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Old 04-14-2021, 10:16 AM
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If the VHF with AIS transponder had been designed as a single integrated radio design one antenna would have worked fine. The problem is that the VHF's with AIS available today are VHF radios integrated in the same housing as AIS transponder modules with a common user interface. Then they end up with two antenna connections, to avoid this an antenna switch would have had to be integrated in the same housing. Such an integrated switch would have had exactly the same challenges as an external.
Old 04-14-2021, 10:40 AM
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Originally Posted by abbor View Post
If the VHF with AIS transponder had been designed as a single integrated radio design one antenna would have worked fine. The problem is that the VHF's with AIS available today are VHF radios integrated in the same housing as AIS transponder modules with a common user interface. Then they end up with two antenna connections, to avoid this an antenna switch would have had to be integrated in the same housing. Such an integrated switch would have had exactly the same challenges as an external.
I think the difference is, with an integrated system, the unit knows in advance when AIS is ready to transmit or when the PTT button has been pressed. As such, it can be much more intelligent about allocating a shared antenna resource. With separate units where VHF doesn't know when AIS will need the antenna, and vice versa, there is far more likelihood of one one stepping on the other. But yes, the same issues around antenna tuning will exist in either case.

Last edited by bobeast; 04-14-2021 at 11:17 AM.
Old 04-14-2021, 10:55 AM
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Originally Posted by abbor View Post
=The problem is that the VHF's with AIS available today are VHF radios integrated in the same housing as AIS transponder modules with a common user interface. Then they end up with two antenna connections...
I have not completely researched this, but I suspect that the reason that devices within one enclosure that house both a VHF Marine Band DSC radio and an AIS transmitter-receiver have inputs for separate antennas for each devie is to comply with the requirements for certification of either device, particularly for the AIS device. It seems like manufacturers are already having trouble getting the two-devces-in-one-housing products certified, even with separate antennas. WIth a fast-acting automatic antenna switch as an integral part of the assembly, there could be more problems in certification.

Once a boater buys a two-devices-in-one-housing combination VHF and AIS, if he hooks it to an external fast-acting automatic antennas switch and there is an effect on performance, that is on him, not on the manufacturers of the VHF or the AIS devices.

At the outset of the AIS protocol, I don't think there was a notion that every recreational boat longer than 16-feet was going to install an AIS, or that there would be a market for a two-devices-in-one-housing product to deliver VHF radio and AIS transmitter-receiver as those two-in-one devices.

As I mentioned earlier, the combined costs of the special broadband antenna plus the fast-acting automatic switch are likely greater than the cost of just adding a dedicated antenna for the AIS transmitter-receiver.

But I do agree, three antennas on a small boat is a bit much.
Old 04-15-2021, 03:33 AM
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Originally Posted by jhebert View Post
I have not completely researched this, but I suspect that the reason that devices within one enclosure that house both a VHF Marine Band DSC radio and an AIS transmitter-receiver have inputs for separate antennas for each devie is to comply with the requirements for certification of either device, particularly for the AIS device. It seems like manufacturers are already having trouble getting the two-devces-in-one-housing products certified, even with separate antennas. WIth a fast-acting automatic antenna switch as an integral part of the assembly, there could be more problems in certification.

Once a boater buys a two-devices-in-one-housing combination VHF and AIS, if he hooks it to an external fast-acting automatic antennas switch and there is an effect on performance, that is on him, not on the manufacturers of the VHF or the AIS devices.

At the outset of the AIS protocol, I don't think there was a notion that every recreational boat longer than 16-feet was going to install an AIS, or that there would be a market for a two-devices-in-one-housing product to deliver VHF radio and AIS transmitter-receiver as those two-in-one devices.

As I mentioned earlier, the combined costs of the special broadband antenna plus the fast-acting automatic switch are likely greater than the cost of just adding a dedicated antenna for the AIS transmitter-receiver.

But I do agree, three antennas on a small boat is a bit much.
I think your guess that regulatory compliance issues have been a major factor in delaying the introduction of combined VHF/AIS transponders is a good one. The rules require a separate dedicated GPS source and GPS external antenna for all AIS transponders. Class A transponders must have dedicated VHF antennas.

I looked up the specifications for the Digital Yacht antenna switch and it is evident why its impact is not discernable in use. It says that it switches in less than 10 microseconds. Again, in real life unless you are especially chatty on the VHF, the interruptions in transmitting or receiving AIS signals is of no consequence especially with a Class B+ unit that transmits more frequently. Worst case, there would be a small temporary error in the displayed position of an AIS target if you missed an updated position report while keying the radio mike and a corresponding error in your displayed position on vessels receiving your signal. At least in the Digital Yacht implementation, it seems to give priority to the VHF radio as a transmitter in the event the switching circuit loses power or fails.

If I can ever find someone who has purchased a dedicated AIS band antenna that is not yet installed, I would be curious to try it on my boat and see if there is any noticeable difference in the performance of the transponder. As I said in my earlier post, I see large ship targets at very long ranges of over 30 miles now ands I know my AIS position can be received by vessels with tall antennas well over 10 miles away. I own two commercial scallop boats and the captains have seen my AIS report on their transponders leaving New Bedford after buying fuel which is 15 miles from my dock and hailed me on the radio.

My feeling is that if you have two VHF radios and one has a dedicated antenna (especially if it is not hard to connect that antenna directly to the second radio) , the shared antenna on the second VHF radio and AIS transponder is a good practical solution to finding a location to install a third antenna that has the requisite distance from the other two antennas.

Old 04-15-2021, 06:47 AM
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Originally Posted by Nomans View Post
...If I can ever find someone who has purchased a dedicated AIS band antenna that is not yet installed, I would be curious to try it on my boat and see if there is any noticeable difference in the performance of the [transmitter and receiver].
An "AIS" antenna is one tuned for best VSWR at 162-MHz. The "standard" VHF Marine Band radio antenna is tuned for best VSWR at 156 to 157.4-MHz. Unless the antenna is a special broadband model, a "standard" antenna will show rather high VSWR at 162-MHz, and an "AIS" antenna will show a rather high VSWR at 156-MHz.

The most likely problem encountered--and often reported--with using a "standard" tuned antenna with an AIS transmitter is the VSWR seen on the transmission line at the transmitter will exceed 2:1. In some AIS transmitters if the transmitter sees a VSWR of more than 2:1, the transmitter power is shut off to avoid damage to the transmitter. And since many brands of AIS devices use the same basic internals (made by SRT Marine System Group), this problem occurs on many brands.

In the case of receiving signals, the presence of a VSWR on the transmission line greater than 2:1 has really almost no observable effect on reception.

The actual VSWR of an antenna is often masked by loss in the transmission line. Without getting too technical, if there is a loss in the transmission line greater than about 1 dB, that loss will make the VSWR at the transmitter end of the transmission line appear lower that it actually is at the antenna. This effect can come into play with use of a "standard" tuned antenna connected by a long and lossy transmission line to an AIS transmitter, resulting in the transmitter seeing a lower VSWR than actual. In that case the VSWR might be below 2:1, and the AIS transmitter does not shut off.
Old 04-15-2021, 06:57 AM
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Re the specified insertion loss of a fast-acting automatic antenna switch as "less than 1 dB": a loss of one decibel is a loss of power by a factor of 0.79. At the 2-Watt power level of a Class B CSTDMA transmitter, a one decibel power loss means the power is reduced to about 1.58-Watts.

The decibel is a unit for comparing power ratios. The "bel" refers to Alexander Graham Bell. The "deci" refers to one-tenth of a bel, a larger unit. The general notion of a one-decibel change in any power is that it represents the smallest detected amount of change.

In regard to acoustic power measurement, there is an old joke about the definition of one decibel: when you ask your teenager kid to turn down the volume on the music he is playing, the amount of decrease will be one decibel--the smallest amount detectable.

Also, as mentioned above, any loss also tends to reduce VSWR--possibly a useful outcome if the antenna is being used with both a voice transmitter and an AIS transmitter and is not a special wide-band antenna. The loss in the switch could help mask higher-than-desired VSWR.

With regard to what effect a loss of signal of one decibel will have on range of communication: about all that can be said is that every increase in signal strength helps until the signal becomes so strong that further increase in transmitted signal does not cause any noticeable improvement in the quality of the received signal. The receiver is said to be saturated. But until that level of signal is reached, every one-deciblel improvement helps. This can be particularly true for digital signals.

For example, when receiving a modern ATS 1.0 digital television signal off-air, the theoretical minimum signal-to-noise ratio at the receiver for a solid picture and sound is 15.2-dB. I can receive some distant signals that are right at the margin of being solid. If the SNR is 16-dB the picture and sound are solid and there is no breakup. If the SNR is 15 dB the picture is unwatchable and the sound comes and goes. If the SNR is below 15, there is often no picture detected. This is a good example of how digital modulation in radio transmission reacts to small changes in signal level.

In the case of AIS data transmission, the signal threshold for least detectable signal includes the allowance for a bit-error-rate of 20-percent. That means one-in-five loss of data. This is presumably acceptable in AIS because the data will be retransmitted in a short time span. So AIS receiver sensitivity is given in input signal strength for data recovery with a bit-error-rate of not more than 20-percent. I don't have data on how fast the data recovery improves with input signal to noise ratio, but I suspect that improvement in data reception occurs rapidly with improvement in received signal, but I cannot quantify the slope of that curve.

Last edited by jhebert; 04-15-2021 at 10:43 AM.
Old 04-16-2021, 05:54 AM
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Originally Posted by jhebert View Post
An "AIS" antenna is one tuned for best VSWR at 162-MHz. The "standard" VHF Marine Band radio antenna is tuned for best VSWR at 156 to 157.4-MHz. Unless the antenna is a special broadband model, a "standard" antenna will show rather high VSWR at 162-MHz, and an "AIS" antenna will show a rather high VSWR at 156-MHz.

The most likely problem encountered--and often reported--with using a "standard" tuned antenna with an AIS transmitter is the VSWR seen on the transmission line at the transmitter will exceed 2:1. In some AIS transmitters if the transmitter sees a VSWR of more than 2:1, the transmitter power is shut off to avoid damage to the transmitter. And since many brands of AIS devices use the same basic internals (made by SRT Marine System Group), this problem occurs on many brands.

In the case of receiving signals, the presence of a VSWR on the transmission line greater than 2:1 has really almost no observable effect on reception.

The actual VSWR of an antenna is often masked by loss in the transmission line. Without getting too technical, if there is a loss in the transmission line greater than about 1 dB, that loss will make the VSWR at the transmitter end of the transmission line appear lower that it actually is at the antenna. This effect can come into play with use of a "standard" tuned antenna connected by a long and lossy transmission line to an AIS transmitter, resulting in the transmitter seeing a lower VSWR than actual. In that case the VSWR might be below 2:1, and the AIS transmitter does not shut off.
My AIS transponder has a diagnostics page that can be accessed by hooking up a laptop. It shows a VSWR of 1.4 with my 17' VHF radio antenna. According to its specifications, the 8dB antenna is tuned for 156.8MHz plus or minus 3 MHz. We used a very low loss LMR400 cable for about a 30 foot run and the connectors are high quality US made and soldered professionally. Reading your description, maybe I should have just gone with three antennas and used a shorter dedicated AIS antenna.
Old 04-16-2021, 07:20 AM
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Originally Posted by Nomans View Post
My AIS transponder...shows a VSWR of 1.4 with my 17' VHF radio antenna. According to its specifications, the 8 dB antenna is tuned for 156.8MHz plus or minus 3 MHz. We used a very low loss LMR400 cable for about a 30 foot run and the connectors are high quality US made and soldered professionally. Reading your description, maybe I should have just gone with three antennas and used a shorter dedicated AIS antenna.
A measured VSWR at the transmitter end of an antenna transmission line of 1.4 (actually 1.4-to-1 or 1.4:1) is really not a particularly bad VSWR for a VHF Marine Band radio or for an AIS transmitter. Anything below 2:1 is useable. Getting below 1.5:1 is generally difficult to accomplish.

Using the specifications of TIMES WIRE, for actual LMR400 cable (not a knock off brand) the loss at 150-MHz for 100-feet is specified as 1.5 dB. This means for 30-feet the loss will be 30/100 8 1.5 = 0.45 dB. That loss is not significant.

Properly installed connectors have so little loss that measurement of the loss in almost impossible. In practice, a connector can be considered to have no loss. Unless the connectors are improperly installed, or of the wrong impedance, or have been exposed to intrusion of water, you can more of less ignore them.

A VSWR of 1.4 means a return loss of 15.56 dB. If the transmission line masks 0.9 dB (two trips, one up to the antenna with the incident wave and one back from the antenna with the reflected wave) then the real return loss was only 14.66 dB, and the VSWR at the antenna was really 1.454:1

In other words, transmission line loss in this case has not caused much effect on the measured VSWR at the transmitter; the LMR400 line loss is small enough that it is not masking a VSWR problem at the antenna.

I am surprised that a very long antenna, 17-feet, with very high gain, 8 dB, has such a good VSWR bandwidth. I infer that the diameter of the radiating elements in the antenna is probably larger than typically found in shorter antennas (in order that the longer antenna have good mechanical strength).

In short, I don't see any problem with using the antenna described above for AIS transmission.
Old 04-16-2021, 07:25 AM
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Originally Posted by jhebert View Post
A measured VSWR at the transmitter end of an antenna transmission line of 1.4 (actually 1.4-to-1 or 1.4:1) is really not a particularly bad VSWR for a VHF Marine Band radio or for an AIS transmitter. Anything below 2:1 is useable. Getting below 1.5:1 is generally difficult to accomplish.

Using the specifications of TIMES WIRE, for actual LMR400 cable (not a knock off brand) the loss at 150-MHz for 100-feet is specified as 1.5 dB. This means for 30-feet the loss will be 30/100 8 1.5 = 0.45 dB. That loss is not significant.

A VSWR of 1.4 means a return loss of 15.56 dB. If the transmission line masks 0.9 dB (two trips, one up to the antenna with the incident wave and one back from the antenna with the reflected wave) then the real return loss was only 14.66 dB, and the VSWR at the antenna was really 1.454. But that loss, In other words, has not caused much effect on the measured VSWR at the transmiiter; the LMR400 line loss is small enough that it is not masking a VSWR problem at the antenna.

I am surprised that a very long antenna, 17-feet, with very high gain, 8 dB, has such a good VSWR bandwidth. I infer that the diameter of the radiating elements in the antenna is probably larger than typically found in shorter antennas (in order that the longer antenna have good mechanical strength).

In short, I don't see any problem with using the antenna described above for AIS transmission.
Thanks for the analysis. As I had noted, in real life on the water both the AIS and the VHF radio sharing the antenna perform extremely well. The bottom section of the antenna is about twice the diameter of a typical 8' marine VHF antenna so maybe it does have larger elements.
Old 04-16-2021, 08:04 AM
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[Duplicate; see below.]

Last edited by jhebert; 04-16-2021 at 09:14 AM. Reason: Post was a duplicate and should be deleted.
Old 04-16-2021, 08:11 AM
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Originally Posted by Nomans View Post
Thanks for the analysis.
I am glad you understood it. So thank you, too.

Originally Posted by Nomans View Post
The bottom section of the antenna is about twice the diameter of a typical 8' marine VHF antenna so maybe it does have larger elements.
Another aspect of marine antennas with long length and fed at the bottom: most of the radiated power is taken by the bottom-most element. The next element in line gets less power. The top element gets the least power. The bottom element probably has the most influence on the VSWR of the antenna as a whole. Thus if the bottom element is larger diameter than typical and ts influence on the VSWR is greater, that probably accounts for why the VSWR bandwidth for the antenna is wider than for typical smaller antennas.

In non-marine vertical antennas with multiple elements, each individual element is fed power from a carefully designed power-distribution harness which tries to get equal power in all elements so as to produce the most gain; the elements are fed in parallel. In long marine antennas the power enters at the bottom, and the elements are fed in series. The power distribution among the elements is very unlikely to be equal in a feed system like that.

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