ANTENTOP- 02- 2003, # 003

Field Strength Meter for the 137 kHz Band

The voltage indicated by the DVM has a linear relation to the field strength. The meter is calibrated so that a field strength of 5 mV/m produces a reading of 1 V on the DVM.

The instrument can be tuned over the range 135.530 to 139.296 kHz. This includes DCF39 on 138.82 kHz which is useful for comparison purposes.

Description of the instrument

The antenna is a ferrite rod from a broadcast receiver with the original long wave coil in place. The rod is centered in an aluminum tube of 32 mm internal diameter and 145 mm length. I made a slot in the tube to prevent it becoming a short circuited turn. One of the photographs show how the rod is kept in place by two discs of perspex that are glued to the rod.

The electronic circuitry is inside a diecast aluminum box of 120 x 95 x 61 mm. The antenna is tuned by capacitors C1, C2 and C3. C2 and C3 were selected so that C1 can tune the antenna to the centre of the band. It then happened to be at its minimum capacitance as one of the photographs shows. R4 was added to widen the frequency response so that it is sufficiently flat over the range of interest. D1 and D2 protect the meter when used near a live transmitter. T1 amplifies the signal without loading the antenna circuit. It is important in a direct conversion receiver that the signal from the local oscillator cannot reach the antenna circuit. The rf amplifier, including C7, is therefore mounted on a separate piece of copper clad circuit board (not etched, I never use PCB's). Where insulated tie points are required I add small pieces of the same material to the board with instant glue.

RRATA Field Strength meter PA0SE

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Schematic is on the next page. The values of both L1 and L2 should be mH and not uH.

C15 should be 220 pF not nF.

The LED is shown upside down.

. The resistor feeding bias to the BB104:s has no value. However 100k is a good

choice which I have used in some other projects

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