Avionics News October 57-10 - 54

THEORY & PRACTICE

INTERNATIONAL NEWS

interaction of these two waves will cause a standing
wave. A standing wave goes nowhere and dissipates
its energy as heat within the transmission line. Figure
3 shows incident, reflected and standing waves in
simplified form.
When everything is working properly, the incident
wave will travel down the coaxial cable with
voltage and current in synchronization. Introduce
a fault, everything goes awry, and its behavior will
depend on where the fault occurred as measured in
wavelength. When transmitting, the conditions seen
at the transmitter could be a near short, a near open,
inductive or capacitive. Changing frequencies changes
the wavelength and the resulting conditions. As a
result, on one frequency, the pilot may notice air traffic
control has trouble hearing him or her. On another
frequency, the radio could open the circuit breaker.
A good example is an open line, exactly one-quarter
of a wavelength from the transmitter. Perhaps a cable
connector came loose. Since the circuit is open, the
wave condition will be at maximum voltage and
minimum current. Nearly 100% of this wave will be
reflected back toward the radio. By the time the wave
gets back to the radio, the current wave will have risen
from minimum to maximum, and the voltage will have
dropped to minimum. Theoretically, the condition is
a series-resonant circuit. Practically, it will behave as
if shorted, which is a condition likely to damage the
transmitter, as if it transmitted directly into a short
circuit. On different frequencies, the apparent short
would disappear, and the system would behave as
if the transmitter was connected to an inductor, or a
capacitor. Of course, a completely shorted or open
coaxial cable is a worst-case scenario.
More likely, a cable will be frayed or corroded,
which will look like a partial open. Or perhaps it is
compressed between two objects, which will look
like a partial short. Still, the symptoms will change
with a change of frequencies. A pilot could notice a
problem on one frequency but none on another.
Next month, we'll bring this theory together with
antenna theory and discuss troubleshooting both
antennas and transmission lines. q

The aeronautical information publication
summarizes these operating requirements
and limits.
The new standards also allow some
uncertified, but technically capable,
ADS-B avionics to be used in light-sport
aircraft, experimental aircraft and a
range of other light aircraft and balloons.
More information about the low-cost ADS-B
can be found in CASA Advisory Circular AC
91-23 v1.0 ADS-B for enhancing situational
awareness, dated July 2020.

Continued from page 19

Continued from page 53

54

AVIONICS NEWS

*

OCTOBER

2020

New Zealand: ADS-B Grant Scheme
payments hit $1m
The New Southern Sky team announced that
the ADS-B Grant Scheme has approved over $1m
worth of claims, providing COVID-19 affected
operators with some welcome financial relief.
Over 720 applications have been received to
enter the scheme and, with more being received
every day, the team is well on its way to
approving the next $1m.
In addition, the month of August saw the total
number of aircraft equipped with ADS-B surpass
the 1,000 mark (1,018). The number of aircraft
equipped with ADS-B since the end of Level 4
lockdown has been incredibly encouraging, with
workshops on the South Island particularly busy.
These two major milestones are reason to
celebrate and is another significant step toward
transitioning to ADS-B as the primary surveillance
system in New Zealand.
"But we cannot sit back now that these
milestones have been hit," said Tom Gormley,
ADS-B Grant Scheme technical advisor. "We
encourage all participants to equip with ADS-B,
including ADS-B In, as soon as possible to ensure
that they can continue to access controlled airspace
and start to reap the safety benefits that ADS-B
brings."
More information about the New Zealand
ADS-B initiative is at nss.govt.nz/ads-b. q


http://nss.govt.nz/ads-b

Avionics News October 57-10

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Avionics News October 57-10 - Intro
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