Z Technology Application Note #1003-1
Measurement Antenna Selection and Use
version of this Application Note
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.
Calibrated, 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
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
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
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
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.
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.
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.
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.
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
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
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.