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EMC Antennas

Antenna Impedance

Most connectorized RF and/ microwave devices have an impedance of 50 W. Free space (generally the transmission media) has an impedance of 377 W. To further complicate matters, most antennas have a free space impedance of their own (a helix 140 W and ½ wave dipole 70 W). The most efficient transfer of power occurs when the antenna matches the transmission media and/or source impedance. Not all sources are 50 W (CATV is 75W and wave-guide 377 W) and not all impedance is real but for explanation purposes we will use a real impedance of 50 W and ignore imaginary contributions.

Matching the antenna and transmission line to the source impedance is important for a transmitter. The most accepted description of that match is the Voltage Standing Wave Ratio of VSWR. A perfect match is 1.0:1, which indicates the source and load impedance are the same. A bad match depends on the application. For very high power applications (EW systems, radar, radio/television transmitters) anything over 1.5:1 may be unacceptable. Wide band test antennas may have a VSWR of 3.5:1 or more. The ratio describes how much power will be transmitted into the antenna and how much power will be reflected back to the source (see table B-1). The VSWR is then an indication of the efficiency of an antenna. An antenna, which has a poor match, will not be an efficient radiator but an antenna with a good match is not necessary an efficient radiator.

Mismatch Effects

In addition to matching the impedance, most transmission sources have coaxial outputs.  This type of connector has a grounded shield surrounding the center conductor, which carries the RF/Microwave energy.  Many antennas are balanced and both sides of the transmission line are active (such as 300 W twin lead for radio and television).  This requires that not only the impedance be transformed but that the unbalanced coax transmission line is transformed into a balanced condition prior to connection to the antenna.  The device that accomplishes this is generally referred to as a balun.  It is important to note that the best match is obtained with a dummy load, which does not generally radiate at all.  This leads to the next topic, efficiency

Gain

Gain is a widely used parameter directly measurable by substituting an antenna with known gain (generally a gain reference antenna) in for an antenna under test (AUT). The output levels of the AUT and the gain reference can then be measured for the same incident field. The gain can then be determined by comparing those measured levels. Gain of an antenna is expressed in dB, 10log10(numerical gain), which is generally referenced to an isotropic radiator (radiates equally in all directions) and expressed as dBi. Typical gains are listed in table 2. The gain expressed for an antenna is generally the maximum or peak gain. This leads to the second part of the gain equation, directivity.

A directional antenna will have a beam that is concentrated in one direction. A typical directional antenna is a standard gain horn with a HPBW of about 20° (in both elevation and azimuth). A highly directional dish antenna may have a HPBW as small as 0.5° in both elevation and azimuth. It is important to note that the HPBW and gain of an antenna are inversely proportional. For the above antennas, the gain for the omni-directional is 2 dB, for the horn 18 dB and for the dish 48 dB. A reasonable formula that relates HPBW and gain is:

Gain (dB) ~ 10 log10 (31,000 / HPBW°E X HPBW°A)

Several examples of typical antenna gains and HPBW are listed in table B, typical antenna gains and beam-widths. 

Antenna Efficiency

Antenna efficiency is a complicated and often misused figure. All antennas suffer from losses. A simple horn antenna for example will not be as efficient as a perfect aperture of the same size because of phase offset. The real efficiency of an antenna combines impedance match with other factors such as aperture and radiation efficiency to give the overall radiated signal for a given input. The best and most widely used expression of this efficiency is to combine overall efficiency with directivity (of the antenna) and express the efficiency times directivity as gain.

Table B, Typical Antenna Gains and Beam-widths

Directivity
The directivity of an antenna is generally combined with efficiency and expressed as gain as described above. The Half Power Beam Width (HPBW), of an antenna, is an expression, in degrees, of the width of the radiated beam between the half-power or 3 dB points (down from the peak of the beam). Many antennas will exhibit one HPBW in azimuth and a different HPBW in elevation written as HPBWA and HPBWE. An antenna described as omni-directional will have equal coverage in all directions. A typical wide band omni-directional (in azimuth) antenna will have a HPBWA of 360° and HPBWE = 50°

Other Important Antenna Specifications and Definitions
Side lobe level- This specification is generally given as a maximum value or not to exceed envelope of maximum side lobe level vs. azimuth. The maximum side lobe level is often dictated by the FCC (or similar agency) for transmit antennas that could interfere with other systems if the side lobe levels were excessive.

Front-to-back ratio- Often listed in dB, this specification is the difference between the peak gain of the antenna and the radiation in the back of the antenna (often 180° from the peak of the beam).

RDP (Radiation Distribution Pattern)- This is a complete antenna set of antenna patterns generally taken at intervals of 2°x2°. The RDP allow for the 3-D reconstruction of the radiation pattern.

Coherent Measurements- The measurement of phase and amplitude verses angle for the antenna measurement. Measurement of orthogonal linear polarizations and the phase difference between them allows for the reconstruction of and incident polarizations.

Polarization
The polarization of an antenna is the orientation of the transmitted (or received) electric field (E field). The optimum polarization for a system depends on the polarization of other antennas in the system. An infinite number of polarizations exist but the most common are linear and circular. For a linear antenna three possibilities generally are seen vertical, horizontal and slant linear. It is important to match linear polarizations for transmit and receive sites. A linear polarization mismatch can result in up to a 20 dB loss (for cross-linear polarization).

Circular polarization is generally given as right hand circular polarization (RHCP) or left hand circular polarization (LHCP). To determine the polarization of a circular antenna, use the right hand rule. Point the thumb of your right hand in the direction of propagation and curl your fingers. If your fingers point in the direction of propagation, the antenna is RHCP (otherwise it is LHCP). Polarization mismatch loss for circular antennas can also be up to 20 dB (for opposite sense polarization). One technique often used in telemetry systems monitoring aircraft is to use a circular terrestrial receive antenna and a linear transmit on the aircraft. This results in a mismatch loss of about 3 dB (linear to circular) but allows the aircraft to maneuver without causing the link to suffer large mismatch losses caused by cross polarization.

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