Welcome To Comant

	How should antennas be installed or mounted ?
	Refer to FAA Advisory Circular 43.13-2b 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.

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.

	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.

	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.

	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.

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.

	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 43.13-2b.

	What type of coaxial cable should be used for antenna installations ?
	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 or RG-400. The FAA may require the use of flame retardant cables for certain installations such as ELT. Check with the appropriate FAA office.

	How long can coax 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.

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.

	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.

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.

	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.

	Do VOR/GS V-Dipoles such as the CI 158C require an in-line balun ?
	No. These antennas have an internal balun.

	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.

Why is there a short from center pin to ground on some antennas ?

Some antennas contain an impedance matching network or 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.

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.

	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.

	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.

	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.

	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.

	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.

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.

	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.

	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.

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.

	What is Return Loss ?
	This is basically the same thing as VSWR. Return Loss is equivalent to VSWR but is expressed in dB.

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.

	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.

	What is polarization ?
	The polarization of an electromagnetic wave is defined by the direction of the electric field.

	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.

	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.

	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.

	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.

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.

	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.

	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.