ANTENTOP- 02- 2003, # 003

Field Strength Meter for the 137 kHz Band

The remaining part of the circuitry is on a second board. Here we find the mixer, an IC type SA612BN, incorporating the oscillator circuit. The ubiquitous NE612 can be used as well.

The values of C10, C11 and C16 were dictated by the choice of coil L2. I used a 2 mH hf choke from my junkbox, as shown in one of the photographs. C10 and C11 are the usual capacitors found in a Colpitts oscillator. C12 isolates L2 from the DC on pin 6 of the mixer. I found the BB104 type dual varicaps in my junk box. Two in parallel were necessary to obtain the required tuning range. The lower frequency was set by selecting C16. The upper limit was found to be a bit high and this was corrected by adding R7. RV1 is the tuning control. (The unmarked resistor between the wiper of RV1 and the varicaps is 120 kohm.)

Selectivity of a direct conversion receiver is determined by a low pass filter in the audio path. A high degree of selectivity is required here because of the extremely strong station DCF39 at 138.82 kHz, only 1020 Hz above the upper limit of the band. I use a RC-filter with three sections, each having a time constant RC=1 mS. At first resistors and capacitors of the same value were used in the three sections and this is the situation seen in one of the photographs.

Later I realised that a better response is obtained when the loading of a section on its preceding one is decreased and this resulted in the values seen in the circuit diagram. The lower limit of the frequency response is set by the time constant RC=(R17 + RV2)*C21 respectively RC=R16*C20. The response of the metering circuit shows a maximum at about 36 Hz and is 3 dB down at 16 and 88 Hz.

The output of the low pass filter is fed to the two sections of a dual opamp type UA747. The upper opamp feeds the headphones. Volume is controlled by RV3 in the feedback path.

The lower opamp feeds a digital multimeter that must be capable of measuring AC in the millivolt range up to about 2 V. Preset resistor RV2 is adjusted when calibrating the instrument. I choose to make the audio output, as indicated by the DVM, 1000 mV when the instrument is placed in a field of 5 mV/m. The reading is linear up to about 10 mV/m maximum (2 V on the meter).

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