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
