Nat NT030, Installation And Operation Manual

Page: 14 / 56

VHF Transceiver Manual SM05 Rev 4.00
Nov 26, 2003 Page 1-4
ENG-FORM: 800-0107.DOT
CONFIDENTIAL AND PROPRIETARY TO NORTHERN AIRBORNE TECHNOLOGY LTD.
2.3.4.4 Antennas
Correct antenna placement and mounting is critical to achieve the best possible
performance. In general, keep all antennas as widely separated as possible and clear
of any large airframe obstructions. Avoid any placement that puts ANTENNAS OF LIKE
FREQUENCIES CLOSE TOGETHER. Especially troublesome will be AM COMM and
VHF FM antennas in close proximity.
Bottom mounted antennas will perform best in flight, but poorest on the ground during
testing. UHF antennas will be severely degraded by 'masking' effects of the fuselage or
stabilizers, and generally give best performance when bottom mounted.
Any blade or whip antenna must be surrounded by a ground plane surface (metallic,
grounded material) equal in radius to the height of the antenna, as an absolute
minimum. Poor grounding will result in severe reflected power and high levels of RFI
throughout the airframe.
Keep cable bends to a minimum at the antenna and use 90 degree elbow adapters if
sharp bends are required. Avoid antenna locations that will become fouled with oil,
water, fuel or dirt as this will degrade performance.
Roof mounts (in close proximity to rotor blades) are permissible without severe
degradation, but should not be adjacent to VHF AM COMM antennas, or cross-talk may
result.
Use caution with cable routing and length on VHF systems. Sharp kinks, bends or
coiled cable may adversely affect performance. Cable runs should be as short as
possible for good performance. Use only high quality mil-rated cable (RG58C/U),
microwave rated cable (RG214/U), or cable with a teflon dielectric for longer runs.
Type 'N' fittings should be used throughout, if possible, to prevent degradation over
time, and to provide the best impedance match.