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Antenna
FAQs
ANTENNA INSTALLATION
GUIDELINES
How
should antennas be installed or mounted ?
What is
a good strategy for mounting communications antennas ?
How much distance should there be between
antennas ?
Do antennas need a ground plane ?
What size should a ground plane be
?
How do I ground an antenna ?
How do you ground antennas on a composite
aircraft ?
Do antennas require gaskets ?
What cable type is required for antennas
?
How long can cables be ?
What connector type is required for
antennas ?
COMANT
ANTENNA OPERATION
How
do “combination” antennas, such as Comant’s GPS/VHF
ComDat™ antennas, work without a minimum required separation
?
My
Marker Beacon antenna has a screw protruding from the base plate.
What is this ?
How does the Comant
CI 601 Duplexer operate ?
Do VOR/GS V-Dipoles
such as the CI 158C need to be “pointed” forward or aft
?
Do VOR/GS V-Dipoles
such as the CI 158C require an in-line balun ?
Do Power Combiners
such as the CI 120-3 have an internal balun ?
Why
is there a short from center pin to ground on some antennas ?
Why does the Power
Combiner show a short from center pin to ground ?
FAA
CERTIFICATIONS
Do
all antennas require TSO ?
Can an
antenna be installed that does not have TSO ?
Can I substitute one antenna brand
for another ?
ANTENNA
REPAIR
When
should I replace an antenna ?
Can antennas
be repaired ?
Can antennas be
painted ?
Can an
antenna be bench tested ?
How are antennas tested ?
ANTENNA
BASICS
What is VSWR ?
What
is Return Loss ?
What is Impedance
or Characteristic Impedance ?
What is antenna
gain ?
What is polarization
?
What is a 1/4 wave antenna
?
What is a 1/2 wave antenna
?
What is a dipole
?
What is a monopole
?
What is reflection
on an antenna and transmission line system ?
What is shadowing
?
What is multipath
error ?
ANTENNA
INSTALLATION GUIDELINES
How
should antennas be installed or mounted ?
Refer to FAA Advisory Circular 43.13-2A and Comant's General Installation
Guidelines for Antennas contained on this Web Site. A copy of the
FAA Advisory Circular can be obtained at www.faa.gov.
Look for Advisory Circulars under the Regulatory/Advisory heading
on the FAA home page. All installations must be done by qualified
personnel and in accordance with Federal Regulations.
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What is a good strategy for mounting
communications antennas ?
Com 1 should be mounted on the top of the aircraft. Com 2 can also
be mounted on the top, provided there is adequate distance (minimum
36 inches) available between antennas. If Com 2 is mounted below,
a bent whip may be required to provide ground clearance. Bent whips
may not provide the best VSWR because of proximity to the ground
plane (aircraft skin), which can cause reflections, plus they lack
overall height. Remember, VHF antennas "like" to be tall and straight
for best operation. Belly mounted antennas on aircraft with fixed
landing gear are also susceptible to shadowing and/or reflections
caused by the gear.
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How much distance should there be between
antennas ?
You should maintain 36 inches as a minimum distance between antennas.
Some antennas, such as SATCOM, may require more. Refer to each manufacturer's
installation guidelines.
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Do antennas need a ground plane ?
Most Comant aviation antennas require a ground plane. The exceptions
are certain Glide Slope and VOR/GS dipole antennas. It can be said
that GPS antennas have a "built-in" ground plane, but GPS antennas
still require grounding to the aircraft.
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What size should a ground plane be ?
For best performance the ground plane should be horizontal and have
a radius of at least 1/4 wavelength of the antenna's operating frequency,
or as large as practical. A good general rule to follow is to assure
that the ground plane radius is at least equal to the antenna height
or length.
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How do I ground an antenna ?
Method 1. With the exception of certain ELT blade antennas, all
Comant antennas can be grounded through the mounting screws. To
obtain the proper electrical bond (grounding), the area inside the
aircraft, where the antenna is to be mounted, must be free of paint
and debris. A backing or doubler plate is placed in the aircraft
interior with the antenna mounting screws affixed to the necessary
nuts and lock washers. The mounting hardware will make contact to
the backing plate, and the backing plate contacts the aircraft skin
(interior), with this, an electrical bond is achieved. This method
accommodates the use of a non-conductive gasket.
Method
2. Determine the area where the antenna will mount to the outside
of the aircraft. Carefully remove paint where the antenna base plate
makes contact to the aircraft skin. Follow the remaining instructions
as described in Method 1. With this method, a conductive gasket
may be required. Be advised that many conductive gaskets contain
metallic materials that are subject to rust or corrosion. Therefore,
be sure to run a bead of RTV around the antenna perimeter to seal
the gap between the bottom of the antenna and the aircraft skin.
Important
notes: With either method, apply alodine to any bare aircraft skin.
With both types of installations, run a bead of RTV silicone sealant
along the edge of the antenna where it meets the exterior aircraft
skin. After completing the installation, check electrical bonding
with an ohmmeter. It should read no greater than .003 Ohms between
a mounting screw and the aircraft skin.
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How do you ground antennas on a composite
aircraft ?
For composite aircraft, use a wire mesh or similar material to create
a properly sized ground plane. Mount the ground plane in the aircraft's
interior where the antenna is to be mounted. Follow Method 1 (above)
for installation guidelines. A good source of material on this subject
is included in FAA Advisory Circular 43.13-2A.
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Do antennas require gaskets ?
Antenna performance can be severely degraded from corrosion caused
by moisture accumulation where the antenna electrically bonds to
the skin of the aircraft. When using Method 1 for grounding, a gasket
is advisable along with RTV around the antenna edges. However, when
using Method 2, a conductive gasket may be required if gasket use
is desired. Since conductive gaskets contain metal, many types are
susceptible to rust and corrosion. Therefore, a bead of RTV sealant
is still required.
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What cable type is required for antennas
?
A good quality RG-58 type is sufficient for most applications and
antennas. Higher frequency systems (DME, Transponder, UHF Radiotelephone)
may require a more "stout" cable, such as RG-142. The FAA may require
the use of flame retardant cables for certain installations such
as ELT. Check with the appropriate FAA office.
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How long can cables be ?
Any cable run should be as short as practical. Take special care
with cables that run from a VOR/LOC/GS set (blade or towel bar)
to the Power Combiner. These cables also need to be as short as
practical, but most importantly, they need to be the EXACT SAME
LENGTH for optimal antenna system performance.
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What connector type is required for
antennas ?
By far the most popular connector type is the BNC Female because
of its low cost and acceptable performance for low frequency antennas
such as VHF or Marker Beacon. As antennas go higher in frequency,
they require a more heavy-duty type of connector. The TNC is popular
for most GPS antennas. Some very high frequency antennas used for
DME, Transponder, or UHF Radiotelephone perform best when coupled
with a TNC, C, N, or HN connector. These connector types show little
loss and are generally more expensive.
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COMANT ANTENNA
OPERATION
How
do “combination” antennas, such as Comant’s GPS/VHF
ComDat™ antennas, work without a minimum required separation
?
All GPS/VHF ComDat® antennas utilize a built-in notch filter that
removes VHF harmonics that are coincident with the GPS frequency.
These 12th and 13th harmonics are "notched" with an 80dB isolation
filter.
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My Marker Beacon antenna has a screw
protruding from the base plate. What is this?
This is a factory set adjustment screw and should not be turned
or adjusted. When mounting this type of antenna, you will need to
provide a clearance hole in the aircraft to accommodate this screw.
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How does the Comant CI 601 Duplexer
operate ?
The CI 601 is designed to provide operation between two transceivers
(118-137MHz) and one antenna. In the de-energized mode, the duplexer
acts as a 3 dB coupler with the output ports isolated by 20 dB.
The unit has white, green, red and black leads. Red is for 12-30
vdc power, while black is for ground. The white and green leads
are connected to corresponding transmit 1 and transmit 2 so that
control voltage, actuated by the microphone circuit, switches the
antenna from transmit 1 or transmit 2. The unit isolates the transceivers
by 30dB when in operation.
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Do VOR/GS V-Dipoles such as the CI 158C
need to be “pointed” forward or aft ?
For best results, the radiating elements should be directed forward,
as the emitted pattern needs to "see" forward and downward. However,
some aircraft owners believe they provide a better appearance if
the radiators are pointed aft. If mounted in this manner, the installer
should flight check for proper antenna performance.
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Do VOR/GS V-Dipoles such as the CI 158C
require an in-line balun ?
No. These antennas have an internal balun.
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Do Power Combiners such as the CI 120-3
have an internal balun ?
No. Comant's Power Combiners, such as the CI 120-3, are coaxial.
Therefore, these combiners are unbalanced and they do not contain
an internal balun.
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Why
is there a short from center pin to ground on some antennas ?
Some antennas contain an impedance matching transformer, which is
a series of windings that short to ground. By measuring resistance
between center pin to ground on an antenna that has this configuration,
you are measuring DC continuity on the primary side of the winding,
and it will appear as a short. However, RF frequencies have a high
impedance path to ground, and it is with this signal that these
types of antennae operate.
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Why
does the Power Combiner show a short from center pin to ground ?
Comant Power Combiners used for VOR/LOC/GS contain phasing windings
that short to ground in order to achieve proper signal phasing.
By measuring resistance between center pin to ground on a combiner
that has this configuration, you are measuring DC continuity on
the primary side of the winding, and it will appear as a short.
However, RF frequencies have a high impedance path to ground, and
it is with this signal that these types of combiners operate.
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FAA
CERTIFICATIONS
Do all antennas require TSO ?
The FAA provides Technical Standard Orders for designated frequencies
of antennas. Not all equipment on an aircraft may fall into a designated
frequency and therefore, an antenna of that type cannot be granted
TSO because a TSO does not exist.
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Can an antenna be installed that does
not have TSO ?
Contact your local FBO (Fixed Base Operator) for established processes
necessary if you are considering equipment for your aircraft that
does not have TSO. The FAA provides several procedures, such as
STC (Supplemental Type Certificate 337), that enable non-TSO'd product
installation.
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Can I substitute one antenna brand for
another ?
In most cases you can, however you may wish to consult with the
appropriate FAA office. A good point of reference the FAA's Advisory
Circular 20-41A which covers this issue in detail. All installations
must be done by qualified personnel and in accordance with Federal
Regulations.
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ANTENNA
REPAIR
When should I replace an antenna ?
The easiest thing to do is blame your antenna for poor reception
and/or needle oscillation from navigation signals. Be sure the antenna
is properly grounded, and that all cables and connectors are clean
and in proper condition. See the .pdf "Comant
Field Repair Policy" on this web site for further information.
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Can
antennas be repaired ?
All Comant antennas are factory sealed and are not repairable. Comant
carries one of the best warranties in the industry. Three years
from date of manufacture. See the .pdf "Comant
Field Repair Policy" on this web site for further information.
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Can antennas be painted ?
Paint de-tunes antennas. Comant antennas are manufactured to take
this into account, and these antennas are passed through final electrical
inspection with their factory-applied paint. Adding or removing
paint at non-specified levels or thickness will alter antenna performance
and may de-tune it out of specification. Many paints contain lead
or other metallic materials, which will severely alter performance.
Antennas that have been altered cannot be warranted by Comant. See
the .pdf "Comant
Field Paint Policy" on this web site.
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Can
an antenna be bench tested ?
Many people will check for open or short circuit from the center
pin to ground as a bench test. Be advised that, depending on their
design, antennas can have either an open or short circuit to ground.
See further explanations below, or contact Comant for information
on your particular antenna model.
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How
are antennas tested ?
Factory tests usually consist of mounting the antenna on a large
ground plane and checking VSWR with a network analyzer. Another
important factory test is at Comant's outdoor antenna range using
a source antenna directed to the UUT (Unit Under Test) mounted to
a positioner and digital pattern recorder to document antenna patterns.
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ANTENNA
BASICS
What is VSWR ?
VSWR is the ratio between the maximum and minimum values of voltage
on the transmission line connected to the antenna. If everything,
from the radio end connector to the base of the antenna is perfect,
and if the antenna is perfectly tuned to the testing frequency,
VSWR will be 1:1. That is, every bit of power that the radio has
to deliver is leaving the radio and being transmitted or received
by the antenna. For frequencies below 200 MHz (VHF, etc), VSWR below
2.5:1 is acceptable. Granted, lower VSWR means more power leaving
the radio but at 2.5:1 the loss does not amount to more than 0.9
dB, even on very long transmission lines. A loss of 0.9 dB is not
a substantial change in actual signal strength.
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What is Return Loss ?
This is basically the same thing as VSWR. Return Loss is equivalent
to VSWR but is expressed in dB.
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What is Impedance or Characteristic
Impedance ?
The characteristic impedance of a transmission line or antenna can
be defined in terms of the ratio of the voltage and current amplitudes,
and the incident and reflected waves. VSWR has a similar general
relationship in its definition. For communication systems, the 50-ohm
characteristic impedance has become the adopted standard. It is
the result of an industry compromise between maximum and minimum
voltage and current handling of a transmission line. Most aviation
communications antennas are matched to 50-ohm systems. The VSWR
is an indication of how well the antenna is matched to a 50-ohm
characteristic impedance system. A VSWR of 2.0:1 (9.5dB return loss)
corresponds to approximately 90% of the RF power being transmitted/received.
With a 3.0:1 VSWR (6.0dB return loss) approximately 75% of the RF
power is being delivered to and from the antenna.
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What is antenna gain ?
The relative increase in radiation at the maximum point expressed
as a value in dB above a standard "reference" antenna, usually a
1/4 wave monopole (2.15dBi) or a 1/2 wave dipole (5.0dBi). This
reference is a calibration to 0dBi for an "isotropic" radiator.
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What is polarization ?
The polarization of an electromagnetic wave is defined by the direction
of the electric field.
Horizontal polarization: The electrical field is parallel to the
ground.
Vertical polarization: The electrical field is perpendicular to
the ground.
RHCP: Right Hand Circular Polarization.
LHCP: Left Hand Circular Polarization.
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What is a 1/4 wave antenna ?
A quarter-wave antenna is a vertical monopole that has its physical
length equivalent to one-fourth wavelength of the transmitted or
received frequency.
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What is a 1/2 wave antenna ?
An antenna that has its physical length equivalent to one-half the
wavelength of the transmitted or received frequency. The most widely
used antenna is the half-wave antenna, commonly called a dipole
antenna.
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What is a dipole ?
An antenna that has two quarter-wave antennas placed back-to-back
and is fed with a balanced transmission line. The most common dipole
is the half-wavelength dipole. These antennas are ground plane independent.
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What is a monopole ?
The quarter-wavelength monopole is the most common antenna configuration.
It requires a ground plane with at least a quarter-wavelength radius
for proper operation.
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What is reflection on an antenna and
transmission line system ?
In a perfect antenna system, all the energy supplied to the antenna
would be 100% transmitted or received. However, in actual practice,
some portion of the transmitted/received energy is reflected back
to the source with a resultant decrease in energy. The more energy
reflected back, the more inefficient the antenna. The condition
of most antennas can be determined by measuring the power being
supplied to the antenna (forward power) and the power being reflected
back to the source (reflected power). These two measurements can
be directly related to antenna VSWR.
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What is shadowing ?
Antennas need to "see" with a direct line-of-site to the source.
Antenna patterns can be disrupted by landing gear or vertical stabilizers
as examples. When mounting antennas, try to locate in areas where
shadowing is kept to a minimum.
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What is multipath error ?
For GPS antennas, multipath errors are caused by the interference
of a signal that has reached the receiver antenna by two or more
different paths. Usually caused by one path being bounced or reflected.
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