To All from PA0SE
There seems to be some interest to modify my
design for a portable field strength meter for use on 500 kHz. The
instrument is described in G3LDO's boek
The LF Experimenters Handbook.
But the circuit diagram has some errors.
The description can also be found
at
http://www.veron.nl/tech/lf/fsm/pa0se.htm. Better use
that one.
I used a ferrite rod antenna from a transistor radio with its
LW coil. I expect the coil is too large for tuning the antenna circuit to 500
kHz.
You could try a MW coil or a LW coil with turns removed.
The
article also describes how the instrument can be calibrated using a pair of
Helmholtz coils. The coils are fed from a signal generator in series
with a 50 ohms resistor. I found that my coils have an inductance of 1.2
microhenry. At 500 kHz that gives a reactance of 2.7 ohms.
This can
be completely be neglected against the 50 ohm resistor that is in series
with the reactance..
So the current through the coils is the generator
output, divided by 50 ohms. Note: the two coils are in parallel so each of
them carries half the current. The article describes how to handle
this.
One has to be careful about what the meter on the signal
generator indicates. On some models the meter indicates the EMF. When loaded
with a resistor equal to the internal resistance of the generator the output
voltage is half the EMF.
But on other generators the meter indicates the
voltage
at the terminals (potential difference) when the generator is
loaded with a resistor equal to the internal resistance of the
generator. This PD is taken as half the EMF.
A simpler way to
measure field strength is to use a single turn untuned loop, connected to a
calibrated selective voltmeter. I did it that myself on 136 kHz after I was
presented a Wandel & Goltermann SPM-12 selective level meter that can also
be run off a car battery.
In a earlier e-mail to the reflector I
included the equation that gives the relation between the field strength and
the voltage induced in the loop.
But my provider or the one of the
reflector did not accept it because the message "scored 5.8 spam points"
as the Mail Administrated said.
I had not realised I had used dirty
words.
This is the method that I use, the tricky part is getting the loop
antenna calibrated accurately, more complicated than you think, for absolute
accuracy.
