Project 11 - Field Strength Meters

Project of the Month

A couple of field strength meters.

 

This is two projects. A simple field strength meter to tell if your radio / antenna is working, and a full blown powered meter to detect RF fields in your vicinity.


Field Strength Meter 1

 

This simple field strength meter is ideal for testing if your antenna or handheld radio is working. I use this exact meter everyday for testing handheld radios. It only has a range of about 30cm, much less on a 500mW (half-a-watt) handheld, but it tells you if it is transmitting. The sensitivity depends on the sensitivity of the meter you use, so try a few different meters if you have a few available. I have found that level meters out of old tape decks have good sensitivity. If you are going to buy a meter, get a 1mA (milli-amp) meter.

 

Make a coil out of some stiff wire. Thickness doesn't matter, nor does it matter whether it's insulated or not. One and a half turns about the size of a 20 cent piece. Leave the ends about 10cm long. Solder a 1nF (0.001uF) ceramic capacitor across the meter. Solder one end of the coil to the negative (-) of the meter. Connect the other end of the coil to the anode (a) of a germanium diode. Connect the cathode (k) of the germanium diode to the positive (+) of the meter. That's it.

The coil antenna picks up RF radiation, the diode rectifies it, the capacitor filters it, and the meter displays it. Dead simple.

   


Field Strength Meter 2

 

This one is quite a bit more complex than the first meter, but it is great for proving once and for all where that pesky button-pushing / music-playing is coming from. We will build this in two sections. The first section is the antenna amplifier. This part is handling the RF, so the circuit must be built compact, with leads kept as short as possible. The second part is the DC amplifier, which is built on a piece of veroboard.


Step 1

Select a housing for the project, and fit the antenna socket. I have used a plastic box from Altronics, part number H-8975. I have fitted the power switch next to the antenna socket, but where you put yours is entirely up to you. Cut some copper sheet to fit into the bottom of the housing. This will act as a shield for the circuit, and a ground plane for the antenna. Solder the antenna socket ground to the shield. If you use a metal box, the copper sheet will not be necessary.

 

Step 2

 

Build the antenna amplifier circuit. In order to keep the leads short, and all the ground connections close to the antenna socket, I decided to solder the components directly to the copper sheet. The picture below shows the first few components mounted. If you build the antenna amplifier on veroboard, keep leads as short as you can, and cut tracks after the last component on the track. Also shown below is the veroboard version of the antenna amplifier that I built before deciding on the shield mount. The transistor is mounted on the copper side of the veroboard because the legs did not fit through the holes.

 

The transistor and the detector diode determine the frequency range of the field strength meter. The transistor is a BFR91, but these are not common anymore. Prime Electronics has them in stock, but you can substitute it with any transistor that can handle UHF frequencies. The 1N5711 from Dick Smith is a good diode, but if you can't get one, you can use any high-speed-schottky diode, or failing that, an ordinary germanium diode has an adequate frequency range. If you use a high-frequency transistor and diode, this field strength meter will detect signals well into the GHz range, and can be used to detect wireless LANs, mobile phones, and wireless surveillance equipment.

 

How it works - Antenna Amplifier

 

All the components connected to the transistor Q1 form a normal RF amplifier, which amplifies signals of any frequency coming in the antenna socket. When the transistor is amplifying, the collector (c) rises and falls in voltage - at the frequency of the incoming signal. The rising and falling voltage pushes and pulls current through the 10nF (nano-farad) capacitor that is connected to the collector. This is called forward current and reverse current.

 

The cathode (k) of the detector diode is held at a fixed voltage of 0.835 volts, called the reference voltage, by the voltage divider formed by the 4K7 resistor in the DC amplifier and the 1K5 resistor on the cathode of the diode. The 4K7 and 560R resistors across the diode feed the reference voltage back onto the anode (a) of the diode, so that when there is no RF, the voltage on both sides of the diode are the same. When RF is amplified by the transistor, and passed to the diode by the 10nF capacitor, forward current through the diode is passed to ground through the 10nF capacitor on the cathode of the diode. Reverse current, however, has nowhere to go and pulls the voltage on the anode of the diode down. There is now a voltage difference between the anode and cathode of the diode. Read 'how it works' for the DC amplifier circuit to see what happens next.


Step 3

 

Build the DC amplifier. When building the circuit into the case, make sure you leave room for the meter. If you can, use a schottky diode for the reverse polarity protection diode D1. Schottky's have lower forward voltage drop, so you will get more life out of the battery. IC1 is a 5 volt voltage regulator, LM2931. We could use a 7805, or the low-power version 78L05, but the LM2931 is a very-low-power, low-dropout regulator which will extend battery life greatly. Mount the two 10uF tantalum capacitors as close as possible to the input and output leads of the voltage regulator. Voltage regulators tend to be unstable if there is long leads or no capacitors on their input / output.

 

There are many op-amps with the same pin-outs as the LM308 and LM358, but don't be tempted to substitute either of them. These op-amps were chosen because they have particular properties that we need in this circuit. The LM308 has a low input offset voltage, and the LM358 common-mode range includes ground. If you use any other op-amps, this circuit will not work properly.

 

The meter is, again, a level meter out of an old tape deck. If you are going to buy a meter, get a 1 mA meter.

How it works - DC Amplifier

 

A low-voltage schottky diode protects the field strength meter against reverse battery connection. While the antenna amplifier runs directly from the 9 volt battery, the DC amplifier runs from a regulated voltage. Otherwise the readings will be dependent on the battery voltage. The regulated 5 volts is supplied to the two op-amps, and the 4K7 resistor for the reference voltage.

 

The first stage of the DC amplifier, IC3, is called a 'differential amplifier'. That is, it amplifies the difference between the voltage on it's negative and positive inputs. The ratio between the input resistor and the feedback resistor determine the gain of the amplifier. In this case the input resistor is 10K and the feedback resistor is 1M, so the gain is 100. The input signal goes into the negative input of the op-amp, so this is an 'inverting' amplifier. If the voltage on it's input rises, the voltage on the output falls. The reference voltage is connected to the positive input, so with a gain of 100, if the voltage on the negative input falls lower than the reference voltage by 1mV (millivolt), the output will rise above the reference voltage by 100mV. The differential amplifier is amplifying the difference between the 'ref' and 'sig' inputs.

 

The 'battery test' push button switch shorts the 'sig' input to ground, which simulates a massive signal. The op-amp output will swing to maximum, and the needle on the meter will swing right over to full scale deflection. If the needle does not go all the way to maximum when this button is pressed, the battery voltage has fallen too low for reliable readings.

 

The negative side of the meter (the actual meter) has to be at the reference voltage, but we can't just connect it to the cathode of the detector diode. The meter is too big a load, and will cause the reference voltage to fluctuate. Readings will be meaningless. We will 'buffer' the reference voltage by passing it through an amplifier (IC2a) that is configured to not amplify. This circuit is called a 'voltage follower'. It's called that because it's output voltage is always exactly the same as it's input voltage. But, the op-amp doesn't put any load on the voltage on it's input, and will hold the voltage on it's output rock steady while taking all the load of the meter.

 

The output of the first stage of the DC amplifier feeds into the positive input of the second stage of the DC amplifier, IC2b. This is a 'non-inverting' amplifier. The gain of the second stage is set by the ratio of the feedback resistor to the reference resistor. With VR1 set to maximum resistance the gain is 101, at minimum the gain is 1.

 

VR2 adjusts the meter current for full scale deflection.


Step 4

 

Make sure you have a good battery in the field strength meter. Press the 'batt test' button. Adjust VR2 until the meter is full scale. The only other adjustment is VR1, the sensitivity. Adjust VR1 to the sensitivity you want and that's it!

Shown here is my meter, on maximum sensitivity, detecting my mobile phone from 1 metre away.


Final Notes

 

If you require more sensitivity, change VR1 to 1Meg ohm. This will give the second stage a maximum gain of 1000. If you want a range control, remove VR1 and substitute it for a rotary switch and four resistors of 1K, 10K, 100K, and 1M, and label the switch positions 1000, 100, 10, and 1.

 

As it turned out, my analogue meter did not fit inside my case, so I fitted a LED digital meter. I figured this would be better because I wanted to be able to use it at night, and backlighting the analogue meter would flatten the battery too quickly. This was a mistake. Dancing numbers are harder to read than a needle, especially when mobile, so I built another field strength meter, this time with a LED bargraph display using an LM3915.


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