The antenna, its mounting and connecting coaxial cable is critical to the performance of the
radio. It has a direct effect on the range for both transmission and reception.
As to type of antenna, we recommend one designed for aircraft usage. It is possible to use a
simple whip antenna, but this must then be ‘cut’ for correct match and will only operate
correctly over a limited part of the aircraft range of frequencies. An antenna designed for
aircraft use provides correct operation over the full range of aircraft frequencies.
Where should you mount it? Have a look at similar aircraft. Talk to other owners. In general, it
should have the best possible all round view of the horizon – whether above or below the
body of the aircraft. Also, it must have a ‘ground plane’ – if it is not a dipole type. A ground
plane is a metal surface (ideally) with a minimum diameter of about 1.3m (50inches) having
the antenna mounted in the middle. This is easy in a metal aircraft. In aircraft of wooden and
composite construction it is usually provided by a separate metal sheet. If your antenna
manufacturer has recommended dimensions for the plate, follow them.
If your aircraft has digital instruments mount the antenna as far away as possible from these
devices as they may radiate small amounts of interference that your radio can receive. This
becomes more critical if your aircraft is made from wood or fibre glass which does not provide
any form of shielding.
Coaxial cable to connect the antenna to the radio should be type RG400. RG58 is an
acceptable substitute. If you make up the cable yourself – and even if you have it made up for
you – an acceptable first test of the cable can use an ohmmeter. Test for continuity of each of
the shield and the inner conductors end-to-end. Then, with the cable disconnected from the
antenna, check that there is no connection (i.e. an open circuit) between the inner conductor
and the shield. Ensure that your connections are secure and will not corrode over time
resulting in bad performance in the future.
A full test of the complete antenna installation requires a directional power meter. A Bird
Thruline Model 43 (with an appropriate ‘slug’) is a good example. An accepted standard is
that returned (reflected) power is less than one tenth (10%) of outgoing (forward) power if the
installation is operating correctly. This standard should be applied at both ends of the coax. At
the radio end, forward power will indicate the output power of the transmitter – about 6-7
watts for both the V6 and the V10. At the antenna end, forward power will be less than at the
radio end due to loss in the coax. It should still be more than 4 watts (for 6 watts output from
the radio). For an aircraft antenna, reflected power should be less than 10% of this value for
any frequency in the band – from 118MHz to 136.975MHz. Note that a check on reflected
power at the radio end will show even less than the 10% of 4 watts (or so) – again due to loss
of the reflected power going back through the coax. The 10% reflected power criterion at the
radio end is therefore not a true indication of satisfactory installation.
The antenna is a reciprocal device. What is good for transmission is also good for reception.
So a satisfactory result for the reflected power test using the transmitter indicates that the
antenna will also be working correctly for the receiver.
SETUP
Now is the time for that list you made so long ago during the Planning stage. Switch on the
Radio and enter all the configuration settings.
After configuration, the FIRST ADJUSTMENT to make is MICROPHONE GAIN.
This is critical. The Intercom VOGAD/VOX cannot work properly if the Mic Gain is not
correctly adjusted. If the gain is set very low, then the unit increases the gain automatically
during transmit so that the signal 'sounds loud enough'. Of course, in order to hear the
intercom it is natural to turn up the volume for the intercom. This means that the sidetone
(during transmit) sounds too loud (the microphone sensitivity 'increases'). With the correct
(higher) microphone gain the volume for the intercom will have a lower setting and the sound
of the sidetone is now 'in balance' with the intercom. The microphone gain setting must be
done by the user (according to how loudly he speaks) in the aircraft with his own headset.
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