The Monopole Antenna
The radiation pattern of a monopole
antenna is omni-directional and radi-
ates equally in all directions. The
monopole antenna can detect leakage
from all sides except the top and bot-
tom. Therefore, this antenna is most
sensitive to detecting leaks that are
parallel to its sides. Because signal
leakage often occurs in a spherical
pattern, the monopole antenna
should be used most effectively for
general location of a leakage source
(e.g. the VMA-3 used for vehicle patrol
applications).
The near field probe antenna is appre-
ciated most for its convenience; it
doesn’t take much space. The most
important drawback to the near field
probe is its inefficiency in locating the
source of the leak. This type of anten-
na is often criticized because of its
limited performance as a result of its
omni-directional pattern.
ACTERNA recommends the use of the
HD-1 dipole antenna, especially for
new and inexperienced field techni-
cians. The vehicle mount monopole
antenna does not provide the direc-
tionality needed to conduct accurate
leakage readings consistently. By
using the HD-1, the new and inexperi-
enced technician will be able to devel-
op more leakage detection confidence
as a result of dependable, high quality
equipment that will contribute to sys-
tematic accurate approach to leakage
detection.
The Dipole Antenna
As discussed, the radiation pattern of a
dipole antenna looks like a figure eight
and the maximum radiation is located
perpendicular to the center of the
dipole. To measure leakage signals,
the dipole antenna must be held at the
same polarization (wave angle) as the
leak. The sides of the antenna’s ele-
ments are most sensitive to RF signals
and the antenna ends are the least
sensitive. During leakage detection,
the antenna should be slowly rotated
about the vertical axis until the maxi-
mum signal level is found and the
meter reading has peaked.
In the United States, the half-wave
dipole is the only acceptable antenna
approved by the FCC for determining
legal leakage levels and Cumulative
Leakage Index (CLI) data collection
(See Appendix A). The dipole anten-
na was selected by the FCC because
of its well-defined electrical current
stability, directivity and ease of use.
3-17
ACTERNA VMA-3 vehicle mount antenna (left)
NOTE:
VMA-3 requires adequate ground plane to function properly
Near Field Probe (right)
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