna) or old FM Radio Antenna
1 x Instrument Cabinet
1 x Potentiometer Knob
Construction
I am not going to go into detail as the diagram provides more than enough information to construct the meter successfully.
Images (Click on images for larger view.)
Video
