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Old 14-12-2009, 18:36   #1
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VHF Radio with Channels 87 and 88

I need a vhf radio with channels 87 and 88 so I can do AIS. The idea is the output audio is plugged directly into my computer's microphone input and software decodes the digital stream. The sound card's line out can then also programmed to send a digital stream to the radio to transmit speed and position data to other vessels.

My handheld vhf radio can tune to channel 87, 88, and 88a. I do not understand what channel 88 aa is. There is also no microphone or speaker plugs, so I imagine it is unsuitable for this use. Could most vhf radios that tune to channels 87 and 88 work for this, or are those actually different frequencies than what AIS uses?
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Old 15-12-2009, 13:55   #2
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Originally Posted by geckosenator View Post
... I do not understand what channel 88 aa is...
VHF channel 88A (157.425Mhz Tx & 157.425 MHz Rx) is assigned to Commercial, Intership traffic only.

AIS uses the high side of the duplex from two VHF radio channels, 87B and 88B.
Channel A (87B):161.975Mhz
Channel B (88B): 162.025Mhz
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Old 15-12-2009, 15:45   #3
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Yes, channels 87 and 88 will work but you will need to take the audio directly from the discriminator output, before the audio filter stages of the receiver. Using the standard filtered audio output probably will not work or work unreliably at best.

Eric
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Old 15-12-2009, 16:00   #4
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Note that there is a GNU library which does just that, as described here

The described this is one-way though, do you think you can get it to send also?

AFAIK the newer radios do not permit switching to channels 87 and 88 as these are reserved now. The cheapest black box decoders are so affordable, perhaps its hardly worth your while.
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Old 15-12-2009, 18:06   #5
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Modern radio's do allow switching to channels 87 and 88 but are receive only. You certainly cannot transmit AIS or anything else on these channels (actually, you can transmit, but it won't be on the AIS frequency). AIS requires a different modulation method called GMSK which requires special circuitry not found in your standard VHF marine radio. I have used an Icom M504 to recieve AIS via direct discriminator output to my computer running software to decode the data stream.

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Old 15-12-2009, 18:12   #6
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You have to keep your radio on that channel though, or do you use dual watch to stay on 16 as well?

NB Discriminator output as described here..
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Old 15-12-2009, 18:24   #7
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Yes, you must keep your radio on an AIS channel. I am not a boater, I'm a marine electronics service technician. I have radio stations in my shop and only did this AIS hook-up out of curiosity. You cannot hear the data stream as it is "in the noise" and therefore would not activate squelch circuitry so if you tried turning the squelch on and scan other channels, it would never pick up on the AIS signal.

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Old 21-09-2010, 18:28   #8
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Eric,
I've searched through your posting looking for more info on putting a tap on the discriminator in an old ICOM M100 VHF.
In someones posting by there was a comment indicating you'd put a tap on the discriminator in a M100.
On Discriminator uitgang there are instructions for putting a tap on a ICOM M80 but it isn't the same circuit board in the M100 even though the front controls are very similar.
Thus, I'm still looking for more info on where to put a tap. Any suggestions on where to find circuit data for the M100?

Dave
Hampton, VA
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Old 22-09-2010, 10:44   #9
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The M100 uses the exact same circuitry but the resistors are labeled r131 and 132.

Eric
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Old 22-09-2010, 19:25   #10
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Eric,

Thanks for the info. I'll look for R131 and R132 and connect a 10uF 25V condenser like they used in hooking up the M80. Running the data to the 3.5mm mono jack for the AIS data feed to my computer audio input.

Thanks again,
Dave
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Old 07-12-2010, 18:50   #11
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The M100 uses the exact same circuitry but the resistors are labeled r131 and 132.

Eric
Eric,

I appreciate your help - I've looked all over the circuit board and have found R132 but can't find R131. Should it be near IC5 like the the ICOM M80?

Dave
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Old 08-12-2010, 04:13   #12
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Sorry, Icom labels it's parts on the schematic differently than how they are labeled on the circuit boards. On the far right side of the main board, about midpoint, the junction of R32, R31, R46 and C35 is where you want to tap off of.

Eric
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Old 08-12-2010, 05:27   #13
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You cannot of course do AIS sending using a standard VHF radio. listen only is possible.

Dave
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Old 14-12-2010, 02:41   #14
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i use the Discriminator Output AR-108/COM307 with success and creating the discriminator output isn't that difficult if you are diy guy with the correct tools.
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