Cobra 148gtl Cb Tranciever Manual

How To  Use Your40-CHANNEL, 
CITIZENS BAND 
SSB/AM 2-WAY MOBILE RADIOModel 148 GTLContents Page
The CB Story ....................................................................................
1
Section I: Introduction ......................................................................2
Section II: Specifications ..............................................................3, 4
Section III: Installation ........................................................5, 6, 7, 8
Section IV: Operation...

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 cover
age of all 40 channels shown below.
ChannelChannel Frequency 
in MHz
126.965
226.975
326.985
427.005
527.015
627.025
727.035
827.005
927.065...

RECEIVER
SensitivitySSB: 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.
SelectivityAM: 6dB @ 3 KHz, 50 dB @ 9 KHz. 
SSB: 6 dB @ 1.1 KHz, 60 dB @ 2.3 KHz.
Image RejectionMore than 65 dB.
IF FrequencyAM: 7.8 MHz 1st IF, 455 KHz 2nd IF. 
SSB: 7.8 MHz.
Adjacent-Channel Rejection60 dB AM & 70 dB SSB.
AM and SSB RF Gain Control40 dB adjustable for optimum signal 
reception.
Automatic Gain Control (AGC)Less than 10 dB...

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 transceiv-
er may be operated with vehicle...

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...

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 trans-
ceiver.  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...

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 qual-
ity 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,...

Section IV Operation (Continued)
Thus when a voice is used in place of a whistle or tone, in the proper listening
mode the voice will be received correctly whereas in the incorrect mode, the
voice will be translated backwards and cannot be made intelligible by the voice
lock control. When listening to an AM transmission, a correct sideband is heard
in either mode since both upper and lower sideband are received.
Once the desired SSB mode has been selected, frequency adjustment may be
necessary in order...

Section IV Operation (Continued)
Fig. 2.Microphone Cable Preparation
To wire the microphone cable to the plug provided, proceed as follows.
Fig. 3. Microphone plug wiring.
1.Remove the retaining screw.
2.Unscrew the housing from the pin receptacle body.
3.Loosen the two cable clamp retainer screws.
4.Feed the microphone cable through the housing, knurled ring and washer as
shown Fig. 3B.
5.The wires must now be soldered to the pins as indicated in the above wiring
tables. If a vise or clamping tool is...

18
Section V Maintenance and AdjustmentThe COBRA 148GTL transceiver is specifically designed for the environment en-
countered in mobile installations. The use of all solid state circuitry and its light
weight result in high reliability. Should a failure occur, however, replace parts
only with identical parts. Do not substitute. Refer to the schematic diagram and
parts list.
NOTE
If the performance described in the OPERATION and MAINTENANCE 
AND ADJUSTMENT sections is not obtained, review the operating...