Introduction
The power radiated by an antenna is equal to the radiation resistance
multiplied by the antenna current squared. Measurement of antenna
current can be done in the 137 kHz band by for instance a thermocouple
ammeter or other means. The unknown factor is the radiation resistance.
Computer programmes for antenna simulation can produce a value
for the radiation resistance but proper modelling the antenna
is not always easy. Another problem is the influence of the earth.
The ground constants are seldom known and even if they are it
is not certain that the computer program applies them in the correct
way.
A more reliable
way of determining radiated power in the 137 kHz band is by measuring
the field strength near the station but outside the near field
region. A distance of 1 km is probably sufficient to reduce the
influence of the near field on the measurement sufficiently and
2 km is definitely safe.


At such a distance we are in the far field of the antenna
but near enough so that the field strength does not depend on
the type of ground. When a strength of the electric field of E
mV/M is measured the radiated power follows from a simple equation:
P=0.0111(E*d)^{3} in which; (1)
P in watt
E in mV/m
d in kilometers
* means multiplication
The equation produces the power really radiated by the antenna, in other
words the power "dissipated" in the radiation resistance.
Note that this is not the same as ERP. By definition
ERP is the fictitious power to be fed to a half wave dipole
in free space that produces the measured field strength. As
Rik, ON7YD, has pointed out in his email of June 25 a short vertical
(and our 137 kHz antennas are always short) has a theoretical
gain of a factor 1.83 (2.62 dB) over a half wave dipole in free
space.

So if you want to know your ERP multiply the power
given by equation (1) by 1.83 (or add 2.62 dB). But apart from
a regulations point of view I see no advantage in using ERP. The
actual power radiated by the antenna is what counts.
I have a feeling that some amateurs talk about their
"ERP" when they mean "radiated power". Maybe
I'm wrong; I hope so.
Most
field strength meters do not measure the electric but the magnetic
component of the electromagnetic field. But this is no problem
because in the far field of the antenna (where we measure) there
is a fixed relation between the electric and the magnetic field
components:
E/H=120*pi ohm=377 ohm (2) in which:
E in V/m and H in A/m.
The
portable field strength meter to be described is a direct conversion
receiver with two audio output signals. One is fed to headphones
for tuning the meter to the signal to be measured. The other output
feeds a digital multimeter.
