T7A09 from the Technician License Course Section 5.0, Signal Propagation:
Which of the following devices is most useful for VHF weak-signal communication?
A. A quarter-wave vertical antenna
B. A multi-mode VHF transceiver
C. An omni-directional antenna
D. A mobile VHF FM transceiver
VHF weak-signal communication? What’s that mean, exactly? Usually this phrase refers to long distance communications, beyond the local area, and sometimes via ionospheric skip propagation, using the VHF bands of 6-meters, 2-meters, and 1.25-meters. Let’s consider how this is different from the more well-known local simplex and repeater operations.
Local communications by simplex and repeaters is most commonly conducted with FM mode, and the majority of these communications tend to be phone or digital. FM is terrific for clear, locally propagating signals, but when you want to get your VHF signals out to greater distances the best bet is to use modes other than FM. Why?
First, FM tends to degrade rapidly with weakening signal strength as compared to other modes such as single sideband (SSB). That is, when an FM signal becomes weak with distance the audio quality craps out rather suddenly. A single sideband signal will become noisier with severe signal weakening but the audio will remain present and readable more so than with FM. SSB tends to provide significantly better long distance performance.
Secondly, the bandwidth of an FM signal will range from 5 to 15 kHz or greater, whereas the SSB bandwidth is about 3 kHz or less. For the same transmitting power the SSB signal will have a higher average amplitude across the transmit band since the power is distributed over a narrower range of frequencies. Thus, FM has an inherent average signal power disadvantage that contributes to its range limitations. As compared to the very narrow bandwidth of CW (~150 Hz), FM is far more disadvantaged.
The power advantage of SSB, CW, and many digital modes over FM makes these other modes more attractive for long distance propagation, especially when ionospheric skip propagation is feasible such as 6-meter sporadic E operations. With skip propagation not only does the extreme distance reduce received signal strength, but the polarization of the signal will no longer be unchanging or known as with vertically polarized local FM ops. The shifting or scrambled polarization further reduces signal strength at the receiving station.
As such, it is advantageous to operate on modes other than FM when weak signal VHF communication is desired or required. Clearly, a common VHF FM transceiver cannot accomplish this. A multi-mode transceiver is most useful. Many transceivers on the market offer 6-meter band multi-mode capability, and a narrower set provide multi-mode ops up to the 2-meter and even UHF 70-centimeter bands. This latter category of transceivers is particularly useful for VHF contesting in which weak signal, long distance contacts using SSB, CW, or digital modes are highly coveted on all the bands above 50 MHz.
The answer to Technician Class question T7A09, “Which of the following devices is most useful for VHF weak-signal communication?” is “B. A multi-mode VHF transceiver.”
Related Questions: T3C02, T3C04, T3C06