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Mermaid Hunter
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5,674 Posts
How are you getting data to your PC? Do you have a serial port or a USB converter? Do you have C-Map configured to get data from the correct port? Do you have the speed on the PC serial port set correctly? Does C-Map have a configuration item to tell it to look for AIS?
 

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Mermaid Hunter
Joined
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5,674 Posts
In-band splitters are fundamentally bad and should be avoided.
These blanket statements are not helpful to novices. There isn't anything "bad" about splitters. Some are better than others.
A blanket statement in response to a blanket statement? *grin*

There are lots of things bad about splitters. I've held forth on this subject before and will again.

The points against splitters are twofold. First there can be attenuation of the transmitted or received signals. This is usually specified in the technical data of the splitter. It is possible to build a splitter with very low attenuation. Second, is the time sharing of transmission as the VHF and AIS cannot transmit at the same time. This second issue is not a big problem for most cruising boats.
Not "can be attenuation" - WILL be attenuation.

You can certainly find splitters with very low noise pre-amps before and after the switching circuitry with additional amplification on each leg. Those are pricey units.

Commonly available splitters attenuation is 2.5 - 3.0 dB. Add .20 - .25 dB each for the connectors on each side and it adds up. You already knock down your signal 4.5 dB / 100 ft (at least - not all 8X is equal and 58 is worse) in the coax. Why give up more that you don't have to?

Let's ignore coax loss since that is common to both alternatives. Add a splitter and both the VHF and AIS will take a hit of 3.0 - 3.5 dB, a reduction in received signal strength AND transmitted of half. Over clear water line-of-sight is more important than power but if you are in a winding channel or dealing with propagation over hills or through trees such as around a point or past industrial buildings in a port you'll want all the power you can get to take advantage of refraction and reflection.

There are also points in favor of splitters. Ease and speed of installation are a good thing. Usually the mast head antenna can be used which due to height can more than make up for any splitter loss.
From a practical point of view the value of height is over-rated, particularly with regard to large commercial traffic. I'll post the numbers when I figure out a way to convert the table I have in a useful form.

So maybe we should just list the pros and cons and let the reader decide what is "good" or "bad" for themselves.
I stand by my statement that in-band splitters are fundamentally a bad idea. That is different from an FM/TV splitter (a notch/bandpass filter since FM radio is between TV channels 6 and 7) or a VHF/FM splitter (VHF 150 - 165 including weather and FM 88 - 108 MHz). In-band is entirely different. After all the AIS channels ARE VHF channels (87B and 88B).

Adding a second independent antenna on the pushpit, a radar pole, arch, or spreader is not that hard, particularly if you DIY. People--including novices--should know that there is a greater detriment to the splitter approach than the manufacturers promulgate.
 
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