Avionics Radio Wiring

Posted by ProPilot in Aviation Parts on 10 4th, 2008 | no comments

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Avionics Radio Wiring
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Avionics engineers and avionics installations.

When aircraft mechanics plan new avionics installations or avionics modifications for an aircraft, the installers and technicians will be presented with the proposed installation. They will input their concerns and expectations. This will go towards increasing the reliability and maintainability of an installation.

Damage to surrounding wiring and connectors could have occurred without the avionics engineers knowledge. High-quality installations prevent problems caused by vibration, moisture, chafing, RF interference, and other sources of trouble.

Aircraft mechanics installing avionics radios.

The initial part of avionics installations involve building wiring harnesses, then roughly routing the harness into the airplane to see how it will fit. During the rough routing, the installer should note all points that will require special attention to avoid chafing and RF interference. As the actual routing takes place, the installer should take care of those potential trouble spots by installing clamps, protecting wiring harnesses with plastic spiral wrap, and installing caterpillar grommets in lightening holes. To avoid RF interference, the harness should be routed to clear high current cables and any other wiring that could interfere with each other electronically.

To prevent chafing, all wiring harnesses, plumbing, and installed equipment should not come into hard contact with the structure or with each other. At no time should equipment or equipment racks come into direct contact with nearby structures. Because the aircraft skin and structure can flex during flight, establish at least a quarter inch or more for adequate clearance around the proposed and adjoining equipment.

If wiring harnesses lye gently on smooth aluminum skin with no sharp edges, the likelihood of damaged wiring is slim, but possible. The key is relative motion and pressure. The greater the pressure and relative motion, the greater the potential for insulation breakdown. To be safe, it's preferable to clamp the harness to protect it from rubbing against the skin.

In the avionics bays at the nose of aircraft, shock-mounted equipment racks should take into account the movement of the radio during normal aircraft operations. The same is true of wiring harnesses clearance from plumbing lines, especially those carrying oil, fuel, and oxygen. There should be no physical contact between adjoining plumbing, wiring, or structure. Never use plumbing for primary support. It's okay to use standoffs to separate harnesses from plumbing, but at no time should the plumbing carry the weight of the harness.

Wires are insulated, but the insulation isn't impervious to damage from sharp edges, heat, or excessive pressure such as that imposed when wires are clamped with nylon ties against a metal surface. Designers must work to a consistent standard for wiring harness installations.

A mantra for aircraft mechanics is don't limit clamping provisions. A sufficient quantity of clamps is necessary to prevent harness droop between clamps. Don't route wiring harnesses to come into contact with sharp surfaces or ride against any moveable surface. Provide antichafing if necessary. Do not design location and space requirements without allowing for service loops (adequate slack in harness that will allow maintenance). Don't design wiring harnesses to route in hot areas without adequate thermal protection. Don't route harnesses in areas that are subject to chemical damage without protective conduit, such as landing gear wells and engine compartments. Rack mounting. The shelf is prepared with inserts that are first drilled, edge-filled for strength, inserted, and injected to prevent the inserts from coming loose and to add additional strength.

Aircraft mechanics should follow these rules on mounting radio receivers, transmitters, amplifiers and computers. Hard-mounted equipment can be installed as close as necessary to other equipment except for clearance needed for cooling (usually 1/4-inch). Rack-to-equipment contact should take bonding into consideration when depending on tension contact. Paint should be removed from the radio where tension contact is expected to touch bare metal. Provide harness supports at the back of the rack to alleviate stress on wiring and connectors that could cause difficult-to-troubleshoot failures down the line. Allow sufficient distance between the radio and the aircraft's skin. Normal airflow and aerodynamic stresses on the skin can cause changes in this clearance. The goal is to avoid contact between the skin and the radio. Fasteners for holding radio racks in place should be secured with locking devices (either a lockwasher or locknut) to prevent vibration from allowing screws to loosen. This is especially important where radio racks sit above flight controls. If a loose radio rack could impinge on flight controls, it is a good idea to add supports to the rack as a backup to prevent flight control interference. Provide sufficient space for wiring harnesses and coaxial cable connectors. Coaxing cables must enter the mating connector on the equipment in as straight and natural a routing as possible to prevent connector damage.

Provide protection from moisture. The time to find out your windshield is leaking is before you spent out on a new radio installation. Make sure all fasteners used in the installation can handle stresses that will be imposed.

If the installation is in a pressurized aircraft, all areas that penetrate the pressure vessel must be sealed to prevent cabin pressure leakage. One small leak might not affect pressurization, but a number of small leaks could cause a significant drop in the ability to pressurize the cabin.

aviation-database.com has lots of resources for the aircraft industry. The web is a vast source of information. Aviation-database collects the industry into one huge database of contacts. aircraft mechanics and avionics engineers are supplied by TAC Europe just one of the companies featured who supply licensed and qualified technical staff.

Bnc Connector

Use

The BNC connector is used for RF signal connections, for analog and Serial Digital Interface video signals, amateur radio antenna connections, aviation electronics (avionics) and many other types of electronic test equipment. It is an alternative to the RCA connector when used for composite video on commercial video devices, although many consumer electronics devices with RCA jacks can be used with BNC-only commercial video equipment via a simple adapter. BNC connectors were commonly used on 10base2 thin Ethernet networks, both on cable interconnections and network cards, though these have largely been replaced by newer Ethernet devices whose wiring does not use coaxial cable. Some ARCNET networks use BNC-terminated coax.

Specifications

BNC connectors exist in 50 and 75 ohm versions. Originally all were 50 ohm and were used with cables of other impedances, the small mismatch being negligible at lower frequencies. The 75 ohm types can sometimes be recognized by the reduced or absent dielectric in the mating ends. The different versions are designed to mate with each other, although the impedance mismatch in the cable may lead to signal reflections. Typically, they are specified for use at frequencies up to 4 and 2 GHz, respectively.

75 ohm BNC Connectors are primarily used for video and DS3 Telco central office applications whereas 50 ohm are used for data and RF. There was a convention in the BBC that BNC connectors used for video were always 50 ohm probably because a stray 50 ohm BNC plug would damage a 75 ohm socket if connected in error. Many VHF receivers used 75 ohm antenna inputs, so they often used 75 ohm BNC connectors.

History

The connector was named after its bayonet mount locking mechanism and its two inventors, Paul Neill of Bell Labs (inventor of the N connector) and Amphenol engineer Carl Concelman (inventor of the C connector). Other backronyms the BNC has picked up over the years include: aby Neill-Concelman, aby N connector, ritish Naval Connector, ayonet Nut Connector, ayonet Naval Connector. The basis for the development of the BNC connector was largely the work of Octavio M. Salati, a graduate of the Moore School of Electrical Engineering of the University of Pennsylvania (BSEE '36, PhD '63). He filed a patent in 1945 (granted 1951) while working at Hazeltine Electronics Corporation for a connector placed on coaxial cables that would minimize wave reflection/loss.

Similar connectors

The BNC connector is much smaller than the earlier N or the C connectors.

A threaded version of the BNC connector, known as the TNC connector (for Threaded Neill-Concelman) is also available. It has superior performance to the BNC connector at microwave frequencies.

Triaxial BNC connector

BNC connectors are commonly used in NIM electronics, but they are now often replaced by LEMO 00 miniature connectors which allow for higher densities. For higher voltages, MHV and SHV connectors are typically used. MHV connectors are easily mistaken for BNC connectors and can be made to mate with them by brute force. The SHV connector was developed as a safer alternative to MHV connectors and will not intermate with ordinary BNC connectors.

In the USSR, BNC connectors were copied as SR-50 (Russian: -50) and SR-75 (Russian: -75) connectors. These connectors have slightly different dimensions (as a result of recalculating from Imperial to Metric system), but are generally interchangeable with BNC, sometimes with force applied.

Twin BNC (also known as twinax) connectors use the same bayonet latching shell as an ordinary BNC connector but contain two independent contact points (one male and one female), allowing the connection of a 78 ohm or 95 ohm shielded differential pair such as RG-108A. They are capable of operation at 100 MHz and 100 volts. Twin BNC connectors will not intermate with ordinary BNC connectors.

Triaxial (also known as triax) connectors are a variant on BNC which carry both a signal and guard as well as ground conductor. These are used in sensitive electronic measurement systems, particularly of Keithley manufacture.[citation needed] Early ones were designed with just an extra inner conductor, but later tri-axial connectors also include a three-lug arrangement to rule out an accidental forced mating with a BNC connector. Adaptors exist to allow some interconnection possibilities between tri-ax and BNC connectors.

Female BNC connector

Cables with BNC connectors

Adapter between a female BNC connector and banana plugs

Pulse generators with BNC connectors and cables

Male 75 ohm BNC connector

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Avionics Radio Wiring

Bnc Connector

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