Mentor radio MB User manual

MODEL MB SOLID-STATE VHF
AIRCRAFT BAND RADIO RECEIVER/TRANSMITTER
INTRODUCTION
The Mentor Model MB receives and transmits on up to six discrete channels in the vhf
aviation band between 118 and 136 Mhz. It is a base transceiver for air carriers, airports,
fixed-base operators, corporate flight departments, hospital helipads, etc. Extensive use is
made of integrated circuits and the latest MOS and bipolar technologies. The advantages
of the solid-state design include high inherent reliability, reduced size and weight, low
power consumption and minimum heat dissipation. The unit is self-contained except for
the antenna, coaxial cable and the microphone. Optional features include a key-operated
on-off switch, remove operation and relay rack mounting. It operates from a 105-125 volt
50/60 Hz power line, or from 210-250 volts by selecting that range on the power entry
module on the rear. Power cords are available for nearly any country in the world.
CIRCUIT DESCRIPTION
The receiver is a single-conversion superheterodyne with a triple-tuned dual gate MOS-
FET r.f. amplifier stage preceding a dual gate MOSFET mixer. The separate local oscilla-
tor is crystal controlled, using 3rd overtone crystals operating at one-third the injection
frequency. In radios having two or more channels, the channel selector switch simultane-
ously connects the proper crystals into the receiver local oscillator and transmitter oscilla-
tor circuits. This switch also applies a d.c. voltage to receiver and transmitter r.f. tuned
circuits, which are electronically tuned by voltage-variable capacitors (varactors). Re-
ceiver selectivity is primarily determined by an eight-pole monolithic crystal filter con-
nected between the mixer and i.f. amplifier. Automatic gain control is applied to the r.f.
amplifier and the first of the two integrated circuit i.f. amplifier stages. A conventional
diode detector is followed by noise limiter, audio preamplifier and squelch circuits. An
automatic leveling circuit (ALC) gives added gain for weaker audio signals, but less gain
for strong signals, providing a relatively constant speaker level and preventing unpleasant
blaring from strong, heavily-modulated signals. This circuit also compensates for varia-
tions in voice strength when modulation during transmissions. Following the volume con-
trol, three stages of audio amplification deliver up to 5 watts audio to an internal 8 ohm
speaker. Separate i.f. and r.f. AGC amplifiers allow the receiver to handle a very wide
range of signal strengths with little change in speaker loudness.
The transmitter consists of a crystal-controlled oscillator, a frequency tripler, a buffer-
amplifier stage and a 4-stage broadband r.f. power amplifier, the last three stages of which
are collector (amplitude) modulated. The channel frequency is generated using precision
3rd overtone crystals operating at one-third the final frequency. At the transmitter output a
low-pass filter network reduces harmonics to meet U.S. FCC regulations. Additional cir-
cuits at the output include an electronic transmit-receive (T-R) switch, r.f. level sensing,
and a reflected-power sensing bridge (antenna SWR indicator circuit) that shows the con-
dition of the antenna system.
1101242-1 (rev. 6-98)
MENTORRADIOCO

MODEL MB VHF AIRCRAFT BAND
RADIO RECEIVER/TRANSMITTER
INSTALLATION
Select a desk, counter, shelf or table top on which to place the Model MB, convenient to
the operator but inaccessible to unauthorized persons. The MB is also available with op-
tional relay rack mounting. Allow several inches space behind and above the unit for air
circulation; this is especially important if heavy use is anticipated. Plug the line cord into a
properly grounded standard 3-wire electrical outlet. If an extension cord is needed, a 3-
wire type should be used. For operation outside of North America, an appropriate line
cord must be used, and to prevent damage to the radio be certain that the voltage selector at
the power inlet is set for 220 volts if that is the voltage available at the site.
The performance of both the receiver and transmitter is affected considerably by the an-
tenna and coaxial cable installation. For greatest range in all directions, the antenna should
be as high as practical, consistent with regulations concerning heights of structures near
the airport. If possible the antenna should be higher than nearby buildings to prevent
shielding or “shadowing”. Antenna height is especially important for communications
with lower altitude aircraft, or if there is higher terrain within desired communications dis-
tances.
The antenna for the Model MB should be located well away from other communications
antennas if at all possible. Strong nearby signals from transmitters on other channels can
block or interfere with reception in any radio receiver. If there are other aviation bad
transmitters and antennas on the airport, the minimum recommended distances between
these and the MB antenna depend on the frequency separations, and are as follows:
frequency separation distance between antennas
less than 0.5 MHz 0.5 miles (800 meters)
0.5 to 2 Mhz 1000 ft. (300 meters)
2 to 5 Mhz 500 ft. (150 meters)
over 5 Mhz 300 ft. (100 meters)
If it is impossible to locate the MB’s antenna sufficiently far from other antennas, and in-
terference results, consult with Mentor Radio Co. (440-942-2025).
The coaxial cable (“coax”) connecting the antenna to the Model MB causes signal attenua-
tion proportional to its length in both receive and transmit modes, so that a short cable is
desirable. This factor often conflicts with the need to locate the antenna high, away from
structures and distant from other antennas. For longer cables, coax types with lower signal
loss should be used. Type RG-58 coax is suitable for lengths up to 50 feet (15 meters).
Type RG-8, or similar, can be used for lengths to 150 feet (45 meters). For more than 150
feet, consult with Mentor Radio Co. about very low loss cables.
1101242-2 (rev. 6-98)

MENTOR RADIO CO.
Very low loss coaxial cables are thicker, heavier and stiffer and are therefore harder to
work with and install. For lengths that exceed 150 feet (50 meters), consider remote op-
eration.
The coaxial cable should be terminated at the radio end by a “UHF” type connector, some-
times called a “PL259”. The connector should be installed by a person skilled in this op-
eration. This connector plugs into the mating connector (type “SO239”) on the rear of the
MB—be sure to turn the ring with the internal threads CW until the connection is firmly
made. At the antenna end, if there is no similar connector, any unshielded center lead
should be no longer than 2 inches (5 cm.)
A number of acceptable microphone choices exist. Whether of the hand held aircraft style
or a table or desk “dispatchers” style, a transistorized dynamic microphone element is
much preferred over the older carbon type elements which have poorer performance and
shorter lives. Many readily available microphones, such as a “condenser”, un-amplified
dynamic and ceramic types will not work with the model MB. The microphone plug must
be of the 3/16 inch diameter 2-circuit type, as commonly used in aircraft. The microphone
plug should be inserted completely and firmly into the microphone jack at the lower left of
the front panel of the MB.
If the Model MB is to be operated remotely, refer to additional installation instructions that
accompany the various kinds of remote accessory equipment.

1101242-3 (rev. 6-98)
MENTOR RADIO CO.
MODEL MB VHF AIRCRAFT BAND
RADIO RECEIVER/TRANSMITTER
OPERATION
The Model MB is turned on and off by the rocker switch near the lower right corner of the
MB front panel. When the MB is on the green lamp built into the switch will light.
The volume control knob (marked VOL) adjusts speaker loudness to the level preferred by
the operator. Since the on-off switch is not combined with the volume control, the volume
setting may be left unchanged when the radio is turned off. The volume control does not
affect transmitter operation.
The squelch control allows the operator to eliminate undesirable receiver background noise
when no signal is being received. Turning the knob (marked SQ) fully counterclockwise
“unsquelches” the receiver, allowing background noise and very weak signals to be heard.
Some operators may prefer to leave the receiver unsquelched at all times. For those who
prefer to use the squelch, periodic unsquelching can serve as a receiver test, since a large
reduction in background noise might indicate receiver malfunction. To squelch the back-
ground noise, rotate the SQ control clockwise only as far as necessary to just stop the
noise. Further rotation may result in not hearing more distant aircraft. There may be some
circumstances in which an operator does not wish to hear more distant aircraft, such as
when such aircraft are communicating with a different ground station. In this situation the
control may be rotated fully clockwise. The squelch control does not affect transmitter
operation.
The signal strength meter (S-meter) is useful for checking receiver sensitivity and the rela-
tive strengths of aircraft transmitters whose positions are known. For example, if several
aircraft at a specific ramp or taxiway position produce S-20 readings, while another pro-
duces only S-3, the latter's transmitter or antenna is probably not operating properly. Simi-
larly, if most aircraft calling “5 miles out” produce an S-7 reading, another aircraft there
that barely moves the S-meter needle probably has a poor transmitter or antenna. How-
ever, if an aircraft calls and produces a good meter deflection (S-3 or higher) yet sounds
weak, the aircraft’s transmitter is producing a good carrier signal but has poor modulation
(perhaps from a bad microphone). The S-meter can also be used to study the effects of
shadowing of the base station antenna due to nearby buildings, structures, terrain, etc., by
having a cooperating aircraft fly a constant radius circle around the airport and calling at
different locations. If the Model MB receiver is working properly and the operator is fa-
miliar with “average” S-meter readings for aircraft at various distances, then the meter can
be used as a crude indicator of the aircraft’s distance from the station. The S-meter can
also suggest loss of Model MB receiver sensitivity, if the readings from all aircraft become
“less than usual”. Intelligent and thoughtful observations of the S-meter can often suggest
the causes of communications problems.
1101242-4 (rev. 6-98)

MENTOR RADIO CO.
The dial markings of “s-units” on the S-meter are somewhat arbitrary. If the MB receiver
is operating normally, the following data are approximately correct:
reading signal strength microvolts
S1 weak 3
S3 fair 6
S5 good 12
S7 very good 25
S9 strong 50
above S9 very strong 50-50,000
It takes about an S1 signal to “break squelch” at the minimum squelch setting. If the op-
erator observes some small needle deflections, but hears no sound in the speaker, he can
unsquelch the radio to determine whether the calls are directed to his station.
If the Model MB has more than one channel, the rotary channel switch on the front panel is
used to select the desired channel. Transmit and receive frequencies are switched simulta-
neously. Changing the switch position connects a different pair of crystals (one each for
receiving and transmitting) and readjusts all tuned circuits for the new channel. There may
be small channel-to-channel variations in receiver sensitivity (as may be indicated by the
speaker noise level) but this will not affect communications significantly. In case of a
communications problem, check that the frequency selector switch has not been inadver-
tently changed to a different channel.
An aviation type noise-canceling microphone is recommended. To transmit, press the
push-to-talk (ptt) switch on the microphone. Hold the microphone close (1/4 inch or 1/2
cm) to your mouth and speak directly into the center of the microphone’s acoustical open-
ing, at a moderate voice level.
While the MB’s audio leveling circuits compensate for variations in microphone output,
“microphone technique” is extremely important for good clear transmissions. Aviation
microphones are purposely designed to be highly directive and are very sensitive to the
distance from the speaker’s mouth. This is done to reduce pick-up of background noise.
It is impossible to overemphasize the importance of holding these microphones close to the
mouth and speaking clearly.
MICROPHONE TECHNIQUE
1. Hold microphone close to mouth (1/4 inch or 1/2 cm)
2. Enunciate clearly
3. Speak with average loudness-not softly (but don’t shout)
If a non-noise canceling pedestal (dispatcher’s) microphone is used, it is not as essential to
hold the microphone close to the mouth. These microphones will pick up more back-
ground and room noise, including the voices of other persons talking, radios playing, etc.

1101242-5 (rev. 6-98)
MENTOR RADIO CO.
The three indicator lights at the upper right on the front panel are all of the high brightness
light-emitting diode (LED) type. They operate only when transmitting. From left-to-right
the indicator functions and colors are: transmitter carrier (yellow), modulation (green) and
antenna (red).
The transmitter carrier indicator circuit samples the transmitter radio frequency power out-
put at the Model MB antenna connector. Thus the circuit is more than just a “stuck micro-
phone” indicator. If the lamp is on, the transmitter is producing a radio frequency carrier
signal. If the microphone p-t-t switch is pressed, and the lamp does not turn on, the trans-
mitter is not working properly.
The modulation lamp should light only when the operator speaks into the microphone.
Due to the nature of voice modulation, this lamp will “flicker” as you talk. Failure to light
indicates a problem in the microphone, the microphone connector or the modulation cir-
cuits. Note that the lamp will not light if the microphone is held too far from the speaker’s
mouth.
The red antenna lamp warns of a bad antenna system. It lights when transmitting if there is
more than 10% reflected antenna power. This is equivalent to an SWR (standing wave ra-
tio) exceeding 2:1. Most good antenna installations have an SWR less than 1.6, with re-
flected power less than 5%. If the ANT lamp lights, a person skilled in antenna installa-
tions should inspect the coaxial cables, connectors and the antenna itself, and correct any
problems found. Note that some antennas must be cut to a certain length to match the fre-
quency in use. If the Model MB has two or more widely spaced channels, a “broadband”
antenna may be necessary to achieve low reflected power on all channels.
Note: The Model MB is designed to withstand high SWR. It
is not necessary to cease transmissions if the ANT lamp
comes on. However, the system should be inspected as soon
as possible, since performance is impaired. In particular,
make sure the antenna connector on the rear of the MB has
not been inadvertently removed or loosened.
Thoughtfully used, the three transmitter indicator lights can suggest the causes and solu-
tions to many communications problems.

1101242-6 (rev. 6-98)
MENTOR RADIO CO.