Antenna currents and field strength

June 1, 2004

Anything metallic can be made to carry RF current and to radiate some sort of RF signal. But what quality of signal will it be? It can depend on the construction and size of the tower, as well as the antenna tuning-unit circuits. RF currents induced in a nearby metallic structure can deform a nominally circular radiation pattern and affect coverage in positive or negative ways. While this is going on the RF antenna ammeter may indicate the licensed antenna current, but listeners may be hearing a distorted signal.

Engineers tend to talk glibly about RF current and field strength in millivolts per meter (mV/m) but sometimes we don't get the millivolts per meter that we expect from the power delivered to the antenna. We talk about the antenna resistance, but sometimes the antenna resistance and the radiation resistance are noticeably different. When we supply RF current to our antennas we expect to get a definite return for the power expended. If this doesn't happen it is sometimes difficult to find a reason, especially if it is a nondirectional station.

Clarity of thought

Sometimes we lose sight of the relationship between current in an antenna and the resulting field strength value. The level of our electric field is measured in terms of millivolts per meter, or volts per meter depending on the requirements. This term expresses the voltage that would be induced in a one meter long wire placed parallel to the lines of flux. This induced shortage results from the movement of the flux across the wire.

Halfwave antenna showing the relationship between current and resistance.

Two fields are developed around an antenna. The induction field exists in the immediate vicinity of the antenna, and consists of lines of flux that connect back into the antenna twice each cycle. This is not the field that the engineer generally wants. In fact, it is this induction field that often produces the unwanted effects of high currents or voltages in close by metallic objects. It is a reactive field in which the electric and magnetic fields are 90 degrees out of phase.

The radiation field consists of detached lines of flux that move out as an electromagnetic wave. This is the emission that is of greatest interest to the radio engineer. It is this field that contains the real power because the electric and magnetic fields are in phase with each other. The intensity of this field is inversely proportional to the distance from the antenna, and diminishes far less rapidly than the induction field; this is the radio signal.

If we want to compare developed field strengths we must compare them at the same distance. Originally, engineers used one mile as a convenient distance. This distance is sufficient to be well outside the induction field and provides a good solid measuring signal.

What's your sine?

We are all familiar with the normal sinusoidal distribution of current in an antenna. Although, under some circumstances, non-sinusoidal current distribution may have to be assumed because unusual antenna configurations such as top loading can affect current distribution.

The radio engineer is generally most interested in the impedance of an antenna and the real part of this term, which is the radiation resistance. This resistance multiplied by the square of the antenna current tells us the power developed in the antenna circuit.

Much of our broadcast work involves vertical radiators around 90 degrees to 120 degrees of electrical height. We try to measure the antenna current at the base of the tower. It is common practice to place an RF ammeter or other current-measuring device in the output leg of the base-matching network. We then measure the antenna base impedance at this point and use the resistance thus obtained to calculate the power into the antenna.

When we use a bridge to measure antenna impedance, what we actually measure is the antenna impedance (the R term), plus any resistance in the actual antenna connections and structure, and the ground connections and system. Dirty and corroded connections, small-diameter cables and connections that have corroded to small conductors, rusted connections between tower sections and inadequate ground system with damaged ground screen and broken or missing radials will add to the I2R losses. Power that should be radiated will be dissipated in heat. Therefore the value of R used as antenna radiation resistance actually consists of Rant + Rmisc. The FCC allows a certain amount of system loss to be included in directional antenna operations.

An antenna's measured impedance depends on exactly where along the antenna it is measured. The indicated current in the antenna varies with the measuring location.

Consider the radiation resistance of a half-wave antenna that is about 73.1Ohm. This is the value that can be measured at the center of the antenna. However, this is the only position that will produce this resistance. All the other points on the antenna will exhibit a lower current and higher resistance. This can be calculated by multiplying the current measured at the center by the sine of the distance from the end of the antenna. However, you must specify the reference part of the antenna. This is called the point of maximum antenna current, but it is not always the same thing as the resistance of the driving point of the antenna, which is the point at which we place our impedance bridge.

Remember the textbook introduction to vertical radiators in which a quarter-wave radiator is shown above ground with an image below it? The total length of the real antenna and the image equals one-half wavelength. An antenna one-half wavelength long with one amp at the center will produce about 37.5mV per meter at a distance of one mile from the antenna. The radiation resistance of this antenna is around 73Ohm.

If we take half of this antenna, i.e. a quarter-wavelength also with one amp at its base, we shall find about 37.4mV per meter at a distance of one mile. The radiation resistance (the R term in the antenna impedance) will be half of the ideal half-wave dipole, or about 36.6Ohm. With 1A flowing the power will be 36.6W.

In terms of millivolts per meter with 1kW power at 1 mile:

37.42 x the square root of 1000/36.6 = 196 mV/m

If we increase the power to 10kW, we multiply the field strength at one mile by the square root of the power increase. Therefore,

196 x the square root of 10 = 196 x 3.16 = 619.36mV/m

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