NPX136D Series Panel Mount P25 VHF Transceiver
SM68 Installation and Operation Manual
Section 2 Rev: 1.00
Issue
4 Page
2-3
ENG-FORM: 805-0118.DOT
CONFIDENTIAL AND PROPRIETARY TO NORTHERN AIRBORNE TECHNOLOGY LTD.
recommended that VHF FM coax runs be widely separated, or be made using triaxial cable, with the outer
shield bonded to the airframe at one end only (transceiver end). In communication-intensive applications,
poor cable routing and shielding may drastically compromise over-all system performance. Symptoms may
be spurious squelch opening, RFI (Radio-Frequency Interference), and garbled reception.
RF cables must be neatly terminated (solder or crimp), and tested for shorts prior to system check-out
(
not
while connected to the radio or antenna). Keep cable bends to a minimum at the antenna. Avoid
sharp bends in the coax cables (minimum 3" radius) to prevent severe reflections. If sharp bends are
required, use 90
°
elbow adapters.
Unless otherwise noted, all wiring shall be a minimum of 22 AWG, except power and ground lines, which
shall be a minimum of 20 AWG. Reference the Interconnect drawing for additional specifications. Check
that the ground connection is clean and well secured, and that it shares no path with any electrically noisy
aircraft accessories such as blowers, turn and bank instruments or similar loads. Power to this unit must
be supplied from a separate circuit breaker or fuse (fast blow), and not attached to any other circuit
breaker without additional protection. Verify that the selected circuit breaker size and wire gauge are
adequate for the installation using the techniques specified in AC43.13-1B Change 1, Paragraphs 11-47
through 11-51 and 11-66 through 11-69.
Grounding and bonding should be in accordance with the original aircraft manufacturers Maintenance
Instructions or AC 43.13-1B Change 1, chapter 11, section 15.
2.4.5
Antennas
WARNING:
To limit exposure to radio frequency fields that exceed exposure limits for
people occupying the aircraft, install the antenna such that it is mounted
either
on a ground plane that is between the antenna and the occupants of the aircraft,
or
further than 32 inches (80cm) from the occupants of the aircraft.
Correct antenna placement and mounting is critical in order to achieve the best possible performance. In
general, keep all antennas as widely separated as possible and clear of any large airframe obstructions.
Installation of the antenna should be in accordance with the aircraft manufacturer’s instructions for
continued airworthiness or AC 43.13-1B Change 1, chapter 11, section 15 and AC 43.13-2A chapter 3. If
possible, the antenna should be located a minimum of 12 ft from aircraft navigation receiver antennas and
a minimum of 4 feet from aircraft communications and ELT antennas. Be careful not to choose
separations that closely approximate ¼, ½ or whole-number multiples of the navigation or
communications system wavelength.
Note
: Avoid any placement that puts antennas of like frequencies close together.
Bottom mounted antennas will perform best in flight, but poorest on the ground during testing. Antennas
may be severely degraded by 'masking' effects of the fuselage or stabilizers, and generally give best
performance when bottom mounted.
To reduce interference from rotor modulation and other undesirable stimuli, any blade or whip antenna must
be surrounded by a ground plane surface (metallic, grounded material) with a radius equal to or greater than
the height of the antenna. Under the same conditions, the antenna is also much less likely to cause
interference to other aircraft systems (e.g. coupling into audio system, fluctuations in instrumentation, etc).
Poor grounding will result in severe reflected power and high levels of RFI throughout the airframe.
