DSC VHF attacks GPS!

[dafteddy]

Have a nice new NASA DSC VHF and a Magellan Meridian GPS.
Both work fine - on their own! Switch on the VHF and the GPS instantly looses all signals. Since the idea is that the DSC function is that it works with the GPS, then the set up as is, does not cut the mustard. Help! A friend tried his Garmin 48 close by and the same problem occurs. Is the NASA DSC VHF producing harmonics, 'birdies', screening or ????

Red to red and green to green, all is safety, go between!<P ID="edit"><FONT SIZE=-1>Edited by dafteddy on Sun May 12 20:34:28 2002 (server time).</FONT></P>

 

[Guest]

How close are the arials? this can cause interference but should be detailed in the manual. If not have you though about power line interference, what happens if you earth the radio chassis? (run a wire to a keel bolt as a trial)

Roly, Voya Con Dios, Glasson, Lancaster http://www.voya.pwp.blueyonder.co.uk//MW1/Intropage.htm

 

[Chris_Stannard]

Assuming the VHF antenna is at the masthead and the GPS is on the pushpit somewhere, which would be a normal configuration, and that the antenna cables are both shielded, it does not sound like an antenna problem. Since the received signal on your VHF is tiny there should not be enough power anyway to cause problems with the GPS. Have you got all the connections right in accordance with the two manuals. If the data grounds are not preoperly connected this might be a part of the problem.
It sounds like the VHF has a noisy processor which is not properly shielded. This is more likely to be the case if the two sets are close together or if the antenna for the GPS runs close to the VHF. You might try putting some kitchen foil, attached to earth, between the VHF and the GPS and its antenna lead. You should also make sure that the data cable between the GPS and the VHF is screened data cable. If this proves to be the case complain because the VHF is not up to the CE standard that it is marked to compy with.

Chris Stannard

 

[dafteddy]

I think that the problem may be that the GPS is a hand-held (with a 0.5 M NMEA interface lead to the DSC radio.) The radio is screening, or grossly interfering with the signals to the GPS. Is there a fix? In its usual operating position, the GPS works fine. Switch on the radio and the signal bars on the GPS melt away instantly.

Red to red and green to green, all is safety, go between!

 

[HaraldS]

Do you have the problem without the NMEA connection? I mean just holding the battery powered GPS near the VHF? Also Iread your note that you do have the problem without actually transmitting on the VHF. Right?

 

[cgull]

Meridian GPS

I have recently purchased a Meridian and find it excellent to use How do you like yours ?
I had in mind to buy a handheld VHF DSC radio to interface,sounds like this may not interface
Also with a handheld VHF which I would use on my own boat and also charter yachts without their own DSC what MMSI number do I use?

 

[billmacfarlane]

Looks like you'll have to go through a process of elimination to get to the bottom of this one .
1. If you think that the aerials are crossing somewhere , do you have an emergency VHF aerial that you can plug into the VHF direct ? If so please try - or simply disconnect the VHF aerial and then turn on both units and see if the problem is still there .
2. You'll need to eliminate the NMEA cabling ( or not ) . Can you disconnect the NMEA cables and use both units standalone ? Does the problem still happen ? If not you know where the problem lies .
3.Someone has already mentioned earthing . How is the VHF earthed if at all. As someone has mentioned earlier , try an earth to a keel bolt ( temporarily ) , and see if that cures it. If it does , you'll need a permanent earth fitted , but that's another subject.

 

[dafteddy]

Re: Meridian GPS

cgull

Meridian is an excellent set in virtually all respects. Could use an external aerial connection though. It will (or should) interface with any DSC set. The 'portability' of DSC ident numbers is a problem. I don't think that are usable in the way that you intend.

Red to red and green to green, all is safety, go between!

 

[dafteddy]

Yes, the problem is there whether NMEA is connected or not. Also, the set need only be on receive. As soon as you switch on the radio, the signal bars on the GPS just 'melt away!'

Red to red and green to green, all is safety, go between!

 

[dafteddy]

Thanks to one and all for replying. Will list all the suggestions in the best logical order that I can and try them all.

Sorry to say, NASA dismiss the whole idea that their new baby is the problem.



Red to red and green to green, all is safety, go between!

 

[HaraldS]

Somehow I get the impression that eletromagnetic interference problems are growing lately. Just from what I see on this forum it seems so. I'm wondering how these devices pass conformity testing, and one can understand why the airlines got so cautious with electronics on board.

If two GPS devices of different make fail, one can assume that the interference is quite severe, probably not on the GHZ frequency of the GPS, but strong enough to jam the input amplifier.

It could be that some peculiarities of your installation help to increase the problem:

The NMEA acble coming off the VHF could act as an antenna, even when not connected to the GPS. If you haven't done so, try without a NMEA cable on the VHF.

I gather your GPS was battery powered, so that feeding interfernce via the DC power system could be ruled out. But if you didn't try that, and it works on battery, chances are high you can filter the DC input to the GPS enough to make it work again.

If you indeed powered your GPS with its own battery, then still the VHF could feed back into the DC power system and use it as an antenna.

Your VHF might have a separate ground, that is not DC-, in that case a very good and short ground connection to ship earth may help, one that doesn't carry DC.

You could try a car radio noice filter the other way round in the supply of your VHF. The side which normally goes to the 12V supply, should be connected to the VHF and the side that would go to a car radio, to the power supply side.

If all fails, bring the device back, if they don't take it back, threaten to send it to a EMC test lab.

Good luck!

 

[Oldhand]

I'm not surprised by your problem. My MLR SP24 GPS when mounted in its bracket near ICS's Nav6 Navtex display also has it s reception wiped out. In this case it is caused by radiated EMC from the Nav6 display and not conducted through wiring. I'm waiting for a fix from ICS which we proved could be achieved after an aluminium foil wrapping experiment. You may wish to try a similar test.

 

[halcyon]

Wonder if manufacturer's are testing there product before selling them with CE marks, or is the requirement for EMC radiation to slack.

After all the equipment is intended to work along side other electronic equipment.


Brian

 

[ParaHandy]

Re: Tested?

The VHF channels centered at 121.150, 121.175, and 121.200 MHz, have a 13th harmonic within the GPS bandwidth and, the VHF channels centered at 131.200, 131.250, and 131.300 MHz have a 12th harmonic within the GPS bandwidth. A harmonic, Harald, is an nth order frequency (eg 121 to the power n) and at that order, very low power.

No apologies for "banging on" but did YM actually apply gps signal to any of the VHF DSC sets they recently tested?

 

[HaraldS]

Re: Tested?

Quite possible. I would assume the receiver would default to the DSC calling channel which is CH70 at 156.525 MHz. I don't know what IF they are using, but would guess 10.5 MHZ, beacuse the filters are cheap to get. That would nean the oscillator runs at 167.025 or 146.025.

The GPS L2 band whic carries the C/A code is at 1227.6 MHZ.

Harmonics of the above oscillator frequency would be: 1336.2 MHz or 1168.2 MHZ

Channel 70 without oscillator offset would get close at1252.2 MHz.

Also possible they run the PLL at higher ferquency and divide down and the noise comes from there.

But it could also be the microprocessor.

I think the plastic housings of the new cheap devices are part of the story, though I hope they would at least vapor coat them.

Somewhere I read that since about a year or two, devices approved anywhere in Europe are automatically approved in every EU country, and that now private institutions and in some cases the manufacturers can declare conformity!

Time for the magazines to hit these guys over the head.

 

[Twister_Ken]

Persevere with NASA

You mentioned truying this with a Garmin 48 and getting the same problem.

FWIW my set-up is a Simrad 2 box DSC VHF and a plumbed-in Garmin 48. Worked well once I'd sussed Garmin's slightly dubious way of using the DC ground cable as one leg of the NMEA circuit (and that never caused the GPS to lose signal, just not to talk to the DSC). Both sit on a bulkhead no more than 6 inches apart. So I'd say it's emissions from the NASA which are the problem. Get insistent with them.

 

[tome]

NASA CE approval

I bought a NASA Navtex last year and was surprised to see that it wasn't CE marked. My understanding is that all electronic and electrical goods have to be so marked if sold within the EC, and have to comply with specific EMC requirements. This includes compliance testing. I'd imagine that you could put the wind up NASA if you were to ask about their mandatory compliance with EN50081-1 emission standards!!

 

[dafteddy]

Re: Persevere with NASA

Twister_Ken

Yes, indeed. I am in the process of formulating my 'approach' to NASA based on the excellent responses that I have received from 'those as knows' on this and other boards.

Red to red and green to green, all is safety, go between!

 

[halcyon]

Re: Tested?

Testing for EMC under CE mark requires someone to question complience, the manufacture then has to show test results, but he can just but CE on, issue a certificate and leave at that. Years ago when I looked into it the cost of testing was very high, order of £10-20,000 a unit, depending on complexity.



Brian

 

[HaraldS]

Re: Tested?

As bad as the previous bureaucrazy was, it seems that this new liberal approach isn't working out too well. We shall see if this corrects itself when people start returning dievices, and magazines and bulletin boards spread the bad news.