Field Strength Meter

A field strength meter is the only objective way to determine whether a change to your antenna system has made your ground-wave signal stronger or weaker. Really, there's no point trying to optimize your antenna system if you have no way of knowing whether the changes you make are improving or worsening the most important parameter, namely the intensity of your signal.

As shown above, a crude field strength meter can be made from a micro-ammeter, a diode, a choke, and an antenna 2 or 3 feet tall. This circuit may be adequate for some applications, such as judging the results of antenna tuner adjustments.

If more sensitivity is needed, a circuit that includes some amplification is called for.

The circuit above is a relative (not calibrated) field strength meter powered by an AA battery. It must be built in an aluminum box, and the ground symbols on the schematic represent a good connection to the box. This circuit probably would not work well at frequencies above 30 MHz.

Parts:
C2: .001 uF ceramic disk
D1: 1N34
J1: jack for antenna; the antenna must be insulated from the case
Q1: 2N107 or GE-2, etc. (This is a pretty old circuit design. Any equivalent PNP transistor should do the trick.)
R1: 50K potentiometer
R2: 1K, 1/2 watt
R3: 1K, 1/2 watt
meter: milliammeter, 0 to 1 mA range
antenna: a 30 to 50 centimeter piece of stiff wire, or a generic telescoping antenna
L1 and C1 are chosen so that they form a resonant tank circuit at the frequency of interest.

To use, turn on transmitter and meter; adjust C1 for peak reading. Turn off transmitter and adjust R1 for a perfect zero reading on the meter. Turn on transmitter (with no audio modulation) and go to your monitor point(s) and take readings. Your monitor point(s) should be many meters/yards away from your antenna, but obviously must be where the signal is still strong enough to register on your instrument. After taking a set of readings, you can experiment with your antenna or transmitter system, then take another set of readings and determine whether your signal strength was increased or diminished. Keep your readings in your engineering logbook. Make note of whether it's been dry or rainy in recent days; moist soil might give slightly better propagation.

You might find that the meter reading changes slightly depending on whether you are touching the case and how close you are standing to it. Be consistent and always hold the meter the same way when you are using it. You might consider attaching a non-conductive handle to the case. Don't put the meter on a car or near any metal object while taking a reading.

If you take a lot of readings at various distances from the antenna, you can write the readings on a map and visualize your signal as it relates to the local landscape.

The ARRL Antenna Book contains a more advanced FSM design.

 
return to Engineering Dept.