Field Strength Meter Monday: General Overview

Having an RF field strength meter on hand is perhaps one of the best and most multi-purpose piece of test equipment any electronic hobbyist, ham radio operator or not, should own if they are doing anything with wireless signals under 1 MHz to over 3,000 MHz!

In 2006, the Britain based Shefford & District Amateur Radio Society released an LED bargraph relative
field strength meter kit for its members and others to copy. Clones of this kit can now be found on E-Bay. 

Brief Overview

Calibrated and relative field strength meters serves different purposes. A calibrated one is much harder and expensive to design unless its for a narrow frequency range and calibrated against a known signal source at all critical design frequencies for linearity purposes. 

The output measurement of a calibrated meter would best show the user in a standardized measurement unit such as decibels per meter (dBm), micro-volts (mV) or milli-watts (mW).

Analog meter showing dBm and micro-watt measurements

A relative field strength meter differs in that the user would first take a first measurement of a baseline signal reading before making any changes to the signal source or measuring device which may impact an increase or decrease in signal visualized on the field strength meter.

Measurement units can be a range scale, such as 1 to 5 on an un-calibrated or relative field strength meter.

Having a broad banded relative field strength meter is very useful, but also having some basic form of calibration makes it slightly more useful for certain applications.

A basic "unbiased"  and un-calibrated relative field strength meter

There are very few components needed to construct a basic relative RF field strength meter and there is not even a  power source needed to worry about to make it work. The below circuit will detect RF signals up to and slightly over 500 MHz.

The circuit theory is rather simple. Signals being picked up by the antenna are coupled through a 100pF capacitor (C1) to the junction point formed by two matched 1N34 diodes. The 10K ohm resistor (R1) and .001pF capacitor (C2) provide the function of a voltage divider to "smooth" out the signal generated by the diodes. Variable resistor R2 provides a level of control to prevent the ammeter from being fully deflected by a strong signal. It also provides some attenuation functionality when dealing with ovwrly strong signals. A value of 50k ohms for R2 and a 50 mA meter are suggested to round out a parts list.  None of these component values are critical and is what makes a relative field strength meter fun to experiment.  The antenna length should be close to a 1/4 wavelength of the signal being monitored or some user standardized random length, like 1 foot.

Essentially, all the above circuit does is takes RF energy, converts it into a rectified voltage and is then read on a visual meter. 

This circuit and many like it have been around for over 100 years. The only major advancement with this basic circuit concept has been in the type of diodes used based on sensitivity.  1N34 or any similar small signal germanium based diode has been the choice of many, but is not the only option out there.  

The other change is how to visualize the signal. You could use a moving analog meter, a digital meter, an LED bargraph or anything else you can think of to show the signal visualized in some way.

Using a very common 1N914 diode will provide a usable circuit, but is not as sensitive as the 1N34 diode.  You could even use 2 low voltage LEDs or certain transistors like a generic 2N2222, which essentially act as two diodes connected together in the circuit above. One transistor to experiment with would be the 2N2222 NPN transistor.  There are also PIN diodes and Schottky diodes that can also be used which are thw most sensitive when properly biased.. You may be surprised what else can be used to detect RF signals.

An RF signal will need to be much stronger to power the junction of a 2N2222 transistor or PIN diode, so a more advanced circuit is needed that adds in a 1.5 volt or greater power source to "bias" the transistor and thus creating a slightly more sensitive circuit compared to the 1N34 based circuit. 

This circuit is more sensitive because of the 2N2222 acting as an amplifier after the single 1N34 detector diode. This circuit will detect signals from a further distance when using the same length antenna compared to the simpler unbiased circuit.  This more advanced circuit will work up to about 500 MHz, but may show other readings as high as 3000 MHz.  It will likely NOT be useful for measuring WiFi at 2.4 GHz because those signals are NOT the same as traditional ones broadcast by those involved in ham radio using modes like FM, SSB and CW, but there are ways to design a circuit to detect 2.4 GHz signals as well as 5.8 GHz.

Calibrating a relative field strength meter

There are ways to calibrate circuits such as the above by using some filtering before the signal detector to limit the frequency range before it is detected by the field strength detector, but it gets a little complicated and is why calibrated measurement tools are somewhat expensive since they need to show the same measurement across the entire frequency range accurately. Last year, HVDN released an article discussing measurement units which closely ties to different frequency ranges.

What values and circuit design works at 7 MHz, 14 MHz or 27 MHz will not work the same as at 146 or 440 MHz.

Calibrating a field strength meter makes it almost closer to a wave or watt meter which can measure much stronger signals directly. A field strength meter is typically used for indirect measurement.

Build, purchase or modify?

If shopping for a used field strength meter on Ebay for example, do not confuse a meter that has a knob showing different frequencies and think it is calibrated. It is still a relative field strength meter, even if the meter shows mW, dBm, etc.  At some point, some commercial models, such as those sold as CB SWR/FS meters may have been calibrated against a known signal source to provide a full scale reading based on common industry levels, but how do you know if it is accurate still?

The QRPGuys Digital Field Strength Meter can detect RF energy over the VLF-500MHz range with sensitivities from -80dBm to +10dBm. It uses the popular Analog Devices AD8307 logarithmic detector/amplifier

There are certain meters that have well documented modifications to increase functionality or accuracy.  There is alot to learn about design, use, purchasing of F.S meters.

The HVDN Field Strength Series

Beyond this and the other article mentioned, future articles will cover:

• Part 2: 3 things to do with a basic relative field strength meter
• Part 3: Updating a basic field strength meter for the modern world
• Part 4: Building an Arduino Nano or ESP8266 based modern field strength meter
• Part 5: Calibrating your Arduino Nano based modern field strength meter
• Part 6: Using your Arduino Nano based modern field strength meter
• Part 7:  Remotely monitoring a modern field strength meter
• Part 8:  And what about Wi-Fi and drones plus signal measurement?

This series will be updated every Monday, starting April 30th 2018, so please subscribe to the HVDN Notebook for updates about field strength measurement and other articles.

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