Radio Does Not Transmit, Radio Transmission and Flutter

Radio Does Not Transmit Far Enough

^ Since atmospheric conditions have a direct effect on transmitting range and vary from day to day, the diagnosis must include a comparison of transmitting range with a known good mobile system. (Be sure the power antennas are fully extended.)
^ After confirming that suspect system is not transmitting far enough, use a SWR meter to determine if fault is in the antenna system, or the CB chassis:

Measure SWR on Channels 1, 19 and 40.
^ A High VSWR on only one of the three tested channels usually can be corrected by retuning the antenna, per shop manual instructions.
^ A Low VSWR with poor transmitting range indicates that the CB chassis must be removed for service.

RADIO TRANSMISSION

To better understand reception characterstics, the following background information will assist in diagnosing customer concerns:

FIGURE 20:

Reception Characterstics

^ The AM signal, being of comparatively low frequency, has a long wave which can bend around obstacles and skip along the ground. This bending characterstic also means that the AM signal will be reflected by a layer in the atmosphere called the ionosphere and bent back to earth. These characterstics combine to give AM long range reception (Figure 20).
^ FM range is limited to 20-25 miles, except for high power stations. Monaural FM stations have greater range than stereo FM. Range for FM does not depend on time of day. As the station signal gets weaker, volume stays about the same, but noise increases. FM signals follow a "line-of-sight" path and are not reflected by the ionosphere (Figure 20).

FIGURE 21:

FLUTTER

^ FM flutter can be best described as repeated pops and hissing bursts heard in the speaker during an otherwise good broadcast. Flutter can be caused by numerous reasons: antenna design, fringe areas, etc. and is not the receivers fault. Flutter may be noticed, not only in fringe areas, but near the station because of the "line-of-sight" characterstic of FM radio waves. FLUTTER occurs when a building or large structure is between you and the station. AM has no flutter (Figure 21).