Cobra 148GTLE User Guide
OPERATING INSTRUCTIONS FOR YOUR
40 CHANNEL
CITIZENS BAND
SSB/AM 2-WAY
MOBILE RADIO
Model 148 GTL
Nothing comes close to a Cobra™
PRINTED IN CHINA ©2002 COBRA ELECTRONICS CORPORATION 480-046-P
6500 WEST CORTLAND STREET
CHICAGO, IL 60707 USA
Serial No.
Date of Purchase
Dealer Name
Keep this manual for detailed information
about your Cobra CB Radio System.
SAVE YOUR SALES RECEIPT, THE
CARTON AND “PACKING” FOR
POSSIBLE FUTURE USE.
If You Think You Need Service,
please contact your local dealer.
The Cobra®line of quality products includes:
CB radios
microTALK
®
radios
Radar/Laser Detectors
GPS
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®
Traffic Warning Systems
Accessories
HighGear
™
Accessories
How To Use Your
40-CHANNEL,
CITIZENS BAND
SSB/AM 2-WAY MOBILE RADIO
Model 148 GTL
Contents Page
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
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
Section I Introduction
FREQUENCY RANGE
The COBRA 148GTL transceiver represents one of the most advanced SSB/AM
two-way 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 coverage 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 27.055
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.
2
Section II Specifications
GENERAL
Channels 40 AM, 40 LSB, 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 -22° F to +122° F (-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 Tra nsmit: 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).
(6 cm (H) x 20 cm (W) x 23.5 cm (D)).
Weight 5 lbs. (2.27 kg).
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.
3
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 1/2watt 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 8 ohms.
(Not Supplied)
(SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE)
4
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.
5
Section II Specifications (Cont.)
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.
6
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.
2. 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).
7
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.
8
Section IV Operation
CONTROLS AND INDICATORS
There are thirteen controls and three indicators on the front panel of your
COBRA 148GTL.
A. CONTROL FUNCTIONS
1. OFF/ON/VOLUME (inner dual concentric). Turn clockwise to apply power to
the unit and to set the desired listening level. During normal CB operation,
the Volume control is used to adjust the output level obtained either at the
transceiver speaker or the external speaker, if used.
2. SQUELCH (outer dual concentric). This control is used to cut off or eliminate
receiver background noise in the absence of an incoming signal. For
maximum receiver sensitivity it is desired that the control be adjusted only to
the point where the receiver background noise or ambient background noise
is eliminated. Turn fully counterclockwise then slowly clockwise until the
receiver noise disappears. Any signal to be received must now be slightly
stronger than the average received noise. Further clockwise rotation will
increase the threshold level which a signal must overcome in order to be
heard. Only strong signals will be heard at a maximum clockwise setting.
3. RF GAIN CONTROL (inner dual concentric). Used to reduce the gain of the
RF amplifier under strong signal conditions.
9
10
Section IV Operation (Continued)
4. SWR CAL CONTROL (outer dual concentric). In order for you to achieve
maximum radiated power and the longest range, it is important that your
antenna be in good condition, properly adjusted and matched to your
transceiver. The Built-in SWR (standing wave ratio) meter lets you easily
measure your antenna condition. To operate this function, connect your
antenna to the transceiver antenna output connector. Select a channel near
the middle of the band such as 21 or the channel you plan to use most
frequently. Turn the power on and set the meter function switch to the CAL
position. Press and hold the microphone push-to-talk button and using the
SWR CAL control, adjust the meter to read the CAL position indicated on the
meter face. Then, without releasing the microphone button, switch the meter
function switch to the SWR position and read the SWR indicated. The lower
the figure, the better, with 1 being ideal. Generally speaking, readings up to
3 are acceptable, but over 3 indicates that you are losing radiated power and
antenna adjustment may be advisable.
5. DYNAMIKE. Adjusts the microphone gain in the transmit and PA modes. This
controls the gain to the extent that full talk power is available several inches
away from the microphone. In the Public Address (PA) mode the control
functions as the volume control.
6. VOICE LOCK. Allows variation of the receiver operating frequencies above
and below the assigned frequency. Although this control is intended
primarily to tune in SSB signals, it may also be used to optimize AM signals
as described in the Operating Procedure paragraphs.
7. DIM/NOR/BRT SWITCH. Controls the brightness of the meter and LED
channel indicator for optimum intensity for day or night-time driving.
8. CHANNEL SELECTOR. This switch selects any one of the forty Citizens Band
channels desired. The selected channel appears on the LED readout directly
above the Channel Selector knob. Channel 9 has been reserved by the FCC
for emergency communications involving the immediate safety of life of
individuals or immediate protection of property. Channel 9 may also be used
to render assistance to a motorist.
9. 0FF/ANL/NB + ANL SWITCH. In the ANL position only the automatic
noise limiter in the audio circuits is activated. When the switch is placed
in the ANL + NB position, the RF noise blanker also is activated. The
RF noise blanker is very effective for repetitive impulse noise such as
ignition interference.
11
Section IV Operation (Continued)
10. PA/CB SWITCH. Selects the mode of operation. In the CB position, the PA
function is disabled and the unit will transmit and receive on the speaker that is
connected. In the PA mode, incoming CB transmission will be heard through the
PA speaker. This allows you to monitor messages while outside of your vehicle.
To use the PA feature, a speaker having a voice coil impedance of 8 ohms and
a power handling capability of at least three watts should be used. This speaker
must be plugged into the PA SPKR jack at the rear of the transceiver. If the
public address feature is to be used primarily for outdoor applications, the use
of a weatherproof horn type public address speaker is recommended. The
durability of this type speaker plus the inherent efficiency of such a speaker
will provide more than adequate results when combined with the high audio
output level available from the COBRA 148GTL. With the PA speaker
connected as outlined previously, be sure that there is physical separation
between the microphone and the speaker itself. If the speaker is located too
close to the microphone, acoustic feedback will result when the public address
system is operated at high volume. A directional type outdoor speaker reduces
the amount of isolation required. Some experimentation will determine the
minimum amount of isolation required for a given sound level from the public
address system.
NOTE
PA volume is controlled by adjusting the DYNAMIKE knob to
the desired volume.
11. S-RF/CAL/SWR SWITCH. When in the S-RF position, the meter swings
proportionally to the strength of the received signal. When transmitting, the
meter indicates relative RF output power.
When in the “CAL” position, the SWR meter can be calibrated by adjusting the
“SWR” control to the “CAL” mark on the meter face.
When in “SWR” position, the standing wave ratio is measured.
12. MODE (LSB/AM/USB) SWITCH. This switch is used to select AM, LSB or USB
mode of operation. Unless the station with which communication is desired is
equipped with SSB, the AM mode is normally used. The mode selector switch
changes the mode of operation of both transmitter and receiver simultaneously.
Turn to “Receiving SSB signals” for a further explanation of single sideband.
13. TONE SWITCH-HI/NOR/LOW. This switch is used to shape the audio
response to the operator’s preference. Bass is increased in the LOW position
and treble is increased in the HI position.
Section IV Operation (Continued)
B. INDICATOR FUNCTIONS
1. S-METER. Swings proportionally to the strength of the incoming signal.
2. RF METER. Swings proportionally to the RF output power.
3. SWR METER. Swings proportionally to the ratio of standing wave voltage and RF
output. Used to properly adjusts the length of the antenna, and to monitor the
quality of the coaxial cable and all RF electrical connections. If there is any
degradation whatsoever in any of the above, due to humidity, salt, spray, vibration or
corrosion, the SWR meter reading will rise, thereby indicating that a problem exists.
To calibrate, switch to the “CAL” position, transmit in AM Mode by pressing the (PTT)
mic switch, and adjust the SWR control to the “CAL” mark on the meter then switch to
“SWR” position for the SWR measurement (Note: CB must be in AM mode).
4. CHANNEL INDICATOR. Numbered LED indicates the selected channel you
wish to operate on.
5. RECEIVE/TRANSMIT INDICATOR. The receiver/transmit LED indicator is
located next to the channel indicator. When in receive, the LED will be green.
When in transmit the LED will be red.
6. PRESS-TO-TALK MICROPHONE. The receiver and transmitter are controlled
by the Press-To-Talk switch on the microphone. Press the switch and the
transmitter is activated, release the switch to receive. When transmitting, hold
the microphone two inches from the mouth and speak clearly in a normal
“voice”. The radio comes complete with low-impedance (500 ohm) dynamic
microphone. For installation instructions on the other microphones see
ALTERNATE MICROPHONES AND INSTALLATION section.
OPERATING PROCEDURE TO RECEIVE
1. Be sure that the power source, microphone and antenna are connected to the
proper connectors before going to the next step.
2. Set PA-CB Switch to the CB position and turn unit on by turning VOL control
clockwise on COBRA 148GTL.
3. Set the VOLUME for a comfortable listening level.
4. Set MODE switch to the desired mode.
5. Set the RF gain control fully clockwise for maximum RF gain.
6. Listen to the background noise from the speaker. Turn the SQUELCH control
slowly clockwise until the noise JUST disappears (no signal should be present).
Leave the control at this setting. The SQUELCH is now properly adjusted. The
receiver will remain quiet until a signal is actually received. Do not advance
the control too far, or some of the weaker signals will not be heard.
12
Section IV Operation (Continued)
7. Set the CHANNEL selector switch to the desired channel.
8. Adjust the VOICE LOCK control to clarify the SSB signals or to optimize
AM signals.
OPERATING PROCEDURE TO TRANSMIT
1. Select the desired channel of transmission.
2. Set the DYNAMIKE control fully clockwise.
3. If the channel is clear, depress the Push-To-Talk switch on the microphone
and speak in a normal voice.
RECEIVING SSB SIGNALS
There are three types of signals presently used for communications in the Citizens
Band: AM, USB, and LSB. When the MODE switch on your unit is placed in the
AM position, only standard double-sideband, full carrier signals will be detected.
An SSB signal may be recognized while in the AM mode by its characteristic
“Donald Duck” sound and the inability of the AM detector to produce an
intelligible output. The USB and LSB modes will detect upper sideband and lower
sideband respectively, and standard AM signals.
SSB reception differs from standard AM reception in that SSB receiver does not
require a carrier or opposite sideband to produce an intelligible signal. A singlesideband transmitted signal consists only of the upper or the lower sideband and
no carrier is transmitted. The elimination of the carrier from the AM signal helps to
eliminate the biggest cause of whistles and tones heard on channels which make
even moderately strong AM signals unreadable. Also, SSB takes only half of an
AM channel, therefore two SSB conversations will fit into each channel expanding
the 40 AM channels to 80 SSB channels. The reduction in channel space required
also helps in the receiver because only half of the noise and interference can be
received with 100% of the SSB signal.
An SSB signal may be received only when the listening receiver is functioning in
the same mode. In other words, an upper sideband signal (USB) may be made
intelligible only if the receiver is functioning in the USB position.
If a lower sideband (LSB) signal is heard when the receiver is in the USB mode,
no amount of tuning will make the signal intelligible. The reason for this may be
understood if you consider that when the modulation is applied to the
transmitter’s microphone in the USB mode, the transmitter’s output frequency is
increased whereas in the LSB mode the transmitter’s output frequency is decreased.
The result in listening to the receiver is that when the MODE switch is in the
proper position (either USB or LSB), a true reproduction of single tone of
modulation will result, and if the tone is increased in frequency (such as a low-
13
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