An antenna is simply a conductor or conductor array, usually metal. But how it is situated and terminated that determines its behavior in transmitting and/or receiving electromagnetic radiation.
Let’s start with an antenna that is configured to transmit. An oscillating voltage applied at a terminal will cause current to flow through the antenna. A transmitting antenna can be wired in various ways, but no matter how you look at it, there must be a return path for the electrons to flow back to the power source. This can be puzzling, especially when we consider an antenna for reception. (Often, as in a two-way radio, the same antenna is used for both transmission and reception.) Various models have been proposed:
• Any transmission line that feeds an antenna has two conductors and the antenna actually begins where the two conductors diverge.
• A transmitter output has two separate conductors, coax core and coax shield. A balanced antenna system has two wires.
• An antenna doesn’t need a return path. It is like a resistor that dissipates heat or a lamp emitting light.
• A whip antenna in itself is not an antenna. The antenna is the whip plus ground. An antenna has two poles, qualifying it as a circuit.
These explanations are not 100% compatible, and they are speaking different languages, or at least different dialects. It is not clear whether the antenna is the load or a conductor connected to the load. Perhaps a good way to look at it is that the RF portion of the transmitter and receiver comprise a single circuit with the space between them an internal conductor. The energy that travels through this space temporarily takes the form of electromagnetic radiation.
Transmitting and receiving antennas may be either omnidirectional or directional. Omnidirectional antennas, as the name implies, transmit and receive equally in all directions, at least on a horizontal plane. A directional antenna can be constructed to transmit or receive in a specific direction as from a small, distant target. An example is the satellite dish antenna, used as a receiver for television broadcasts. With a small angular target, this type of antenna has high gain, a term usually associated with an amplification device that receives a relatively weak signal at the input and higher power from an external supply. But gain as a concept is equally applicable to a directional antenna.
A vertical or whip antenna is omnidirectional. It consists of a single rod, usually one-quarter of a wavelength long relative to the frequency to be received or transmitted. It follows from this that simple antennas are built to the quarter-wavelength of the frequency they are meant to handle. Because of size constraints, it is not economical to build a directional antenna for low-frequency transmission or reception.
The optimum length for an antenna is set by the frequency it is meant to radiate. Any length will radiate any frequency, but antennas that have a length that is an integral multiple of the wavelength will need a lot less voltage or current to radiate.
Specifically, resonance happens at the right fraction (and multiples thereof) of the wavelength of interest. Most resonant systems (including antennas) have a relatively narrow frequency range at which they resonate. Resonance results in much higher currents or voltages in the antenna than what is fed into it. Without this resonance, it would take high voltages or currents to ultimately radiate the same power.
Near the resonant frequency, the antenna looks resistive. At the resistive frequency of the antenna, the voltage and current are in phase, so real power is delivered to the antenna, which it then radiates. (Also, for reasons we won’t get into, the active length of the antenna is a bit longer than its physical length.)
The dipole antenna has a more complex appearance than the whip monopole, but functionally it is simple. (Strictly speaking, a monopole is a dipole antenna where the earth functions as the mirror image of the monopole. So the monopole plus the mirror image together comprise a half-wavelength at the radiated frequency.) The dipole was the first antenna, invented by Heinrich Hertz in 1886 to demonstrate the existence of radio waves as theorized several years earlier by James Clerk Maxwell in his electromagnetic theory of light.
The generic dipole consists of two conductive metal rods situated on a common axis, end to end. It is convenient to connect the transmission line’s twin leads to the two adjacent ends, but these connections can as well be made at other points along the rods. So configured, the two rods are resonators, with oscillating electrical current taking the form of a standing wave located across the dual elements. The total length of the two rods is one-half wavelength, so the assembly is smaller for higher frequencies.
One widely used antenna wiring configuration is a dual transmission line with equal voltages of opposite polarities connected to the two elements. This is known as balanced transmission. One of its advantages is common-mode rejection, which equates to noise-free operation. The same objective can be attained by feeding the dipole antenna or receiving from it through a coaxial cable equipped with a balun.
This strange little word is derived from “balanced to unbalanced.” The device can take many forms. It accomplishes its objective often, but not always, by means of transformers and magnetic coupling. It converts an ungrounded balanced signal to an unbalanced signal with one side grounded, or vice versa.
Baluns have many uses, including impedance matching by means of transformers. In most cases, the slight signal current involved allows small component sizes. The isolation transformer balun, with electrically separate windings, has the advantage that circuits with differing potentials to ground can be joined with no fear of ground loops. In all cases, ground does not necessarily mean an actual connection to earth. Just a chassis ground, the zero-volt point to which signals are referenced, will suffice.
Additional conductive elements sitting adjacent to a dipole antenna can make it more directional and give it more gain. Though gain rises in this manner, it should be understood that in no case is there an increase in power relative to the amount supplied by the transmitter. Instead, radiated power is concentrated into a usable region so that conservation of energy is not violated. One example is the phased array, where a number of antenna elements are electrically connected to make a single device.
One variation is a log periodic dipole array. This configuration involves dipole elements of differing lengths. The end product is characterized by moderate directivity and wide bandwidth. Before the advent of satellite dish TV, these arrays were prominent on rooftops in rural areas. The dipole elements are all active because they are electrically connected to one another and to the transmission line.
Another variation, having a similar appearance, is the Yagi antenna. In this array, only one element has a direct electrical connection to the transmission line. The other elements, called parasites, are placed in close proximity. They affect the electromagnetic field and in that way have an indirect influence on the electrical performance of the entire array. In still another variation, parasitic elements known as directors sit between the active element and the transmitter, and one or more parasitic elements called reflectors are placed on the back side. Here again, only the active element is electrically connected to the transmission line.
An individual antenna has the same electrical qualities whether it is transmitting or receiving. This principle is known as reciprocity and refers to impedance, bandwidth, gain, radiation pattern, resonant frequency and polarization. For reciprocity to apply, materials that comprise the antenna and transmission line must be linear as well as reciprocal.
There are myriad types of antennas and subsystems. Antennas not located within the transmitter or receiver connect through a transmission line. Generally, the antenna is not too remote, to minimize transmission line losses. An important aspect of transmission line design is impedance matching. If there is a junction within or at either end of the line, where impedance abruptly changes, there will be signal reflections, which give rise to data collisions and corruption, or noise.
All antennas are characterized by a few performance metrics: input impedance, polarization, directivity, gain, radiation efficiency, and the radiation pattern. All but the first property can be measured either on an antenna range or in an anechoic chamber. An anechoic chamber is a room constructed to eliminate all reflections at the frequencies of interest. If tests take place on an antenna range that is not an anechoic chamber, the range must first be analyzed for any sources of reflections (for example, from the ground). Then these reflections are eliminated before the test (sometimes easier said than done on outdoor ranges).
The general approach to antenna tests is to position a known transmitter and antenna at a known distance away from the tested antenna. The tested antenna is connected to a known receiver. The magnitude of the transmitter output is known, as is the loss of any cabling. Similarly, the sensitivity of the receiver attached to the antenna under test is also known. The path loss over the distance between the two antennas can be calculated.
With this setup, the amplitude of the signal measured at the receiver will indicate the gain of the antenna under test. The antenna radiation pattern can be determined by placing the antenna under test on a rotating platform and measuring the magnitude of the received signal at increments as the platform rotates.
One point to note is that at large distances from a transmitting antenna, the phase front of the transmitted wave is shaped like a sphere. For extreme separations between the transmitting and receiving antennas, the radius of curvature is so large that for all practical purposes the phase front can be considered planar over the aperture of a practical antenna. This is why the sending and receiving antennas must be spaced some distance apart. As the antennas are brought closer together, a point is reached where the short radius of curvature causes an appreciable separation between the wavefront and the edges of the antenna aperture.
Antenna VSWR (voltage standing wave ratio) is a measure of how much energy sent to the antenna is reflected back. Its measurement usually involves a directional coupler, signal generator, and the spectrum analyzer mode of an oscilloscope. The signal generator puts out a known signal into the coupler that feeds to the coupler output port. A baseline reading is taken by connecting a dummy load of a known impedance (usually the same impedance as the target impedance of the antenna) to the output. The amplitude of the reflected and direct signals are noted. Then the antenna under test is connected to the output. The difference in the reflected wave amplitude with the antenna attached and the reflected wave during the baseline is the figure of interest. It is usually expressed as a ratio of the input amplitude over the reflected amplitude. A perfectly matched antenna/cable system will have a VSWR = 1. Real VSWRs are typically in the 1.1 to 1.2 range.
Finally, there are a few other specialized measurements for more exotic antennas. Active
phased arrays, for example, have a transient response that arises when switching between beam directions as well as switching between frequencies. This response is a function of internal antenna interactions such as coupling and VSWR, active circuitry, and components such as phase shifters and attenuators. These sorts of measurements are quite specialized and generally depend on the specifics of the antenna being measured.