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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 ?
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
TESTING
When
should I replace an antenna ?
Can
an antenna be bench tested ?
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 ?
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
REPAIR
Can
antennas be painted ?
Can
antennas be repaired ?
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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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
TESTING
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.
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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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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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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 dipole (2.15dBi) or a 1/2 wave monopole (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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ANTENNA
REPAIR
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.
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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.
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