Cobra Electronics 148 GTL User Manual

How To Use Y our

40-CHANNEL, CITIZENS BAND SSB/AM 2-WAY MOBILE RADIO

Model 148 GTL

Contents Page

The CB Story ....................................................................................

Section I: Introduction......................................................................2

Section II: Specifications ..............................................................3, 4

Section III: Installation ........................................................5, 6, 7, 8

Section IV: Operation ................................................................9 - 17

Controls and Indicators ....................................................................9

A.Control Functions..............................................................9, 10, 11

B.Indicator Functions ......................................................................12

Operating Procedure to Receive......................................................12

Operating Procedure to Transmit ....................................................13

Receiving SSB Signals ..............................................................13, 14

Alternate Microphones and Installation ..............................15, 16, 17

Section V: Maintenance and Adjustment..................................18 -21

FCC Warning ..................................................................................18

Section VI: Appendix ..........................................................19, 20, 21

Ten Code ........................................................................................19

A few Rules That Should Be Obeyed ..............................................20

How Your CB Can Serve You ..........................................................20

Use Channel 9 for Emergency Messages Only ................................21

Warranty Service Instructions..........................................................22

Limited T w o Year Warranty..............................................................23

Cobra CB Radio Accessories ..................................Inside Back Cover

OPERATING INSTRUCTIONS FOR YOUR

40 CHANNEL

CITIZENS BAND

SSB/AM 2-WAY

MOBILE RADIO

Model 148 GTL

Section I Introduction

FREQUENCY RANGE

The COBRA 148GTL transceiver represents one of the most advanced SSB/AM twoway radios ever designed for use as a Class D station in the Citizens Radio Service. This unit features advanced Phase Lock Loop (PLL) circuitry, which is used in the AM mode and in the upper and lower single sideband modes, providing complete cover age of all 40 channels shown below.

Channel Channel Frequency

in MHz

1 26.965 2 26.975 3 26.985 4 27.005 5 27.015

6 27.025 7 27.035 8 27.005 9 27.065 10 27.075

11 27.085 12 27.105 13 27.115 14 27.125 15 27.135

16 27.155 17 27.165 18 27.175 19 27.185

Channel Channel Frequency

in MHz

21 27.215 22 27.225 23 27.255 24 27.235 25 27.245

26 27.265 27 27.275 28 27.285 29 27.295 30 27.305

31 27.315 32 27.325 33 27.335 34 27.345 35 27.355

36 27.365 37 27.375 38 27.385 39 27.395 40 27.405

20 27.205

The COBRA 148GTL has a vastly superior receiver which includes an RF gain control and noise blanker circuitry effective in both AM and SSB modes., and an automatic noise limiter effective in the AM mode. The receiver also features increased protection against cross modulation and strong adjacent channel signals.

To obtain maximum performance please read carefully the descriptions and operating instructions in this manual.

The Citizens Band lies between the shortwave broadcast and 10-meter Amateur radio bands, and was established by law in 1949. The Class D two-way communications service was opened in 1959. (CB also includes a Class A citizens band and Class C remote control frequencies.)

FCC regulations permit only “transmissions” (one party to another) rather than “broadcasts” (to a wide audience). Thus, advertising is not allowed on CB Channels because that is “broadcasting.”

NOTICE:

Replacement or substitution of transistors, regular diodes or other parts of a unique nature, with parts other than those recommended by Cobra, may cause violation of the technical regulations of Part 95 of the FCC Rules, or violation of Type Acceptance requirements of Part 2 of the Rules.

Citizens Band (CB) Radio operators are no longer required to obtain an FCC license to operate their CB equipment or provide station identification. An operator of a CB radio station is required to comply with the communications act and with the rules of CB Radio Operation.

Serial No. Date of Purchase Dealer Name Keep this manual for detailed information about your Cobra CB radio.

SAVE YOUR SALES RECEIPT, THE CARTON AND “PACKING” MATERIALS FOR POSSIBLE FUTURE USE.

The CB Story

RECEIVER

Sensitivity SSB: 0.25 µV for 10dB (S+N)/N at greater

than 1/2-watt of audio output. AM: 0.5 µV for 10 dB (S+N)/ at greater than 1/2-watt of audio output.

Selectivity AM: 6dB @ 3 KHz, 50 dB @ 9 KHz.

SSB: 6 dB @ 1.1 KHz, 60 dB @ 2.3 KHz. Image Rejection More than 65 dB. IF Frequency AM: 7.8 MHz 1st IF, 455 KHz 2nd IF.

SSB: 7.8 MHz. Adjacent-Channel Rejection 60 dB AM & 70 dB SSB. AM and SSB RF Gain Control 40 dB adjustable for optimum signal

reception. Automatic Gain Control (AGC) Less than 10 dB change in audio

output for inputs from 10 to 100,000

microvolts. Squelch Adjustable; threshold less than 0.25 µV. ANL Switchable. Noise Blanker RF type, effective on AM and SSB. Voice Lock Range ±2.5 KHz. Audio Output Power 4 watts into 8 ohms. Frequency Response 300 to 2500 Hz. Built-in Speaker 4 ohms, round. External Speaker (Not Supplied) 8 ohms; disables internal speaker

when connected.

PA SYSTEM

Power Output 4 watts into external speaker. External Speaker for PA (Not Supplied) 8 ohms.

(SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE)

Section II Specifications (Cont.)

Section II Specifications

GENERAL

Channels 40 AM, 40LSB, 40 USB. Frequency Range 26.965 to 27.405 MHz. Frequency Control Phase Lock Loop (PLL) synthesizer. Frequency Tolerance 0.005% Frequency Stability 0.001% Operating Temperature Range -30° C to + 50° C Microphone Plug-in dynamic; with push-to-talk

switch and coiled cord.

Input Voltage 13.8V DC nominal, 15.9V max., 11.7V

min. (positive or negative ground).

Current Drain

Transmit

: AM full mod., 2.2A. SSB 12

watts PEP output, 2A.

Receive

: Squelched, 0.25A

Maximum audio output, 0.6A. Size 2 3/8”(H) x 7 7/8”(W) x 9 1/4”(D). Weight 5 lbs. Antenna Conductor UHF, S0239. Semiconductors 3 field effect transistors, 45 transistors,

63 diodes, 6 integrated circuits, 1 two

color light emitting diode. Meter (3-in-1) Illuminated; indicates relative output

power, received signal strength,

and SWR.

TRANSMITTER

Power Output AM, 4 watts.

SSB, 12 watts, PEP. Modulation High-and low-level Class B, Amplitude

Modulation. Intermodulation Distortion SSB: 3rd order, more than -25 dB.

5th order, more than -35 dB. SSB Carrier Suppression 55 dB. Unwanted Sideband 50 dB. Frequency Response AM and SSB; 300 to 2500 Hz. Output Impedance 50 ohms, unbalanced Output Indicators Meter shows relative RF output power

and SWR. Transmit LED glows red when

transmitter is in operation.

Section III Installation (Continued)

IGNITION NOISE INTERFERENCE

Use of a mobile receiver at low signal levels is normally limited by the presence of electrical noise. The primary source of noise in automobile installations is from the generator and ignition system in the vehicle. Under most operating conditions, when signal level is adequate, the background noise does not present a serious problem. Also, when extremely low level signals are being received, the transceiver may be operated with vehicle engine turned off. The unit requires very little current and therefore will not significantly discharge the vehicle battery.

Even though the COBRA 148GTL has ANL and NB controls, in some installations ignition interference may be high enough to make good communications impossible. The electrical noise may come from several sources. Many possibilities exist and variations between vehicles require different solutions to reduce the noise. Consult your COBRA dealer or a 2-way radio technician for help in locating and correcting the source of severe noise.

ANTENNA

Since the maximum allowable power output of the transmitter is limited by the FCC, the antenna is one important factor affecting transmission distance. Only a properly matched antenna system will allow maximum power transfer from the 50 ohm transmission line to the radiating element. In mobile installations (cars, trucks, boats, etc.), an antenna system that is non-directional should be used. A vertically polarized, quarter-wavelength whip antenna provides the most reliable operation and greatest range. Shorter, loaded-type whip antennas are more attractive, compact and adequate for applications where the maximum possible distance is not required. Also, the loaded whips do not present the problems of height imposed by a full quarter-wavelength whip.

Mobile whip antennas utilize the metal body of the vehicle as a ground plane. When mounted at a corner of the vehicle they are slightly directional, in the direction of the body of the vehicle. For all practical purposes, however, the radiation pattern is nondirectional. The slight directional characteristic will be observed only at extreme distances. A standard antenna connector (type SO239) is provided on the transceiver for easy connection to a standard PL 259 cable termination.

If the transceiver is not mounted on a metal surface, it is necessary to run a separate ground wire from the unit to a good metal electrical ground in the vehicle. When installed in a boat, the transceiver will not operate at maximum efficiency without a ground plate, unless the vessel has a steel hull.

Before installing the transceiver in a boat, consult your dealer for information regarding an adequate grounding system and prevention of electrolysis between fittings in the hull and water.

Section III Installation

LOCATION

Plan the location of the transceiver and microphone bracket before starting the installation. Select a location that is convenient for operation and does not interfere with the driver or passengers in the vehicle. In automobiles, the transceiver is usually mounted below the dash panel, with the microphone bracket beside it.

MOUNTING THE CONNECTION

The COBRA 148GTL is supplied with a universal mounting bracket. When mounting the bracket and radio to your car, make sure it is mechanically strong. Also provide a good electrical connection to the chassis of the vehicle. Proceed as follows to mount the transceiver:

1. After you have determined the most convenient location in your vehicle, hold the COBRA 148GTL with mounting bracket in the exact location desired. If nothing will interfere with mounting it in the desired position, remove the mounting bolts. Before drilling the holes, make sure nothing will interfere with the installation of the mounting bolts.

2. Connect the antenna cable plug to the standard receptacle on the rear panel. Most CB antennas are terminated with a type PL-259 plug and mate with the receptacle.

3. Connect the red DC power input wire (with the fuse) to +13.8V DC. This wire extends from the rear panel. In automobile installation, +13.8V DC is usually obtained from the accessory contact on the ignition switch. This prevents the set being left on accidentally when the driver leaves the car and also permits operating the unit without the engine running. Locate the accessory contact on most ignition switches by tracing the power wire from the AM broadcast receiver in the car.

4. Connect the black lead to -13.8V DC. This is usually the chassis of the car. Any convenient location with good electrical contact (remove paint) may be used.

5. Mount the microphone bracket on either side of the transceiver, using the two screws supplied. When mounting in an automobile, place the bracket under the dash so the microphone is readily accessible.

Section III Installation (Continued)

D. Try a different location on your car (keeping in mind the radiation pattern

you wish). E. Is the antenna perfectly vertical? F. Try a different location in your neighborhood. Stay away from large metal

objects when adjusting (metal telephone or light posts, fences, etc.).

NOTE

The COBRA 148GTL will operate into an SWR of 2 to 1 indefinitely and sustain an SWR of 20:1 for a maximum of 5 minutes at rated operating conditions.

EXTERNAL SPEAKER

The external speaker jack (EXT. SPK.) on the rear panel is used for remote receiver monitoring. The external speaker should have 8 ohms impedance and be able to handle at least 4 watts. When the external speaker is plugged in, the internal speaker is disconnected.

PUBLIC ADDRESS

To use the transceiver as a public address system connect an external 8 ohm speaker (4 watts minimum) to the PA SPK. jack located on the rear panel. Direct speaker away from the microphone to prevent acoustic feedback. Physical separation or isolation of the microphone and speaker is important when operating the PA at high output levels.

Section III Installation (Continued)

TUNING THE ANTENNA FOR OPTIMUM SWR

Since there is such a wide variety of base and mobile antennas, this section will strictly concern itself to the various types of mobile adjustable antennas. Because antenna length is directly related to the channel frequency, it must be tuned to resonate optimally all 40 channels of the transceiver. Channel 1 requires a longer antenna than Channel 40 because it is a lower frequency.

Due to the various methods of adjusting antennas for proper SWR we have chosen what we think is the optimum method:

A. Antennas with adjustable screws (set screws).

1. Start with the antenna extended and tighten the set screw lightly enough so that the antenna can be lightly tapped with your finger for easy adjustment.

2. Set your COBRA 148GTL to Channel 21. Press the PTT (Push-to-Talk) switch, and tap the antenna (making it shorter). The SWR meter will show a lower reading each time the antenna is tapped. By continuing to shorten the antenna you will notice the SWR reading will reach a low point and then start rising again. This means that you have passed the optimum point for Channel

21. Extend the antenna a short distance and again follow the procedure above. When the lowest point has been reached, switch to Channel 1 and then to Channel 40 and compare SWR readings. They should be almost equal.

B. Antennas which must be cut to proper length.

1. Follow the same procedure as above, but adjust the length by cutting in 1/8” increments until a good match is obtained.

Be very careful not to cut too much at one time, as once it is cut, it can no longer be lengthed.

3. The whip is easily cut by filing a notch all the way around and breaking the piece off with a pliers.

NOTE

THE PROPER SETTING IS ACHIEVED WHEN THE SWR IS

1.5 OR BELOW, AND WHEN IT HAS THE SAME READING FOR CHANNELS 1 AND 40.

If you are having difficulties in adjusting your antenna, check the following:

A. All doors must be closed when adjusting the antenna. B. Make sure the antenna base is grounded. C. Check your coaxial cable routing (it may be pinched when routed into the car).

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