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You can measure field strength yourself when
you have access to a selective level meter.
Connect the meter to an untuned circular loop of
one turn.
When such a loop with a diameter of D meter is
positioned for maximum signal in a field of E volt per meter and frequency 137
kHz a voltage is induced in the loop of e = 0.00225*D^2*E (e in volt; D^2 means
D squared, * means multiplication).
To get a useful voltage the loop should not be
too small: give it a diameter of 1 m or so.
With a loop of 1.35 m diameter DCF39 produces 8
microvolt at my location.
In theory the loop should not be loaded by the
meter but at 137 kHz the impedance of the loop is so low that it makes hardly
any difference whether the input of the meter is open circuit, 50, 75 or
whatever ohms.
When you don't have a level meter but do have
access to a signal generator with calibrated output you could use it to measure
the input/output characteristic of your LF-receiver and then use the receiver as
a level meter.
I possess a signal generator with a tracking
selective level meter type PSM 5 made by Wandel & Goltermann but the
instrument is mains fed and very heavy so it cannot be used in the
field.
I therefore have constructed a small portable
battery-fed field strength meter. It is a direct conversion receiver with a
tuned and electrically screened ferrite antenna. The receiver has two audio
outputs. One is for headphones to tune the meter to the wanted
signal; the other for a digital multimeter set
to an AC range. The measured voltage has a linear relation to the field
strength.
I calibrated the instrument by placing it in a
known field generated by a pair of so called Helmholtz-coils connected to a
signal generator.
When the description of my contraption is ready
and circuit diagram and photographs scanned I will put it on the
reflector.
A very useful article in this respect was
written by Heinz Scherz, DJ3ON, in CQ-DL of April 1998 with title
"Feldstaerkemessgeraete und Ihre Kalibrierung".
Beware: In the centre column of the first page
you find a formula for the voltage induced in a loop when placed in an magnetic
field. In that formule pi^3 shopuld be pi^2!
73, Dick, PA0SE
JO22GD
D.W. Rollema
V.d. Marckstraat 5
2352 RA Leiderdorp
Tel. +31 71 589 27 34
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