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Old 27-05-2018, 15:40   #1
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AIS Xmit attenuation

I installed a second hand Raymarine AIS250 receiver which worked fine from the AIS perspective. Seemed to recieve OK but Xmit modulation was very much attenuated. SWF showed almost as much power with the AIS as without and the SWR was about 2. A base station could hear the carrier but voice was very much attenuated.
This has really got me tricked. I would have thought the carrier would attentuate and the modulation with it but it seems the carrier gets through the AIS with little loss.
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Old 27-05-2018, 15:48   #2
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Re: AIS Xmit attenuation

I'm confused. What voice are you referring to with AIS? It is a digital signal.
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Old 27-05-2018, 20:53   #3
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Re: AIS Xmit attenuation

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I'm confused. What voice are you referring to with AIS? It is a digital signal.
The AIS250 receiver has an onboard splitter for the VHF aerial. The problem is with the splitter function rather than the AIS function.
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Old 27-05-2018, 20:59   #4
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Re: AIS Xmit attenuation

Have you tested the VHF without the splitter direct to the antenna? Are you confident on the coax connectors VHF to splitter?
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Old 27-05-2018, 21:02   #5
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Re: AIS Xmit attenuation

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Have you tested the VHF without the splitter direct to the antenna? Are you confident on the coax connectors VHF to splitter?
Yes. have had an SWR in line both with and without. With the AIS250 in circuit the power meter show slightly less power transmitted.
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Old 27-05-2018, 22:30   #6
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Re: AIS Xmit attenuation

Crap cables, connectors or the AIS250 itself. Its receive only, and not worth spending money on if you have checked the cables.....
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Old 27-05-2018, 23:13   #7
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Re: AIS Xmit attenuation

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Crap cables, connectors or the AIS250 itself. Its receive only, and not worth spending money on if you have checked the cables.....
Just bought the AIS700. We'll see how that goes. I am curious and having a fair bit of experience with radios to component level, I cannot explain what I am seeing.
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Old 27-05-2018, 23:17   #8
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Re: AIS Xmit attenuation

I haven't installed a 700 yet, but they look great Class B SO (SOTDMA) and a built in splitter, should be a good unit!
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Old 28-05-2018, 00:13   #9
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Re: AIS Xmit attenuation

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I haven't installed a 700 yet, but they look great Class B SO (SOTDMA) and a built in splitter, should be a good unit!
Just had it down on the bench. A little trouble sorting out the USB interface but otherwise looks pretty slick. Will be on the boat tomorrow. I will be using the Nmea 183 interface only because it will be replacing the AIS250. There is some doubt the Nmea 183 connection has the full functionality of the SeatalkNG. However in researching it I thought the AIS700 is good value considering it has a splitter integral to the unit and will have compatibility with the rest of my Raymarine instruments.
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Old 31-05-2018, 05:53   #10
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Re: AIS Xmit attenuation

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I installed a second hand Raymarine AIS250 receiver ...Xmit modulation was very much attenuated.....A base station could hear the carrier but voice was very much attenuated.
It is impossible that attenuation of a modulated transmit signal such as might occur by passing through some sort of antenna coupler could cause the modulation percentage of the signal to decrease.
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Old 31-05-2018, 14:32   #11
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Re: AIS Xmit attenuation

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It is impossible that attenuation of a modulated transmit signal such as might occur by passing through some sort of antenna coupler could cause the modulation percentage of the signal to decrease.
Thankyou. That's what I think. You sound like you know what you're about.
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