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The reason normal receivers have the potential of interfering with aircraft communications is that they
internally produce radio signals as part of the receiving circuitry. As shown in Figure 1, standard receivers
have one or more oscillators that produce radio signals used by the radio to tune to a specific frequency.
Signals from these oscillators that escape the receiver may interfere with communication and navigation
signals.
In contrast, the PABM1 Passive Air Band Monitor does not contain any oscillators or other radio-frequency
sources that may radiate interfering signals. Signals picked up by the antenna are filtered within the
airband and then directly demodulated to produce audio. The reason why this is not done in normal
receivers is that direct demodulation makes it very difficult to tune to a specific frequency, allowing all
signals within a band to come through simultaneously. In the case of our airband monitor this is an
advantage since no knowledge of the frequency used for air traffic control is necessary, thus requiring no
tuning.
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The Airband
The VHF airband comprises the range of frequencies between 108 and 137 MHz and is sub-divided as
shown in Figure 2.
Figure 2 – The civilian airband
The lower 10 MHz of the band, from 108–117.95 MHz, are reserved for navigational aids such as VOR
beacons, and precision approach systems such as ILS localizers.
The upper 19 MHz of the band, from 118–137 MHz, are divided into channels used mostly for AM voice
communications. This is the specific range where the PABM1 is most sensitive.
Air Traffic Control (ATC) is assigned to the 118-122 MHz, 123.6-128.8 MHz, and 132-135.975 MHz ranges.
These contain most control tower, ATIS, ground control, clearance delivery, approach control, departure
control, and Air Route Traffic Control Center (ARTCC) frequencies.
ATIS is the Automatic Terminal Information Service, which continuously transmits recorded aeronautical
information in busy airports and their immediate surroundings. ATIS broadcasts contain current weather
information, active runways, available approaches, and any other information required by the pilots, such
as important notices to airmen (NOTAMs).
The 122-123.575 MHz is assigned to private aviation services, flight service stations, balloons, gliders,
airshow communications, and other miscellaneous aeronautical communications.
