GPS Problem with NASA SX35 VHF

[Guest]

I am in the process of fitting out a small trailer sailer and have fitted a NASA SX35 DSC VHF radio which is interfaced (NMEA) to a Garmin GPS12 to allow the radio to be automatically updated with a position in case of an emergency DSC call.
Because space is limited and because the Garmin GPS 12 is intended to be only associated with the VHF radio (I have another seperate hand held GPS interfaced with a Yeoman Plotter for navigation) I have mounted the Garmin GPS directly on top of the NASA VHF set.
All the cables are therefore reasonably neat, short and the GPS looks neat sat atop the VHF.
However there is an interference problem!
If I switch the GPS on without the VHF being switched on (powered up) it behaves as it should. If I then switch on the VHF, almost imeadiately the satalite signal strength graph bars diminish to zero and the GPS then warns of insufficient satalites being visible! The VHF is just powered up and is not transmitting.
If I then key the mic (transmit) on the VHF, the GPS bar graphs then re-establish themselves!
Anyone heard about this before?
If I take the GPS away from the VHF set (dismount it) but still have everything connected I can re-establish correct satalite reception on the GPS by moving it away approx 1 or two feet.
It's almost like the VHF set is absorbing the satalite signals until it is put into the transmit mode? I have a feeling that this is a problem associated with a lack of shielding of either or both units.
I really don't want to seperate these units but may have to if I can find no other way of preventing this collapse of satalite signal strength.
Has anyone come across a similar problem, if so how was it rectified? I'd be very pleased to get an answer.
The NASA SX35 instruction book gives no mention of a restricted or reccomended distance between a GPS and the VHF set.
I am going to contact NASA about this but any input from this forum's experienced members will be gratefully received.

<hr width=100% size=1>An Ron Beag

 

[LORDNELSON]

Have you tried shoving a sheet of lead between the two units? Barry

<hr width=100% size=1>

 

[Guest]

Thank you for the advise, re "shoving a piece of lead between the two units"
The basic idea sounds OK, its the choice of material. Why lead? Surely this is an expensive and heavy material to use to provide RF (radio frequency) screening. Lead is ideal where ionizing radiation is the problem.
Like I said the idea is sound though. Perhaps a peice of thin aluminium or stainless sheet would be more apt, providing it is bonded to earth or the negative side of the supply. I will certainly try it and if it works I'll report back.
In the mean time I contacted NASA who said that the instruction manual actually advises a minimum distance between the two units. Funny they couldn't remember which page of the manual......I read the whole bloomin thing and there is no advise about the distance between the GPS and the VHF radio anywhere in my copy! Perhaps NASA have updated and amended their instruction manual?
They could not answer the question as to why the GPS satalite bar graph suddenly re-appears on the GPS when the NASA SX35 VHF set is keyed on to transmit.
Surely there are other people who have installed their SX35 in close proximity to a GPS set?

Thanks to all for any further advise and the suggestion above.

<hr width=100% size=1>An Ron Beag

 

[Gunfleet]

I have gps and vhf right next to each other with no probs - but then I have an antenna on the taff rail. See if you can't borrow an external antenna.

<hr width=100% size=1>

 

[Guest]

Hi JohnM
Thank you for your input. Yes I rather think that a remote GPS antenna would probably be the answer......trouble is my Garmin GPS12 isn't designed to take an external antenna.
I tried placing a screening sheet of metal between the two units but it made no difference, the satalite signals were 'gobbled up' by the VHF.

I'm afraid the answer is to relocate the GPS but this means putting it further into the cabin with a resultant degradation of satalite signal strength to its inbuilt antenna.

I'm going to bite the bullet, I'll buy a new GPS, one with the facility to plug an external antenna into it and then I'll advertise my old GPS12 in the For Sale section of this site. (Shame really because this set has been faithfull over the years and other than this recent interference problem it has never given the slightest hint of malfunctioning.)

My thanks to everyone...

<hr width=100% size=1>An Ron Beag

 

[Gunfleet]

Hi Ron. If that's the case I think I'd definitely try to borrow one which will work off an external antenna first, otherwise you stand the risk of being £200 down and having made no improvement!

<hr width=100% size=1>

 

[LORDNELSON]

Yes, whatever metal grabs you, would be interested in what happens if you do try my suggestion. I have to inform you that a friend of mine tried out the NASA DSC VHF and had so much trouble with it that he now has installed another make (Silva) and has no problems at all with the new radio. Good luck, Barry.

<hr width=100% size=1>

 

[HaraldS]

It seems that the oscillator in the VHF is noisy. These days theyare all synthezised and they have to make the Ch70 frequency and whatever else you turned the device to. So there could be different mixes, some of them ending up on the 1575.42 MHz of the L1-Band of the GPS. Obviously that changes when you transmit. Have you tried to set the VHF to other channels on receive and would that have any positive effect?
It's definitely hard to shield this on the outside and should have been done in the unit. Compliance testing has virtually vanished in the last years, and I'm sure a VHF that noisy wouldn't have passed in the good old days. It's sure not the fault of your trusted GPS and if you consider swapping a unit, I'd certainly throw out that VHF instead of the GPS.
I have never personally seen the NASA VHF, but heard of others having similar problems. Is it new? Have you thought of bringing it back?

<hr width=100% size=1><A target="_blank" HREF=http://www.taniwani.de>http://www.taniwani.de</A>