Simple RF Detectors
1. Passive RF Probe
A useful and simple detector probe can be built with a few low cost components and a discarded ball point plastic pen case. The diodes can be Germanium (OA47, OA90 etc) or Schottly (HP2800, HP2835, BA482, 1N5711 etc).
The meter should be 50 or 100 microamps FSD unless some form of DC amplifier is to be used in which case it can be 1mA FSD. A 741 op-amp with its gain set to low (1-10) would be fine with a centre tapped DC supply or a second 741 can be used to provide a virtual centre tap for the meter return. If a DC amplifier is to be used then a DC return must be provided for the probe - about 470K. to
The detector components can be wired together using a small piece of printed circuit board or Vero board for mechanical support or just mounted on the metal ground screen before insertion into the insulated handle. About 1m of any small screened or coaxial cable will be suitable to connect to the detector.
When the probe is complete and tested a small self tapping screw or small amount of adhesive should be used to retain the electronics in the insulated handle.
2. Active RF Probe
This probe is a little more complex than the previous one and requires an operational amplifier to drive the meter. However, better sensitivity can be obtained over a wide frequency range provided that VHF construction techniques are used - short lead lengths, good groundplane etc. The low frequency response will be determined by the 1n capacitor between the attenuator and diode. To use at a lower frequency, increase the capacitor value.
The detector diodes should be VHF Schottky types such as HP2800, BA482 or similar. The input attenuator, which provides an input impedance of about 50ohms and has a 6dB loss should be made using 200mW or more surface mounting resistors for best VHF/UHF results. The 500K potentiometer is used to set the meter zero and should be accessible in use. A general purpose op-amp like the CA3140 will be satisfactory and can be run from a supply as low as +/-5v although the 10Meg and 500K resistors will need to be reduced to about one third of their values shown below.
Assemble the components on a small piece of printed circuit board with the 75 ohm resistors close to the edge. Use a BNC coax socket at the RF input with very short leads to the PCB. The entire assembly can be housed in a small die cast box or a light-weight hand held probe assembly depending on your preferences. The op-amp and meter may be housed with the detector or in a separate box.
The maximum RF input will be determined by the attenuator dissipation and the breakdown voltage of the detector diodes - one volt RMS in 50ohms is 20milliwatts. The completed assembly should be able to detect signals down to a few millivolts if the op-amp gain is high enough although the meter calibration will not be linear. If low frequency operation is required then the 1n capacitor between the attenuator and the left hand diode may need to be increased in value.
