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Field Strength Meter for the 137 kHz Band

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ANTENTOP- 02- 2003, # 003

Field Strength Meter for the 137 kHz Band





By Dick Rollema, PA0SE






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 e-mail 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.

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