ANTENTOP- 02- 2003, # 003

Field Strength Meter for the 137 kHz Band

The instrument is fed by a 9 V battery. The LED is a small one that gives a clear indication of the instrument being switched on when drawing a current of only 2 mA.

To make the gain of the RF amplifier and mixer independent of battery voltage the supply for these stages is stabilized at 6.2 V by a zener diode. R12 was selected so that the zener keeps control for battery voltages down to 7 V. There is no need to stabilize the supply for the opamps because their gain is controlled by negative feedback and therefore hardly depending on the supply voltage. The instrument draws about 17 mA from a new battery.

Calibration

To calibrate the field strength meter the instrument must be placed in a magnetic field of known strength.

This can be produced by a pair of so called "Helmholtz coils".

German scientist Helmholtz already in the 19th century found by computation that a homogeneous magnetic field can be produced by placing two circular one-turn coils of radius r metres parallel to each other at a distance of r metres and with their axes coaxial. When a current I is made to flow through each of the coils in the same direction a homogeneous field of H=1/(1.40*r) (3) is generated in a considerable volume between the coils. H in A/m; I in amps; r in m.

I constructed a pair of Helmholtz coils with r=0.292 m as shown in one of the photographs. The coils are connected in parallel and in series with a 50 ohm resistor.

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