RF Probe for HF

Here is a useful circuit for the RADIO frequency experimenter and HAMs to check thepresence of RF signal and to measure its strength. The device indicates the RF strength on a calibrated micro ammeter. The sensitivity of the probe is in the micro volts range. The circuit is well usable in full HF spectrum ( up to 30 MHZ). An attenuater is also added to measure high signal strength. Attenuater is not needed if the use is restricted to measure signals in the microvolt range only. The probe not only indicates the strength on a meter, but also provides an amplified signal of the given input is at the output. This output can be used for demodulation, etc. The circuit is based on the FM receiver IC LM 3089 manufactured by National semiconductor. Here the IF amplifier part, the level detector section and the tuning output section of the IC are used for our purpose. The IC has a tune meter drive output to indicate the signal strength in dB. A meter with 150 microamperes full-scale deflection is sufficient for indication. A VU meter used for audio system is sufficient is not very critical about accuracy. The meter can be calibrated by using a known source or by using a standard S meter.

Fig 1. Circuit Diagram of Keyer


The attenuater consists of series of stepped attenuater sections having 3 dB, 10 dB, 20db, 20db and 20-dB attenuation. Each attenuater section can be inserted into the circuit by operating the switches in each section Total attenuation possible is then 73 dB. More attenuater sections can be added if required. Good quality double pole switches are recommended for the attenuater switch and watt 5% carbon composition or metal file type are recommended for the resistors. An experienced home brewer can easily construct the attenuater sections by mounting the resistors on the soldering terminals of the switches itself. However a PCB for the same is included in this article. It is highly recommended for the beginner or one who is not very confident to use the PCB for attenuater. The switch used is Philips locking type push switch. It is of low cost and is easily available. A high power attenuater section of 20 dB must be added at the front end to use this meter for measuring RF power amplifier output. Construction of high power 20 db attenuater be done with high wattage resistors and should have sufficient ventilation for the resistors to cool down. A suitable metal enclosure is needed for the attenuater and the probe circuits to prevent stray signal pickup.


Construction is straightforward. Assemble the components on the PCB whose layout is given in the figure. A good quality coax wire should be used for connecting the source to be measured and the probe circuit. Use minimum length for the cable. The diodes 1N4148 are used for protection of the IC from excessive RF voltage. These diodes can be omitted for measuring low voltage RF voltages such as receivers. The tuned circuit that consists of L1 and C1 is added to the circuit for selectively measuring the signal strength. L1 and C1 must be tuned to measure the desired frequency. The tuned circuit can also be omitted. A 9 volts battery is recommended for the circuit. Battery drain is about 30 milli amperes (maximum). A good ground is also recommended for measurement.


The meter calibration is to be done with great care for better results. The pot meter P1 is used for this purpose. If a micro ammeter with lower FSD current then suitable shunt resistors are to be used.

Go Back