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VHF Marine Radio: Asymmetry in Receive and Transmit Range Due to Bad Antennas
|Author||Topic: VHF Marine Radio: Asymmetry in Receive and Transmit Range Due to Bad Antennas|
posted 03-21-2013 10:36 AM ET (US)
I have frequently read postings in which boaters report their VHF Marine Band radio is able to receive but they cannot transmit very far, perhaps only to other stations at very close range. This article exams what is occurring in these situations.
The gain in an FM radio transmitter is limited. When you speak into the microphone, you create an electrical signal from the microphone of about –50-dBm. This electrical signal is amplified to become an output of about 20-watts to the antenna, or a signal level of +43-dBm. This gain is more or less fixed, and represents a total gain of around 93-dB.
The gain of an FM radio receiver is also limited, but typically there is much more gain available. A signal arrives from the antenna at a level of 0.1-microvolt, or –127-dBm. This signal is amplified to an audio signal to the speaker of 4-watts, or +36 dBm. This is a gain of 163-dB.
We see that the receiver is able to apply more gain than the transmitter, by about 70-dB. Now we connect a very bad antenna to the radio, one where there is some extreme problem that reduces the gain of the antenna. Normally we expect the antenna to have unity gain or perhaps +3-dB. Now the antenna has a loss, perhaps a loss of a ratio of 1000:1, or –30-dB. How does this affect our communication?
On transmit, the circuitry in the transmitter senses that the antenna impedance is very far out of range. The transmitter protects itself by reducing power to a safe level. Let us presume the transmitter reduces to 1-Watt. This reduction is a loss in transmit power of 13-dB. Now we add the additional loss of 30-dB from the reduced efficiency of the antenna. Our transmit signal is down 43-dB from normal.
On receive, receiver sensitivity is down only 30-dB, a result of the bad antenna. Now our gain differential of Transmit compared to Receive is 83-dB, that is, the original 70-dB of extra gain in the receiver plus the 13-dB lost gain in the transmitter, with the result that with the damaged antenna the radio can hear with 83-dB more gain than it can transmit. This represents a power ratio of two-hundred-million-to-one. This explains why you can still receive some stations with a very bad antenna, but when you try to transmit to them, they cannot hear you. Your receiver has at least 30-dB of reserve gain to compensate for the antenna. Your transmitter, has no reserve gain, and, in fact, to protect itself, it has shut down its power by 13-dB, further reducing the transmitted signal.
On receive, there is reduced sensitivity, but the receiver can still take in the stronger signals and amplify them—recall there was 163-dB of gain available. The ultimate sensitivity is reduced to about 3.16-μVolts. This is still enough sensitivity to hear a lot of the stronger signals.
On transmit, the effective power is reduced from 20-watts (+43-dBm) by a 30-dB loss in the antenna, and a 13-dB loss in the transmitter shut-down protection, to 0-dBm, or 1/1,000th of a watt. There is no reserve gain available. Your transmit signal is stuck at the 43-dB reduced level.
In a particular path between two stations, you might wonder about the other station's reserve gain on receive. Couldn't the receiver at the other station use some of its reserve gain to make up for the 43-dB loss in the transmitted signal from the station with the bad antenna? Of course, the answer is yes. That station would be able to receive the reduced transmission from the affected antenna, making the offset in the gain only the 13-dB difference in transmitter power output. The communication circuit breaks down when this 13-dB difference becomes too much for the other station to be able to hear the affected transmitter.
If we assume path loss is proportional to 20 x log (d) (which is perhaps typical on a line of sight path), then we can see that a differential of -13dB in transmit power suggests the range of the lower power station is only 0.22 of the range of the other station. That is, given the affect of the 30-dB loss in antenna performance, the normal station can be heard by the affected station at a range of say 10-mile, but the affected station can only be heard by the normal station at a range of 2-miles.
In actual practice, the offset in receive and transmit gain may be greater than 13-dB, as the transmitter may shut down to practically no power output with a bad antenna, instead of the 1-Watt I have suggested. Also, the receiver may be more effective at getting signal from the bad antenna than the transmitter is effective at putting signal into the bad antenna. This further increases the differential between actual transmitted signal and reduced sensitivity from a lossy antenna. If we suppose the bad antenna affects the transmitter by a reduction in power to 0.25-Watt, then the offset in gain is –16-dB, and the range differential is more like 0.15. In that instance the affected station could still hear others ten miles away but could only be heard for 1.5-miles.
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