NASA VHF & Garmin GPS

[Guest]

As promised an update on my problems associated with the close proximity of a Garmin GPS to a NASA VHF SX35 Radio. To recap, I'd positioned the two units closely together (GPS sat on top of radio) but the GPS signals disappear when the radio is switched on (not transmitting, just receiving).
Placing an earthed metal plate between the units (as suggested by one reader) did have a marginal effect but not nearly as good as providing an external GPS antenna placed some distance away from the radio! This was proved by carefully moving the external antenna (a puck type) increasingly nearer or further away from the radio whilst still keeping the hand held GPS in close proximity to the radio. Within about a metre the GPS signals started to show a collapse.

So to retain a hand held GPS in close proximity to the NASA SX 35 so that it (the GPS) remains fully functional you need to supply the GPS with an external antenna which is sited at least a metre away and in my experiments, using a Garmin GPS 76 with a puck antenna full signal strength was obtained and guaranteed with the antenna at about 2 metres distance.

To the respondants of my previous query on this problem, my sincere thanks. If anybody is thinking about installing a similar setup/combination then experiment first before deciding a permanent fix. Make sure your hand held GPS is able to take an external antenna and then site it a good 2 metres away from the radio if your radio is an SX35.

I have now purchased a new Garmin GPS 76 to replace my Garmin GPS 12 which doesn't have an external antenna capability. (Anyone want a GPS12?)

NASA do not warn of this problem, so if like me you have a small boat and you want to use your existing hand held GPS in close proximity due to space resrictions of a small boat.......be warned.

Happy and safe boating to all.

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

 

[wakeup]

No problems with my 35 and a Garmin III PLus

I've installed the Nasa DSC 35 and linked to my Garemin 3 plus which sits about 8 inches away from the Nasa unit. I am not using an external GPS antenna and the GPS signal is not affected?????

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[Porthandbuoy]

Intersting.
I have a geriatric Garmin GPS45 which has performed admirably since I bought it in, um, erm, 1992 or '93. The problem I have is that it takes about an hour to get a fix every time I switch it on. This month's PBO mentions a similar problem if you leave your elderly GPS for 24 weeks without switching it on. Something to do with downloading new almanac data. What the article doesn't say is that when you switch it off it seems to revert to the original almanac.
Perhaps I should send mine back for a software update. I'd write a glowing euology of course.
Anyone from Garmin reading this?


<hr width=100% size=1>"Mariposa"

 

[Oldhand]

The NASA VHF is not the only equipment to wipe out GPS reception on a closely positioned handheld. The ICS NAV6 Navtex dislay unit wipes out by MLR GPS.

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[tome]

I've recently had reports about VHF tranmissions affecting GPS reception on our (professional) survey equipment, as well as SSB which is well known for it's interference. On larger ships, there are several VHF antennas and generally it's possible to isolate the antenna which is causing the problem. I've not heard of a receiving VHF antenna causing such problems and can't think of a reason why this should be.

Marine VHF operates on around 150MHz whereas the GPS L1 frequency is around 1.6GHz so there shouldn't be any significant harmonic interference (it's usually the 3rd harmonic which causes problems).

Most likely it's EMI (electro magnetic interference) due to the proximity of the 2 sets. If your hand-held is battery powered then you could try powering it from the ships supply and putting a ferrite suppresor on both the VHF and GPS power feeds.

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