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 setup 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*106
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
Download the article (Word2) and pictures as
a zip file (223k)
http://www.veron.nl/tech/lf/fsm/pa0se.zip
