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Field Strength Meter for the 137 kHz Band

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ANTENTOP- 02- 2003, # 003

Field Strength Meter for the 137 kHz Band


At 137 kHz the reactance of the coils is so low that it can be neglected against the 50 ohm resistor. Therefore when the coil pair in series with 50 ohm is connected to a signal generator with U volt output the current I through the coils in parallel is

I=U/50. (I in amps).

Note that each of the coils carries half of that current.

To check my set-up I made a single turn coil of 10 cm diameter and connected it via a coaxial cable to a selective level meter. I had calculated that with 1 volt applied to the Helmholtz coils in series with 50 ohm a voltage of 208 microvolt should be induced in the 10 cm coil when held between the Helmholtz coil pair. I measured 210 microvolt! Almost too good to be true. But upon checking everything again I found nothing wrong.

As I wanted the field strength meter to produce a reading of 1000 mV in a field strength of 5 mV meter I applied equation (2) and found that the corresponding magnetic field component is H=13.3*10-6 A/m. Using equation (3) the generator output voltage was found that would result in the wanted field between the Helmholtz coils. The field strength meter was put between the coils and RV2 adjusted for a voltage of 1000 mV on the digital multimeter. That completed calibration.


Measuring field strength

Try to find an open space at least 1 kilometre from the transmitter. Keep the antenna of the meter horizontal and tune the signal to zero beat. Now slowly increase or decrease the tuning slightly for a maximum reading on the DVM. Whether or not you can hear the beat note of about 36 Hz depends on the quality of your headphones. Turn around slowly to find the position for maximum signal. Now walk around a bit. If the reading varies the field is distorted by for instance a metallic fence, a lamppost or underground cables or pipelines. (On 137 kHz the waves penetrate tens of metres into the earth!) When a constant indication is found, multiply the reading in volt by five to obtain the field strength in mV/m.

Find the distance to the transmitter on a map or by other means (GPS!). Apply equation (1) to find radiated power. Multiply by 1.83 if ERP is required.

Good luck!

73, Dick, PA0SE


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A New one DX- LF QSO!


March, 2003



The tube PA worked at opened air at minus 3 up to minus 25 Celsius! 100 watts output is not bad at the very sever conditions...

Page 58


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