FCC ID: QQTM15 Mobile transmitter for Part 87

MODEL M15 VHF AIRCRAFT BAND RADIO RECEIVER/TRANSMITTER INTRODUCTION The Mentor Model M15 receives and transmits on up to six discrete channels in the vhf aviation band between 118 and 137 MHz (25 KHz channel spacing). It operates from a 14 v dc supply, and is intended for both vehicular and airborne application as well, and can also be used as a base station. Its advanced design features and surface mount con-

struction provide compactness, light weight and high reliability. The only external com-

ponents required are a suitable antenna with coaxial cable, an aviation-type microphone and a 4- or 8-ohm speaker. CIRCUIT DESCRIPTION The receiver is a single-conversion superheterodyne with four varactor-tracked r.f. tuned circuits and dual-gate MOSFET transistors in the r.f. amplifier and mixer stages. The local oscillator uses 3rd overtone crystals, and is followed by a MOSFET frequency trip-

ler stage. Receiver selectivity is primarily determined by a six-pole 10.7 MHz crystal filter connected between the mixer and i.f. amplifier. the latter consists of two cascode integrated circuit amplifier stages. Automatic gain control is applied to the r.f. amplifier and the first i.f. amplifier stages. A conventional diode detector is followed by a noise limiter, audio amplifier, squelch gate and an integrated circuit audio power amplifier capable of delivering 4.5 watts of audio power into a 4-ohm speaker. The transmitter oscillator also uses 3rd overtone crystals, followed by varactor-tracked MOSFET tripler and buffer amplifiers. Three untuned broadband stages boost the trans-

mitter carrier power to approximately 5 watts. Amplitude modulation is applied to the last two stages, the "driver" and final amplifiers. The transmitter signal passes through a 7-element Tchebychef low-pass harmonic filter which also contains two PIN diodes functioning as a T-R switch. The transmitter modulator is separate from the re-

ceiver audio amplifier, and includes an audio AGC amplifier that automatically adjusts for variations in operator microphone technique. This circuit also prevents overmodu-

lation. The modulator power amplifier consists of an integrated circuit amplifier fol-

lowed by complementary bipolar transistors. This arrangement eliminates the need for an audio transformer, resulting in less low frequency distortion while minimizing the size and weight of the radio. Mentor Radio Company 1101429 (rev 05-99)

MODEL M15 VHF AIRCRAFT BAND RADIO RECEIVER/TRANSMITTER OPERATION The M15 is turned on by rotating the volume control (VOL) clockwise from the off position. OFF is fully counterclockwise. Turning this control clockwise increases the volume or loudness at the speaker. The best volume setting depends on the ambient noise, location of the speaker and the preference of the listener. In quiet environments, 1/4 turn is often satisfactory; in noisy environments 1/2 to 3/4 turn is usually best. The channel selector switch is in the center of the front panel. It may have up to six positions; normally the number of positions corresponds to the number of crystal pairs installed. However, for single-channel radios, the switch will have two positions, both which select the single available frequency. The squelch control eliminates receiver background noise when no signal is present. In the fully CCW position, the receiver is unsquelched. Advancing the control CW will squelch the background noise. Since using squelch may also result in not hearing weak signals, the operator should use the minimum amount which eliminates annoying noise. If it is anticipated that it may be necessary to receive some very weak signals, use no squelch at all (control fully CCW). An aircraft type transistorized dynamic microphone is plugged into the MIC jack at the right side of the front panel. Be sure the plug is pushed into the jack (receptacle) as far as it will go. To transmit, press the push-to-talk (PTT) button on the microphone as far down as it will go. If a noise-canceling microphone is used, hold the microphone close to the mouth; these microphones are very sensitive to the distance between the operator and microphone. Talk directly into the center of the microphone, in a normal voice. Speak clearly and distinctlymany readability problems are caused by poor mike technique. A red lamp near the microphone jack lights up when the PTT button is pressed to transmit. if this lamp stays on after the PTT button is released, a stuck microphone is indicated. If you cannot release the PTT switch (button), turn off the M15 or remove the microphone plug so your continuously operating transmitter will not prevent other radios from using the channel. With a stuck mike, in an emergency, you can transmit by plugging the microphone back in each time you want to transmit. While the M15 receiver has been designed for congested radio signal environments, very strong nearby transmissions on other channels may bleed through, or in some cases desensitize the receiver. This does not mean that the receiver is operating improperly. If interference is a significant problem, contact Mentor Radio for assistance. Mentor Radio Company 1101430 (rev 5-99)

MODEL M15 VHF AIRCRAFT BAND RADIO RECEIVER/TRANSMITTER INSTALLATION Installation is easy, but due to the great variety of aircraft and vehicles, mechanical installa-

tion is virtually a custom job. Mentor suggests that installation be performed by an avionics or a two-way radio shop technician. Experienced technicians can avoid potential problems and adapt each installation to the users specific needs. All connections must be mechanically and electrically sound if the full reliability of the M15 is to be realized. A U-shaped mounting bracket, supplied with the M15, is useful in most installations, espe-

cially in vehicles. Using this bracket obviates the need to drill holes in the radio cover, a step that must be done only with great care to avoid damage to internal components and to avoid metal chips or screws shorting internal connections. Holes may be drilled in the bracket as needed, with no danger to the radio. Thumb screws through the sides of the bracket fit into the no. 8-32 threads on each side of the M15. The threaded portion of these screws must be no longer than 1/2 inch. Rubber washers hold the M15 firmly when the thumb screws are tightened. The speaker can be mounted in any location that provides the best combination of ease-of-

mounting and good sound. In noisy environments, the speaker should be directed towards the listener; preferably it should be located within one to three feet of the listeners head. In very noisy locations, an optional earphone output plus cushioned earphones may be best. In vehi-

cles, the speaker may be mounted in the place provided by the vehicles manufacturer, or un-

der the dashboard or even overhead. The speaker cone should be protected by a grille. The speaker should be four inches in diameter, or more, capable of handling about 5 watts, and should have an impedance of 4 ohms (an 8 ohm speaker is acceptable where the M15s full audio output is not required). Mentor Radio Co. can provide an optional weatherproof en-

closed speaker suitable for mounting in vehicles. Earphones may be connected in place of the speaker, or in parallel with it. If only earphones are used, it is not necessary to install a shunt resistor in place of the speaker. M15 electrical connections are made via a 9-pin molded nylon connector on the rear (mating connector supplied with radio). Pin numbers are molded into the connector, but are small and it is easier to refer to the attached diagram (dwg. 1101454) to determine pin functions. The mating connector is polarizedthat is, it must be tuirned to the proper orientation or it cannot be pushed on. The two corner notches must be towards the right side of the radio (looking at the front panel). Be sure the two prongs on the panel connector pass through the openings at each end of the mating cable connector. The prongs will snap in to keep the cable connector from vibrating loose. To remove the cable connector, squeeze these prongs toward each other to release their hold; jiggle and pull the connector out. Mentor Radio Co. 1101431-1 (rev 5-99) MODEL M15 AIRCRAFT BAND RADIO RECEIVER/TRANSMITTER INSTALLATION (continued) Drawing 1101454 shows connections for a typical installation, which uses only 4 of the 9 connector pins. (The extra pins are used for special options.) The pins are crimp type, but it is recommended that they also be soldered for best reliability. Wires for +14 volts and ground should be 16 ga. wire. Note that there is a fuse built into the M15, so that an external fuse is not required, although it will provide added protection should the 14 v wire in some way short to the vehicle chassis. The power ground wire should be connected firmly to the metal chas-

sis or frame of the vehicle. The speaker ground wire should connect directly to the speaker. In some cases it will be acceptable to connect the ground of the speaker to the chassis/frame and use a single ground wire for both power and the speaker. The M15 operates from 14 volt dc negative ground only. The radio will operate, with re-

duced specifications, for supply voltages down to about 10.5 volts. The voltage should not exceed 16 volts. A type 2AG 4 ampere fast acting fuse, located on the rear of the M15, pro-

tects the radio and internal and external wiring should a short occur within the radio. The fuse will also blow if the polarity of the supply is reversed. To open the fuse holder, press down on the top of the center section, then pull that section outward to its full extension of about 3/4 inch. There is a place in the fuse holder for a spare fuse, as well as the active one. To simplify vehicular installation, Mentor Radio Co. supplies a harness which includes the mating connector and wires of suitable sizes having lengths that are sufficient for most instal-

lations. RG-58A/U coaxial cable or equivalent should be used between the antenna and the M15. The M15 uses an SO-239 r.f. connector; the mating connector for the coax is type PL-259. The length of the coax is not critical, but a great deal of unnecessary length should be avoided. The antenna itself should be vertically oriented and should have an impedance of 50 ohms. A quarter-wave ground plane type is suggested. After installation and whenever antenna condi-

tion is doubtful, a standing wave ratio (SWR) measurement is recommended. In a good in-

stallation, the SWR should not exceed 1.5 at a frequency near the center of the range of chan-

nels. Vehicular antennas purchased from Mentor Radio Co. include the antenna connector with suf-

ficient coax for almost any installation. Excess coax may be coiled conveniently rather than cut off. If the antenna is a narrow band type (such as the 1/4 wave ground plane), it should be trimmed to a suitable length for the channel frequencies in userefer to instructions ac-

companying the antenna. If the M15 has more than one channel, and the difference in frequency between the highest and lowest channels exceeds 5 MHz, a broadband antenna will give better performance at fre-

quencies farthest from the center of the frequency range. Mentor Radio Co. 1101431-2 (rev 5-99) MODEL M15 VHF AIRCRAFT BAND RADIO RECEIVER/TRANSMITTER LICENSING Federal Communications Commission (FCC) rules require that radio transmitters be licensed. Application is filed on FCC form 404 for aircraft or form 406 for vehicles. IGNITION INTERFERENCE The M15 contains an automatic series-gated noise limiter which greatly reduced interference from impulse noise sources such as engine ignition systems. However, such limiters can never completely eliminate interference from nearby severe ignition noise sources. While it is standard practice to use shielded ignition systems in aircraft, most vehicles do not contain such shielding, and may at times cause annoying interference. Trucks, tractors and older automobiles are usually the worst offenders. In severe cases it may be necessary to add shielding and/or filters to such ignition systems that cause severe interference to the M15. ROTATING BEACON INTERFERENCE The rotating beacons of emergency vehicles may cause significant radio noise interference. This is often caused by worn brushes on the motorreplacing the brushes can stop the inter-

ference. Filters that mount at the beacon are available to eliminate beacon interference. Men-

tor Radio can supply such filters, or they are usually available through local electronic dis-

tributors. OTHER RADIO INTERFERENCE When other radio transmitters are installed in the same vehicle as the M15, those transmitters can interfere with M15 reception. Commercial broadcast FM stations can cause interference in an M15 receiver, especially if the broadcast antennas are less than a few miles away. Men-

tor Radio Co. can provide applications notes and other assistance in solving interference prob-

lems. Mentor Radio Co. 1101431-3 (rev 5-99)