De PA0SNY/AM explanation page
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Some details of HF on the A330 (747 is similar)

The purpose of this page is to give some background on the HF equipment installation
on the Airbus A330. I will not go into deep details, but will try keep it simple yet informative.

Basically the A330 has 2 HF systems, each consisting of an HF transceiver. This is the
Rockwell Collins model HFS-900 with p/n 822-0330-020.

the nose seen from RH side
Both transceivers are mounted
in the avionics compartment, an area just forward of the forward cargo compartment.

The HF transceivers are controled from the cockpit. From three so called RMP's.
These are the Radio Management Panels. From these panels the 'radio' is selected
(HF1 or HF2) and the desired frequency is selected in the 'standby' window. After
that this frequency is transfered to the 'active' window. When pressing the PTT on
the microphone, the antenna coupler matches the transceiver output to the antenna.

a view of the antenna location
From the Avionics Compartment, 52 ohm coaxial cabels run to the tail of the aircraft.

On the lower part of the vertical tailplane, the HF antenna is mounted on isolating studs on the front spar, behind a composite nose.

The output of each antenna coupler goes directly to a piece of metal: the antenna.

view of tail from LH side
Here a pictorial display of the approximate location of the couplers and the antenna
The direct coupling of the two couplers is here well visible. The connecting cable from the coupler  (at the lower side if this picture) is fixed on the isolating stud mechanically, and attaches to the metal piece that we now know as 'the antenna' on the top
the attachment
the piece of metal called antenna
The situation on the tail's front spar: the whole antenna. The length is around three and a half meters (so 10 feet) only
Note: on the 747 classic it is the stick in the wingtip! Same size.
The antenna can be 'tuned' over the range of the system: 3.0 untill 24 MHz

This is less than what the transceiver can cover (up to 30 MHz)

Also note that the transceiver can provide both USB and LSB, but USB is the only mode that can be selected (aviation standard!)

And finally an overview of the HF system. You see here the 'essentials' of the systems.
systems interconnection
RMP = Radio Management panel, one for each pilot and a third for the observer
  AMU = Audio Mangement Unit: arranging audio for all equipment on board
  ACP = Audio Control Panel, one for each pilot and a third for the observer.

On the AMU the HF system is selected 'on' and the RX volume is being set. The AMU has a
 switch to PTT the (HF) system. This switch is doing the same as the PTT switch on the
microphone. Because of my position in the cockpit under such circumstances (on the 'fourth
seat, behind the pilot on the right hand side) I prefer to use the PTT on the hand microphone.

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