Thread: AIS Questions
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Old 04-14-2021, 07:27 AM
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Originally Posted by Nomans View Post
I have never been able to discern any impact whatsoever on my VHF performance from the active antenna switch built in to the AIT5000.
I don't recall making any remarks about how a fast-acting automatic antenna switch would affect performance in a discernible way, and particularly no remarks about a specific model like an AIT5000.

The effect on transmission through a fast-acting automatic antenna switch would be limited to:

--a very short delay in operating the switchover; the transmitting signal has first to be detected so the switch can be operated; this interval must be very short in order to not mutilate the transmission by clipping off the initial portion; since there is probably some delay in the transmitter output signal rising to full intensity, and since the detector can begin to detect the signal at a very low level, the switch-over to providing the transmitter direct access to the antenna can likely be done without harm; for this reason, this delay is probably not "discernable" to the operator.

--introduction of some signal loss by routing the transmit signal through the switching device itself; this loss should be minimal in a well-designed and well-made device; the loss ought to be less than 0.5 dB. Such a small loss is probably not "discernable" to the operator.

The effect on reception through a fast-acting automatic antenna switch would be limited to some signal loss when the antenna is being shared between two receiver inputs, but this can be compensated for with a pre-amplifier in the device that adds enough gain to make up for the loss in the splitting of the signal; the result is there is no discernible effect, and there could be an improvement if the original receiver sensitivity was lacking. But any amplification of radio-frequency signals tends to add noise to the signal. To prevent adding noise from a pre-amplifier, the pre-amplifier must use a very good active amplifying device.

A further requirement for a pre-amplifier in the fast-acting automatic switch device is immunity to overload from strong signals. VHF Marine Band radios are already prone to problems occuring from strong signal inputs to their receivers, so any pre-amplification could cause more harm than good when very strong local signals--possibly even out-of-band signals--are present.

The degree to which making a voice transmission on the VHF Marine Band radio will cause a discernible effect on the operation of the AIS transmitter or AIS receiver is entirely dependent on circumstances and coincidences. If you happen to make a transmission on the voice radio at the same time as another vessel is transmitting its AIS information, then that transmission of AIS data from the other vessel will not be received and that data will be lost.

Also, if your AIS transmitter's slot for transmitting your vessel's AIS data occurs while you are transmitting on the voice radio, that transmission might be lost, unless the designers of the fast-acting automatic antenna switch give priority to the AIS transmitter; if that is the case then your voice transmission will be interrupted to let the AIS transmitter take over the antenna. Such priority for the AIS probably is (and if not probably should be) designed into the fast-acting automatic antenna switch.

I don't make any argument that a fast-acting automatic antenna switch CANNOT be useful. I just think prospective buyers should be aware of what those devices are doing. They are not really sharing one antenna between two transmitters. It is possible for two transmitters to transmit simultaneously into one antenna, but it would take much more circuitry and external gear than could be used on a small boat.

The overall effect on the radio system of using a fast-acting automatic antenna switch is to add complexity. When the device is external to the VHF Marine Band radio and the AIS transmitter-receiver, there are additional transmission line cable and connectors. The device must be powered. Typically if the device were to fail there could be harm to the attached transmitters. There could be significant signal loss on receive.

In a situation where a boat already has one VHF Marine Band radio with a dedicated antenna, then adding a second antenna with a fast-acting automatic switch to connect a second VHF Marine Band radio and an AIS transmitter-receiver is a reasonable configuration. The primary VHF Marine Band radio retains exclusive use of its antenna.

My other remarks about the requirement for the antenna to work with both voice and AIS frequencies is a significant limitation in the system, and any boater planning to use one antenna for both voice and AIS transmitters needs to be aware of the requirement.

Last edited by jhebert; 04-14-2021 at 09:55 AM.