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-   -   Smart Radio Antenna Splitter (for SR161, SR162 AIS receivers) (http://www.cruisersforum.com/forums/f13/smart-radio-antenna-splitter-for-sr161-sr162-ais-receivers-6221.html)

Randomandy 18-12-2006 00:33

Smart Radio Antenna Splitter (for SR161, SR162 AIS receivers)
 
Some have been concerned about the attentuation caused by using a splitter as compared with a dedicated secondary antenna for their AIS. The splitter sold by MilltechMarine for the SR161 and SR162 is spec'd with an insertion loss of < 1dB
VHF Antenna Splitter from Milltech Marine
I'm not qualified to say what that means.

My bigger concerns about the device are that
  • It does not purport to protect from a transmitter more powerful than 25 Watts. Aren't many VHF radios more powerful than that?
  • As an active protector, if any power anomaly occurs on the splitter device, the AIS receiver will be toast. Oh, and the receiver won't tell you it's been fried. It simply won't hear ships anymore. :(

btrayfors 18-12-2006 06:52

I have no experience with this particular unit, and have not had occasion to take one apart to see what's inside.

It appears to be fairly well made (in China). The 25-watt rating isn't a problem, as recreational marine VHF transceivers are limited to 25 watts.

However, attenuation of RF signals is just one of the possible problems, and IMHO not the most important one.

This device requires 12VDC @ < 1 watt, and they warn that the power must never be disconnected when using the VHF transceiver. This is because the 12VDC obviously is used to power a circuit to sense when there is RF from the VHF transceiver, and to break the RF path to the AIS receiver so it doesn't get fried. This could be done with a relay, or a solid state circuit.

For the life of me, I can't understand why anyone would want to interrupt the signal path of a marine VHF transceiver -- an absolutely vital piece of gear -- to its antenna by putting ANYTHING which could fail in that path. Especially, an active device which could "be damaged" if the 12v connection is somehow disrupted or if the electronics/relay fail.

AIS receivers work very well with auxiliary VHF antennas stuck on the stern rail, a spreader, or any other convenient location. To my way of thinking, that would certainly be a better way to go than messing about with a critical communications system.

Bill

Rick 18-12-2006 09:01

Two VHF antennas
 
I agree with Bill. Today's marine VHF radios (including AIS receivers) have protection circuitry which will prevent damage from a near-by VHF antenna which is connected to another transmitter. I would not advise making a direct connection from your marine transceiver to the AIS receiver in order to verify that, tho.

What that means to you is that is makes sense to mount a separate antenna for your AIS receiver that is probably not mounted on your main mast so that if you loose the mast you have an "emergency" antenna that you can disconnect from the AIS receiver and connect to your transceiver for communication. Keep in mind that the AIS receive signal will be strong in the vicinity of your vessel where you are most concerned with other near-by vessels and, therfore, need not be mounted very high up in order to extend the signal horizon.

Randomandy 18-12-2006 10:41

some replies
 
I'll debate some small points.
  • WRT the issue of what is meant by "interrupting the signal path of the VHF transceiver": My concern was that the AIS would be fried by a splitter component failure. In neither state (triggered or untriggered) does the device block the VHF tranceiver's path to the antenna. It only blocks the tap off to the AIS receiver. Keep in mind the VHF receiver doesn't need any reciprocal protection since AIS does not transmit. As such, it is designed with the bias towards protecting VHF voice path in event of failure. I haven't looked inside either though, so I haven't verified.
  • Powerfailure puts the AIS receiver in jeopardy (annoying/expensive), not the splitter itself or the VHF radio (critical). This is obviously the tradeoff to achieve the previous point along with the lowest cumulative energy consumption.
  • Pulling out the splitter is no more difficult than swiching out an antenna. The real challenge is in being able to recognize that something failed in plenty of time if there isn't a lot of radio traffic.

btrayfors 18-12-2006 12:43

"Powerfailure puts the AIS receiver in jeopardy (annoying/expensive), not the splitter itself or the VHF radio (critical)."

Not necessarily. If the splitter fails in the "closed" position, it could well adversely affect the SWR as seen by the VHF transceiver and, in the worst case scenario, could cause it to lower or shut down the transmitter. Following Murphy's Law, this would no doubt happen during an emergency when you really need the transmitter.

With regard to damaging the splitter itself, here's what their own advertising says:

"Note:
When the VHF radio is transmitting, the DC input must not be turned off and the AIS receiver port must be connected, otherwise there is the possibility that the VHF splitter can be permanently damaged by a continuous high power RF output signal (25w or above)."

Either way, it's a BAD idea. Keep the AIS completely separate :-))

Bill

hellosailor 18-12-2006 14:29

I just can't help wonder, if "Milltech" is supposed to be similar to "Military Technology". And with the usual Chinese attention to detail and kwality kontrol, they just didn't notice they'd spelled it with TOO MANY LLs.<G>

A splitter has to cost more than a separate antenna, too. Can't argue much about weight aloft...an antenna that won't be doing any transmitting can be awfully light, too.

Paul Elliott 18-12-2006 18:06

"Milltech" is based on the company owner's name, Doug Miller. I've got no connection other than as a satisfied customer.

By the way, I agree that the AIS receiver ought to have it's own antenna, and it can be mounted on the stern or spreader if convenient. As Rick mentions, it can then be used as an emergency VHF antenna (just be sure to have the right coax connectors and/or adaptors, and patch-cables). This is what I do.

coot 18-12-2006 20:51

Quote:

Originally Posted by Randomandy
Some have been concerned about the attentuation caused by using a splitter as compared with a dedicated secondary antenna for their AIS. The splitter sold by MilltechMarine for the SR161 and SR162 is spec'd with an insertion loss of < 1dB
VHF Antenna Splitter from Milltech Marine
I'm not qualified to say what that means.

1 dB loss is roughly the same as 40 feet of RG8/U coaxial cable or about 20 feet of RG8/X. It's actually not bad for a relay inserted in your antenna line.

Another drawback you didn't mention is that the device costs $120. You can get a VHF antenna that will work just as well for roughly half that, so I'm not sure I see the point.

CSY Man 18-12-2006 21:11

AIS
 
I got one of them SR 161 AIS receivers.
Hooked up to a regular VHF antenna on the stern rail.
No problem, no splitter, no fuzz.

Also have another VHF antenna on the opposite rail for the back up VHF comm, then the primary on the top of the mast.
Then there is the GPS and the NAVTEX anteanna on the same side. (All receiving stuff on one side of the ship to keep distance from transmitting antennas on the other side)
Bolting on VHF antennas and running coax cable is so easy and cheap that even considering a splitter is more work and expense than doing it the right way.

hellosailor 19-12-2006 06:31

Thanks, Paul. Who'd a guessed. Shows what sometimes happens when we make assumptions.<G>

Coot, $60 for a 2-meter antenna is still way too much. Spend the money on better coax, peel back the end and turn it into an end-fed coaxial cable dipole. Net cost? Whatever one meter of cable and a jacket to seal over it costs. Maybe five bucks if you use PVC pipe for that, for a simple, light, robust solution. (With least weight aloft.<G>)

coot 19-12-2006 14:44

Quote:

Originally Posted by hellosailor
Coot, $60 for a 2-meter antenna is still way too much. Spend the money on better coax, peel back the end and turn it into an end-fed coaxial cable dipole. Net cost? Whatever one meter of cable and a jacket to seal over it costs. Maybe five bucks if you use PVC pipe for that, for a simple, light, robust solution. (With least weight aloft.<G>)

You could build your own antenna and so could I, but is that generalizable to the cruising community as a whole?

I expect that most cruisers won't want to build their own antennas. Or they don't know how. They can buy one ready to use, and if it doesn't work they take it back to the store and say "give me one that works". Sometimes that is worth a few extra dollars.

Still, if you really want to save money, building a cheap antenna is very practical for somebody with the necessary knowledge.

hellosailor 19-12-2006 20:51

Good point, Mark. A note in passing--if anyone *ever* needs to rig an emergency VHF antenna because the main one is gone or broken? Yup, the end-fed coax dipole (I supposed I should call it a "peeled banana" so it has market appeal?<G>) is the way you turn whatever is left of the coax into a perfectly functional antenna. Or, it can be built and then rolled up and stowed, as a backup/emergency antenna.


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