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  VHF Marine Radio: Reciprocal Propagation

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Author Topic:   VHF Marine Radio: Reciprocal Propagation
jimh posted 03-28-2012 07:52 AM ET (US)   Profile for jimh   Send Email to jimh  
When radio signals propagate along a path between two points, it is generally assumed that the attenuation in the path is the same for signals traveling in either direction. For example, let us imagine two boats, A and B, at sea, and separated by a distance of 15-miles. If boat A transmits and boat B can receive the signal, then it is reasonable to assume that the condition along the path between the two boats is sufficient to permit the radio signal to propagate from A to B, and the condition should also permit B to send a signal back to A. There typically is not a one-way propagation effect. On this basis one might conclude that if B can hear A, then A ought to be be able to hear B in return. This may or may not be true.

If both A and B are transmitting with the same power, and if their receivers are equally sensitive, then B should be able to hear A as well as A can hear B. In the Marine Radio Service, most all transmitters on recreational boats are 25-watt transmitters, so we should have a case of equal transmitted power.

What if A, for example, as a high gain antenna, say a 9-dB gain antenna, while B has only a unity gain antenna. Will this affect the reciprocal nature of the link? The antenna gain should not affect the outcome, because antenna gain occurs both on transmit and receive. If A's high-gain antenna boosts his transmitted signal 9-dB, it should also boost B's signal on receive by that same amount.

What if A has the antenna mounted much higher than B? Again, the higher mounting will increase the distance to the radio horizon for the antenna, but it does so on both transmit and receive. Again, the effect should work in both directions.

The reciprocal nature of the path leads one to conclude that if B can hear A, then A should be able to hear B's reply. However, this is not always the case. There are a few additional considerations. While in general we can expect that the received signal levels--the amount of actual radio signal power coming out of the antenna feedline at either end--should be the same, this is not a guarantee of being able to receive the signal clearly. If there are other signals present at one receiver which are not present at the other, there could be interference. For example, consider C, a transmitter located near A but far from B. The signal from C might interfere with A's receiver but not affect B's receiver at all. There could also be locally generated signals at A that cause interference. Perhaps A's spark ignition system is creating interference. Or perhaps A's SONAR creates interference. All of these factors will reduce the sensitivity of A's receiver, perhaps to the point when A can not copy B's signal at all.

In addition, something a simple as the setting of the Squelch control on A's receiver could be reducing the sensitivity of A's receiver. If the path loss is such that the received signals are just at the threshold of audibility, a high Squech control setting could cause the receiver to ignore the signal. On board B we might have a very quiet, noise-free, interference-free receiving environment, and we have the receiver Squelch set for maximum sensitivity. We can hear A's transmission just fine. When we call back to A, however, we cannot be heard because A's receiver is in a noisy radio environment, there are interfering signals on the channel, there are local sources of noise and interference, and the receiver Squelch control is set for minimum sensitivity. The result is B can hear A but not vice versa.

jimh posted 04-02-2012 08:54 PM ET (US)     Profile for jimh  Send Email to jimh     
A common problem in VHF Marine Band radio installations on small boats is an apparent contradiction to the notion of reciprocal propagation that I have described above in which a shipboard radio can seemingly hear well but cannot be heard by others. This situation can occur if there is a significant difference in the transmitter power of the two stations in the circuit. There are two common causes for the transmitter power of a radio to be reduced.

If there is a severe antenna problem leading to a very poor match of the antenna to the transmission line, the transmitter power output can be reduced. When the antenna is not well matched to the transmission line, power applied to the antenna is not immediately transferred to the antenna, but instead is reflected back toward the transmitter. This creates standing waves on the transmission line. The presence of a high standing wave ratio on a transmission line may be sensed by protective circuits in the transmitter, which then reduce the transmitter power to avoid damage to the final amplifier stage. This results in a reduction of transmitter power output. In some cases there are no protective circuits and the final amplifer stage can be damaged to the point of failure. The presence of high standing waves on a transmission line also leads to increased transmission line loss. This further reduces the power being delivered to the antenna.

A failure in the transmitter final power amplifier is another source of reduced power output. Such failures can occur if the transmitter has been operated into an open circuit, into a poorly matched antenna with high standing waves on the feed line, or into a feed line that is not connected to an antenna. Transmitter power output will also decrease if the voltage supplied to the transmitter is not well regulated. If the supply voltage sags during transmit, transmitter power output will fall off. The loss of voltage can be due to a weak battery or to high-resistance connections in power distribution

The combined effects of increased transmission line loss and decreased transmitter power cause the actual power delivered to the antenna to decrease perhaps to as little as one-hundredth of the original power or less. Such a significant power reduction (-20dB) will cause the transmitted signal to have a noticeable decrease in its range. The reduced range may not be immediately noticable if the radio is only used to make transmissions over very short radio paths. I recall one experience when sailing for several days in the company of another boat. The radio installation on the other boat was not good, but when the two boats were close to each other, the damaged radio could still be easily heard by the other boat. It was only when the two boats became separated by a few miles that it was discovered that the radio on one of the boats had very low power output and could be heard only at a distance of a few hundred feet.

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