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Avionics Test Equipment Handbook
News & Comments from the Publisher
Kitplane Avionics
Information on planning, wiring, installing and troubleshooting radios, instruments, and electrical accessories for kit-built and other experimental aircraft.
Questions? They'll be answered by highly experienced custom avionics technicians. Send e-mail to:
Index to Topics
Antennas: Mounting Location
GPS: Com Interference
Grounds: Bus Bars
TSO: Required for an Altimeter?
Wire & Breakers: Size
Q "What is a good location for mounting a com antenna on my Kitfox?"
A. The first consideration is to mount the antenna vertically. Signals in the VHF communications band are vertically polarized (transmitted vertically) and maximum range occurs when receiving and transmitting antennas are vertical.
An advantage in most kitplanes is fiberglass construction. Because plastic is transparent to radio waves, the antenna can be located inside the structure. One good location for homebuilts is on the vertical stabilizer (rudder fin). One popular approach is a 36-inch long copper foil, split at the middle for a coaxial cable, cemented inside the vertical stabilizer (rudder fin).
If you're wondering why the length is 36 inches, when production aircraft have 18-inch whips, the reason is the ground plane. In a metal aircraft, the aluminum skin makes up one half the antenna length by providing an electrical ground. The 36-inch antenna, known as a "dipole," establishes its own ground. Otherwise, you would have to install a sizeable ground plane of copper foil at the base of the antenna.
Two precautions: If your airplane is all-metal, com antennas must be externally mounted, as done on production aircraft.
Secondly, some aircraft are designed with carbon fiber composite material. Because the carbon content blocks radio waves, this rules out an internal mounting location. However, some kits have a mixture of materials; a fiberglass fuselage and a carbon fiber tail---allowing an internal fuselage mounting. Some builders have also made a cut-out in the carbon fiber on the rudder fin, installed a fiberglass panel, and fastened the antenna on the inside surface.
If you use a dipole, run the coaxial transmission line away at a right angle from the elements for as much distance as possible. Otherwise, some interaction could distort the radiating signal patter.
Q. " I've heard that my com transmitter can interfere with GPS navigation. How serious is it?"
A. It is serious. When you transmit on the VHF communications band---let's say 118.5 MHz, the transmitter unavoidably emits multiples of that frequency. Known as harmonics, they reach into the L-Band part of the spectrum occupied by GPS, about 12 times higher in frequency (1.5 Gigahertz). These high order harmonics weaken rapidly, but when they land squarely on a GPS frequency there's trouble because GPS signals are extremely low level. This happens only for certain channels, where the multiplication is correct.
If you hold down your transmit button, say, for 10 seconds, and watch your GPS signal strength page, you might see the degradation. During a critical maneuver (an approach, for example) there is a remote, but possible risk of losing GPS navigation after multiple transmissions.
There are several ways to fight the problem. Foremost, locate any com antenna three or more feet from the GPS antenna. Next tactic is to install a notch filter in the com transmission line. Tuned to the offending harmonics, it electrically "notches" them out.
In some instances, the GPS and com antenna cannot be spaced sufficiently apart. Here, the engineers have come up with a "coupler," which installs in the transmission lines of both the com and GPS. Although considerably more expensive than the notch filter, it provides a solution Ina difficult installation.
Q. "I bought a bunch of circuit breakers and cable at bargain prices at Oshkosh and want to know if I can use them in my electrical system.
A. This area has proved very hazardous to homebuilders, but not because of the quality of the merchandise. The frequent problem is improper matching of circuit breaker and wiring. In fact an editor of this website (avionics.com) took a flight with a friend who had just purchased a Lancair. When it came time to lower the landing gear, they were greeted by a strong odor and the sudden appearance of smoke in the cabin. The pilot landed safely and soon discovered why.
Let's say you install a 10 amp circuit breaker, which typically trips open somewhat above 10 amps. But if you run a 22 gauge wire from that breaker to the radio or other appliance, the current capacity, say 5 amps, is limited by the wire. Under these conditions, a 10-amp current flow will not trip the breaker, but will cause the wire to become a toaster. Besides disintegrating it also damages other cables within the same bundle. A major safety hazard and a mess to repair.
In the landing gear incident mentioned above, this is what happened. The kit manufacturer had supplied a 6-inch length of 10 gauge wire from the landing gear power pack. This lead was to be extended by the builder into the aircraft's electrical system. But the builder lengthened that lead with 18-gauge wire. Each time the landing gear was extended, the 18-gauge cable was damaged until its resistance increased and caused a red-hot condition and smoke.
The answer is simple. Consult a chart which shows the current-carrying capacity of each conductor size and length. This is shown in FAA Advisory Circular AC-4313.2a. (For more information on a CD-ROM which contains this information, along with all FAR's, click: FAA Library
Q. "I see that I need electrical grounds for my radios, accessories and other components. Can you explain the best way to do it?"
A. The electrical devices on your airplane are powered over two paths or connections; one carries current to each device, the other carries current away. To save a lot of wiring and labor, one pathway is the metal structure of the airplane, which usually is connected to the negative side of the battery. When radio trays or other devices are bolted to the airframe, they become connected to the negative or ground side of the system. The other connection,from each appliance to the positive side of the battery, is variously called; 12-volt DC, the "A" lead or the "hot" lead.
The grounding problem in a kit plane is the lack of a metal airframe for the negative connection. Each ground lead, therefore, needs to be brought to the negative side of the battery. But if the ground system is not carefully conceived, it can lead to a raft of problems; including noise or howling sounds in the audio system, intermittent switch action, reduced transmission power and oscillations.
The culprits in many of these conditions is the "ground" loop or corrosion. The ground loop appears when two ground wires are connected together with an imperfect bond. It causes a slight amount of resistance at the joint which introduces a tiny, but significant, voltage to appear. This affects sensitive circuits with high amplification, such as audio systems, often causing hum.
Corrosion produces the same effect. As oxides build up in the connection, resistance increases and voltages are generated that cause interference.
The answer is a carefully planned ground system. Check your manufacturer's manual, for example, to see if there are any precautions about grounding. Some require that all mike grounds, for example, be connected to single point.
When planning your electrical system, here are practices which have proved useful. A bus bar is an excellent technique. This is a flat copper bar installed behind the instrument panel and connected to the negative battery terminal through a heavy copper stranded cable (No. 8 gauge, for example, with Tefzel insulation). Radios and instruments are connected to the bus bar with short lengths of copper wire fitted with ring connectors. . For easy attachment, the bar can be fitted with threaded studs or other hardware for No. 6 or No. 8 screws. This should provide a low-resistance, efficient ground system.
When dealing with the starter system, the ground wire must be far heavier. Typically, a No. 2 gauge copper stranded wire should run from the engine case to the negative battery lead and the starter (which can draw several hundred amps during starting.
Autopilot
Q. " How can I make my GPS drive my autopilot?"
A.
First,
your autopilot must have the capability of
coupling to a VOR receiver. Any of the
following autopilots should have the ability to
couple to a GPS; Century, King, Navaid and
S-Tec. The typical installation is shown
below. A double-pole, double-throw switch (DPDT,
with a quarter-inch shaft) is connected with 22
gauge shielded copper wire. You will need the
autopilot and receiver installation manuals
to find the pins designated Nav Right and Nav
Left.
Be sure all connections are well
insulated neatly secured and follow any
additional instructions provided by the
manufacturer. Test the completed system in
the aircraft on the ground to be sure the
autopilot follows the left and right commands of
the VOR and GPS receivers.
TSO: Required for an Altimeter?
Q. Do I need a TSO'd altimeter? It's almost twice the cost and I don't see any difference.
A. This is among the most frequently-asked questions I hear from kit-builders. Although there is no legal requirement to have a TSO'd instrument, I strongly recommend the TSO'd version. After many years of checking and calibrating altimeters on the bench, I see a clear difference. A TSO model will often hold its tolerance year after year. On the other hand, I've pulled new non-TSO'd altimeters out of the box and couldn't bring them into calibration.
Even if you never plan to fly in clouds, at night or in low visibility, do you really want to rely on such an instrument as a primary means of determining altitude? The cost is about $200 more for the TSO version. Thus, consider the non-TSO only if your flying is day VFR with excellent visibility. Maybe this applies to the southwestern desert of the U.S., but in most other regions, you'll need a reliable altimeter
To be continued
Note: For more information on any items mentioned above, or to submit questions or comments click len@avionics.com