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.
