Low Frequency Field Strength Measurements

Most amateur radio operators are familiar with "field strength meters". These are usually simple devices with a whip antenna, a diode, a meter and a handful of parts. Such devices give a relative indication of RF hotspots, and need to be used in the vicinity of the transmitting setup. This article describes a much more complicated portable device for the 130 - 200 kHz range, the process for calibrating it, and gives tips for planning, making and analyzing measurements from 3 to 20 km (or more) from the transmitting antenna.

What are you measuring? Electromagnetic waves are (no surprise) composed of electric fields and magnetic fields. Typical notation gives electric fields as "E" (units of Volts/meter), and magnetic fields as "H" (units of Amperes/meter). At distances of at least a wavelength from an antenna, there is a fairly simple relation between those fields, where the fields are at right angles to each other, and E/H = 377. Radio people rarely speak of magnetic field strength, preferring to use that relationship and stick with electric fields. One notable exception is the area of the hazards of RF radiation near antennas, where E and H fields don't follow that simple expression, and their biological effects may be different. For our purposes in evaluating transmitting setups for efficiency and regulatory compliance, we will mostly stick with E fields. The catch is that the meter being described actually measures H fields! More on that later, however.

Why? Until recently, most amateur radio regulations specified maximum transmitter DC input or RF output power, and an operator was "safe" when staying within those limits. Nothing was usually said about the antenna system, or the field strengths that would be produced. Combinations of legal-limit transmitters and high-gain antennas could produce signals considerably in excess of what would be expected from an average transmitter and a simple dipole antenna. Other radio services, however, have long been regulated by "Effective Radiated Power" (ERP) limits. With the advent of the 60 meter amateur band at HF, and the 2200 meter band at LF, we have now joined the club. The idea is that a reference antenna (usually a dipole) in free space produces definite electric and magnetic fields at a given distance for a given power. Since a dipole is directional, we usually consider the maximum field that it can produce, which is in the direction perpendicular to its axis. For example, a dipole in free space produces 7 millivolts/meter at a distance of 1 km when fed with a power of 1 watt. So if a regulatory agency limits you to 1 watt ERP, you can achieve that by some combination of transmitter power and antenna efficiency that produces a maximum electric field equivalent to 7 mv/m at 1 km. Note that I'm dancing a bit with the word "equivalent." This is in deference to the 2200 meter band, where measurements should not be made that close to the antenna, as there are E and H fields that drop off with the cube and square of the distance, and they will affect your readings.

How do you measure field strength at LF? Field strength readings are taken with an antenna and a receiver that are calibrated together to give results in volts/meter. Here is an article that discusses the choice of a receiver and design of an antenna for FS measurements in the 2200 meter band:
Download (in PDF format) Receivers and Antennas for Low Frequency Field Strength Measurements

How do you calibrate this stuff? The 2200 meter band is very forgiving of construction and measurement techniques. If you have some reasonably common test equipment for LF work, you should be able to calibrate the combination of a small loop and a Selective Level Meter to make accurate field measurements. Here is an article that gives the details on on the setup I use:
Download (in PDF format) Calibrating a Low Frequency Field Strength Meter

Where do you take the measurements, and what do you do with them? OK, now it's time to find out about the real world! You will need advice on selecting good measuring locations, how close to your antenna you can make measurements, and then how to analyze the data once you have it. Here is an article covering that and more:
Download (in PDF format) Choice of Locations and Interpretation of Data from Low Frequency Field Strength Measurements

Results on measurements at WD2XES: An upcoming article will discuss a series of field strength measurements on WD2XES. Stay tuned.

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