Z Technology Application Note #1003-1
Measurement Antenna Selection and Use

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 There are many antenna types available for signal coverage measurement. This application note will discuss some of the factors you should consider in making a selection.

Antenna calibrationCalibrated, Characterized, Neither?
 For NIST traceable measurements, the antenna, cable, and field strength meter must be calibrated individually, traceable directly to the National Institute of Standards and Technology. You will want an antenna or antennas that will cover each of the frequencies you will be measuring. You can use these antennas for the measurement, or use these antennas as a traceable reference to calibrate other antennas for the actual measurement. A ‘calibrated’ antenna will be provided with an individual calibration table that will list measured antenna correction factors, and any necessary physical adjustments, for frequencies of interest. It is also possible to take measurements using antennas not individually calibrated, but used according to manufacturer’s specifications, or constructed to textbook specifications for that antenna model or type. Such antennas are said to be ‘characterized’ rather than calibrated, but they are still useful for locating problem areas for further investigation. A characterized antenna may be calibrated by following good engineering practice to measure and document the antenna’s actual performance. Antennas in the ‘neither’ category are antenna of unknown design. While these antennas may be useful when characterized for a single frequency, they may not behave as expected off the main axis or on other frequencies. Integrated preamplifiers could also be a problem as they may not be perfectly linear, could drift with temperature, have internal AGC gain control, or be equipped with traps to notch out interfering signals.

Horizontal or vertical polarization?
A measurement antenna will generally be designed for linearly polarized signals, and most may be oriented vertically or horizontally. This single polarization design allows the user to measure in any polarization and to investigate any desired or undesired variation polarization.

When changing polarization of a linear measurement antenna, the user should take care to avoid proximity to the ground or other objects that will affect one polarization vs. any other.

Proximity to other objects?
A measurement antenna can only measure actual field strength at the antenna location, so any variation in the signal strength is real. It is, however, useful to avoid as much as possible reflections that add or subtract from the real signal strength in an undocumented way when the measurement is being taken.

The earth has a minor effect on signal strength. As an antenna comes nearer the earth, its impedance will be reduced, usually affecting the reading. It is a good idea to keep the measurement antenna at least one wavelength above the earth. The metal roof of a vehicle is relatively small vs. the height of the antenna above the roof and will have only a minor effect on signal strength. The angle of signal incidence to the roof metal is usually very shallow and significant energy does not affect the reading. The gain would be affected as the roof area became very large relative to height of the antenna above the roof. A quarter-wavelength vertical antenna mounted directly to a metal roof, with its image reflected in the roof metal, would approach the gain of a free-space dipole.

A perfect linear antenna, in the absence of any reflections or multipath will not see any signal in an orthogonal polarization. In practice, the best horizontal vs. vertical isolation can reach about 30 dB.

Antenna directivity?
A directive measurement antenna has good and bad characteristics. It would be good to be able to ignore antenna orientation of the antenna, but if the antenna were non-directional, the measured primary signal could not be separated from strong reflections.

Antenna directivityHorizontal measurement antennas are usually directional, with the ideal dipole having a half power (3 dB) beam width of about 78 degrees. A calibrated Yagi or log-periodic antenna will be more directional and greater care must be taken to keep the antenna in alignment towards the signal source. A vertical dipole, or a vertical whip antenna on a metal ground plane, will be truly omni directional in the absence of any surrounding objects.

The best antenna for single frequency linearly polarized signals seems to be a dipole that can be adjusted to an optimum length. The log-periodic antenna, or some variation, is very useful when measuring a range of frequencies. These antennas are commonly available calibrated traceable to NIST, and are the types most often used.

Antenna ruggedness?
Calibrated antennas are usually intended for use in the laboratory or in a calibration range, and are often not very rugged for day-to-day use. These antennas, especially the large lower frequency antennas, must be handled with care. On the other hand, the construction of these antennas is quite simple and it is obvious when an antenna has been damaged. When repaired and re-calibrated, it is rare to see much change in measured performance.

When antenna damage is likely, you should consider characterizing a more rugged antenna, such as an antenna designed for commercial use under severe weather conditions; or a low cost disposable antenna. 

Antenna mountingAntenna mounting?
Calibration antennas are designed to be mounted on special non-conductive supports in a laboratory measurement range. Z Technology builds a special magnetic mount system that supports a horizontal dipole antenna 24” to 36” above a moving vehicle for DriveTest signal coverage measurements. The calibration dipole antennas for high band VHF and UHF survive very well, even at high speeds. The low VHF dipole antenna will be extended to a length of 8 feet for calibrated measurements on TV channel 2, so mobile measurements for these low channels are often made using a shorter antenna characterized or measured in comparison to the calibrated dipole.

The FCC may require proof-of-performance measurements when directional antennas are used to serve certain areas or protect other broadcast stations from interference. These measurements are taken as a series of 100 ft runs along selected service contours. The measurement antenna is required to be at a 30 ft elevation, mimicking a viewer’s outdoor antenna. Needless to say, operator safety should be the major consideration when making FCC proof measurements, with particular attention to utility lines, overhead obstructions, and stability of the moving antenna and its support. Warning devices are available to alert the operator of close by power lines. Z Technology Field Strength Meters and Coverage Measurement systems can measure under such conditions, but Z Technology makes no recommendations other than to warn the operator to take responsibility for his own safety and the safety of any others nearby.

Summary
When making coverage measurements, know the characteristics of the measurement antenna. Use calibrated antennas for definitive measurements that must be documented for historic records or reported to prove performance. Use field calibrated, characterized antennas where expensive range calibrated antennas are not practical for mechanical reasons. Where calibrated measurements are not required, or for general purpose sampling to discover problem areas, it is possible to use antennas that are neither calibrated or characterized, but it is still important to select such antennas using good engineering common sense.

And finally, safety is the most important issue. Awareness of your surroundings can save your life. Take responsibility for yourself, your employees, and the observing public. Stop, think, and be safe.