Field Strength vs. Signal-to-Noise in VOACAP
Q: I am a beginner in VOACAP. Why is the Signal-to-Noise ratio more important than the Field Strength? There seems to be two schools of thought here.
George Lane: Think of an HF circuit as a telephone link. On one end a modulated signal is introduced which contains information (voice, data, etc). The modulated signal is amplified by the transmitter and raised to RF. The RF is then introduced to the transmit antenna which must efficiently accept this power and launch it into a window in space.
This window is defined by the azimuthal spread of the great circle route (usually plus or minus about 2 degrees for point-to-point circuits or by the width of the broadcast target area). The vertical height of the window is determined by the ionospheric modes which will support that frequency to the intended target area. The trip through the ionosphere attenuates the signal power until it is delivered to the receive location as a power flux density (or as some people say, a field strength).
At this point we must consider the receive antenna pattern. This pattern should be such that it only accepts waves arriving within the ionospheric window defined by azimuth angle and arrival angles of the ionospheric modes (hops). However, the receive antenna accepts some energy at all angles. Any receive antenna over perfect earth will have an average or integrated gain of 3 dB (it receives both the direct wave and also the ground reflected wave - hence twice the power or 3 dB). RF noise power is arriving at the receive site from all directions (atmospheric, man-made and galactic). This noise power is picked up by the receive antenna with 3 dB of gain in the lossless case. The receiver accepts both the signal and all of the noise power within the bandpass filters of the receiver. The ability of the person or device to extract information from the receive signal is dependent on the signal power being sufficiently above the noise floor such that intelligence is detectable.
Field strength or signal power is only half of the story. A poorly designed receive antenna will deny reception of a signal just as though the transmitter had been shut off. A receive antenna in a high noise environment will mask the signal such that no intelligence can be detected.
Early work prior to World War II in broadcasting only used the field strength as a measure of communications success. However, since the 1940's radio engineers have used Signal-to-Noise ratio as the preferred measure of communications success. SNR is directly related to intelligence transfer. Also at great expense very accurate RF noise prediction models have been developed for use in computing the SNR.