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What Determines How Far Your Signal Goes?

In AM, Look to the Ground Below Your Feet

Strong in Your Field (Exam level: CBRE)

In the Oct. 12 issue of RWEE, we asked:
What factor(s) most affect the predicted or measured far-field signal strength of a nondirectional AM station?

a. Transmitter power
b. Antenna power input, antenna height
c. Antenna power input, antenna height, radial count
d. Antenna power input, antenna height, radial count, top loading
e. All other factors being equal, ground conductivity

Before we get to discussion of our question from last issue, might I mention that the time between columns has been one of introspection for me. I realized that I am fast approaching becoming a septuagenarian, which in the scheme of seniority is one down from an octogenarian.

Among other consequences is that you tend to clear your desk rather than postpone things because you might not be getting back to them. You spend more time with your 2-1/2-year-old grandsons, the only ones who truly understand you. Reflections on the past become more numerous simply because there is more past than future, and in life’s rear view mirror, everything is far clearer.

There is a Chinese adage normally attributed to Buddha: “Not having a goal is more to be feared than not reaching one.” Thisis so true, especially as regards growth in our profession. To whip out another pertinent point: Somewhere in the first week of medical school, all the students are told that if you’re not growing, you’re dying.

Might I urge you to set a personal goal to grow. Resolve to become certified or to advance at least another grade or add a specialty certification. Advancement in certification is good for you and our profession — and proof that you’re not retired in place.


Back to the question. The correct answer is e.

Signal strength falls off according to the inverse square law in far fields; that is, it decreases in a ratio to (is divided by) the square of the distance from the antenna. For standard AM, this behavior is consistent in places like outside the earth’s atmosphere, where you might encounter an atom of something every few hundred miles and the radio energy is unaffected by stars, planets and other celestial bodies and magnetic fields.

Here on earth, calculating AM radio field strength in the near field includes factors such as antenna power input. This is the power level from the transmitter after you subtract transmission line losses.

Tower height/antenna length (of which top loading is a component) is another important factor as it affects the efficiency of the antenna. Antennas of ideal height concentrate electromagnetic radio waves toward the horizon and thus provide a stronger signal on the surface of the earth without wasting energy straight up into the sky. Ground radial count and their length again is another efficiency factor.

However, once that signal is launched, the signal has to pass over and then return through the land or sea. In radio parlance, this ground propagation is affected by a factor called conductivity. The value of conductivity sets the signal loss over distance as measured in wavelengths. Put simply, electrical conductivity is the measure of the amount of electrical current a material can carry. AM is unique in broadcasting in that we depend mostly on this signal propagated through the ground.

The graphic representation of the field radiation pattern of a typical 1/4-wave antenna indicates that most of the signal is released along the horizontal (see Fig. 1). The graph for the 5/8-wavelength antenna demonstrates an additional small lobe above the horizon. The horizontal component is the ground wave and highly affected by conductivity. Getting these currents in and out of the ground is the primary reason for all those ground radials and the choice of ideal soil and location. As Dr. Frederick Terman tells us, “The earth is like a leaky capacitor.”

Fig. 1: Taller towers tend to focus radiofrequency signals toward the horizon, an advantage in broadcast AM. As towers are increased in length beyond a full wavelength, their efficiency decreases, making the range of ideal tower heights roughly between 1/4 and 5/8 wavelengths.

The above ground lobe is the skywave, and is radiated towards the atmosphere, where ionospheric reflection (incident angle of reflection, layer hardness and other factors) have the most influence on the signal’s carriage. After sunset, when the ionospheric reflection is greatest, skywaves are receivable at very long distances, although they are subject to random fades and noise. Most AM stations measure their coverage area based on their ground wave.

Electrical conductivity is denoted by the symbol σ and is measured under the metric SI convention in siemens per meter. The term gets its name from the German inventor and industrialist Ernst Werner von Siemens, and whether singular or plural is always written in the plural. In the world of radio, millisiemens (mS) is a more apt unit as it provides whole numbers for a greater range of the values we work with on a daily basis.


Classically, the FCC and the radio engineering community have looked to the M3 chart for a generalized description of the conductivity of the continental United States (see or search online for “FCC M3 map.”).

While the M3 conductivity values are useful for estimating the value of conductivity on a rough basis, in many areas, especially rocky or mountainous terrain, they are not detailed enough for good accuracy.

An important factor in calculating AM field strengths for FCC purposes is that localized actual readings of field strengths can be substituted for the generalized conductivity conditions shown on M3. The ordinary circumstance where this is beneficial to the radio station is where lower than anticipated conductivity would allow a new facility to be shoehorned into a desirable market, or a substantial increase in power can be achieved for an existing facility.

In these cases your local measurements have to be accurate, sufficient in number and properly taken to support your supposition that the facility you are proposing will actually perform as you postulate rather than those conductivities shown on M3.

The actual measurement of AM field intensities — such that they can be submitted and also repeated by others — is a skill I covered in a past article; see

Fig. 1: Taller towers tend to focus radiofrequency signals toward the horizon, an advantage in broadcast AM. As towers are increased in length beyond a full wavelength, their efficiency decreases, making the range of ideal tower heights roughly between 1/4 and 5/8 wavelengths.

Sometimes you have to suffer for your art. The dashing gentleman in the picture is me. In the tundra it is freezing cold, the wind is blowing 120 mph and we are above the Arctic Circle. (OK, the truth? It was chilly that day near New London, and we were more than a mile from a good restaurant.)

Conductivity can range from 5,000 mS for salt water as an extraordinary high (little loss) to a miserable low of near 0.5 mS, such as you have on the moraine field called Long Island. The national average for conductivity is about a 4.

This efficient ground wave carriage over sea water can create havoc for coastal stations such as the now-defunct 1510 in New London, Conn., and the 1510 in Boston when it was located closer to Boston Bay. The area of interference in between was quite notable at times.

Charles “Buc” Fitch, P.E., CPBE, AMD, is a frequent contributor to Radio World. Missed some SBE Certification Corners or want to review them for your next exam? See the “Certification” tab under Columns at

Function Like Bessel Question for next time
(Exam level: CPBE)

What is a Bessel function and what is a typical application?

a. A numerical description of propagation through a solid body, most often used in coax design.
b. A thermal transfer derivative, most often used to enumerate fluid cooling such as around the new solid-state components in liquid-cooled transmitters.
c. A solution to a particular type of equation, used in broadcasting mainly as a determinant of FM modulation levels.
d. The delta change in free air temperature, most often used in broadcasting to enumerate non-linear coax expansion and the potential for shearing.
e. The delta change in wire temperature as a function of uneven harmonic currents, most often used in broadcasting to enumerate the capacity of neutral power conductors.