GPS Interference From VHF

[Robin]

Trying to replace a faulty GPS aerial last weekend I connected the new one to test it and moved it around to find the best location to fit it. (for some reason the replacement from Raymarine has only 10m of cable, it's predecessor had 15m). It actually worked quite well according to the signal strength display down at the chart table, but close to the VHF etc and I know this has caused interference for some people fitting the ne NASA DSC set, so I thought I would try ours (an old Midland 5600). Well 10secs at 25w transmit, I didn't try 1W, and all the satellite signals vanished. OK so the aerial was too close to the VHF set down below, so I moved it outside and tried all over, even where it would normally go on the gantry at the stern. Guess what? 10 secs of transmit = no signal! OK so now I'm in experimental mode, tried the original aerial still on the gantry (it usually works for about an hour before going AWOL) and same result. OK so how about our #2 set, Navman Plotter with built in WAAS GPS this plugs in out on the coachroof, yes this also loses the fix after 10 secs on transmit! So 3 different GPS's in 3 locations all affected. All signals return when transmission stops and a fix quickly found again.
The VHF works fine as far as I can tell, we receive and transmit over long distances without problems (ie Cherbourg to Portland CG).

This is now getting worrying. We have never noticed a problem in practice, so maybe never transmitted long enough in one spurt to actually lose a fix totally (takes probably 30 secs of no signal before the 'no fix alarm' goes. The VHF aerial is masthead mounted and 50ft above deck, the lead runs under the deckhead lining. There could not be more separation between VHF and GPS aerials without buying a bigger boat.

Now if we replace our VHF with shiny new DSC one, it will transmit our position in an emergency, obtaining said position from the GPS - except that if it transmits for more than 10 secs it will probably knock out the GPS signal......


So what now?????


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[Jewel]

Is it possible that your VHF is going unstable after 10 secs. transmit, this is not unusual as transmitter outputs can heat up and change characteristics. This may not be evident and may not effect voice transmission. If your battery is weakish the lower voltage(below 12v may also cause instability). (the 10 secs delay may be on the VHF transmitter or it may be on the GPS which may retry for that period before it gives a no signal indication. - only some thoughts.

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[Robin]

Thanks for that idea. Cerainly it is not low volts since we have huge batteries that are fully charged and indeed were on shorepower at the time. I think the 10 secs delay is the time for the GPS to react to the reduced signal strength and display the change. What you see (on both sets) on the signal strength meters is the column graphs drop down the scale, then one or more vanish, then they all go, shortly followed by the alarm beep 'no fix'. Once the transmit button is released all the signals return almost instantly, shortly followed by a new fix. The 2 GPS's by the way are on separate battery banks.

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[Paul_H]

Could be VHF signal getting onto GPS power supply and upsetting it. Need good earthing of both VHF and GPS supply and possibly some LC filtering.

More likely its harmonics from the VHF Tx deafening the GPS receiver. 10th harmonic of 157MHz VHF is close to GPS freq of 1.57542GHz and could drown out the weak signals from the sats. GPS receiver front ends have very poor selectivity and can easily be desensitised by strong local signals even if theyre not at exactly the GPS frequency.

Cure would be fit a low pass filter in the VHF aerial feed to stop harmonics above the VHF band being radiated.

This has the makings of a serious EMC problem given the importance of GPS with GMDSS. Anyone know if the GMDSS regs specify the cleanliness of DSC VHF installations?



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[Birdseye]

try running the vhf or the gps sets on completely seperate batteries to see if the interference is coming down the common negative. if it is, you might have some success going along to maplins buying some ferrites, and clipping them round the power leads both to gps and radio.

if separate power supplies doesnt get round the problem, them beg borrow or steal a good up to date vhf and try that. if the problem goes away, then a new vhf is called for (repairs will be more than its worth). before you buy a full dsc set, however, read the posts on the stinkie site on this subject. the alarms really are a pain.

finally, try a different vhf aerial. you probably carry an emergency aerial in case your mast falls down. try this on both the old set and the one you've borrowed.

if none of the approaches works, then you need someone far more skilled that me! good luck

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[alan]

After the tests you have performed so far, it would appear that the VHF set is producing spurious EMI (ElectroMagnetic Interference). This could be radiating from the set itself or from the aerial or from the aerial feeder ( ora combination of two or three!!). As VHF works in the 30 - 300 MHz band and GPS receives at L-Band (around 1.5 Ghz), there shouldn't be any direct interference because of filtering in the VHF transmitter and in the GPS receiver. Interference could be caused just by having the two antennas close together (as in one of your tests) and the transmit is swamping the receiver, or by the VHF transmitting spurious in the L-Band which is being picked up by the GPS. Remember that the GPS signal is a spread spectrum type of signal with a relatively low Signal to Noise ratio and whose signal looks like noise when viewed on a spectrum analyser; so any other noise in the same band cause corruption of the CDA code and hence loss of reception.

As a first step I would check all the cabling: in particular earths. In my experience most interference problems are caused by bad earths somewhere.

Following that I would recommend the following steps:

[1.] Perform a test with another VHF set connected to an emergency antenna (if you have one) located away from the GPS receiver e.g. hoisted aloft on a halyard; or borrow a VHF set and antenna from a friend and hoist the antenna aloft.

If the interference goes away then:

[2.] Connect your set to the emergency antenna (friend's antenna) which is hoisted aloft. If the noise is still present then your radio is generating spurious which are NOT being transmitted via the antenna.

If the noise goes away then:

[3.] Connect your friends VHF to your antenna. If the interference is still apparent then your antenna or feeder (coax cable) is at fault.
If the noise goes away then you probably have more than one problem: replace VHF set, antenna feeder (coax) and antenna.

Finding sources of RF interference can be a real problem, the secret is to try and be logical.

Best of luck.

Alan.
Nettuno, Italy.



<hr width=100% size=1>Alan Cloke

 

[steve28]

Ok , heres what you have to do ,
get hold of some ferite chokes, these are normally fitted to laptop power supplies, generally on mose electronic devices.
put them on the power and aerial cables and it will fix the problem.


steve

info provided by raymarine<P ID="edit"><FONT SIZE=-1>Edited by steve28 on 23/10/2003 16:13 (server time).</FONT></P>

 

[steve28]

cpc can supply the chokes :
phone number 08701 202530

part numbers :
4-5 mm cable CBBR6929 £2.68 EACH
6-7 mm cable CBBR6930 £2.84 EACH

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[Robin]

Cor, thanks Alan, did they have this problem with pidgeon post?

I have tried a handheld VHF close to both GPS sets and it doesn't cause a problem but I guess it is only 5W on transmit not 25W. We do have an emergency aerial already fitted on the stern gantry so I can try that, but actually it is on the same 'perch' as the original GPS Aerial. We also have (somewhere) another emergency aerial if I can just remember where...

The masthead VHF aerial is faily new, we took the opportunity to replace it when the mast was down for re-rigging, the coax too is new down to below deck, from there to the set (under the deckhead lining) is original.

How far can EMI travel? The perch GPS aerials must be 20ft away from the nearest piece of VHF coax or the set, though two of the GPS leads are within 3ft of the VHF at the set end. But then the combined plotter/GPS is 10ft away, on a separate battery and yet is also affected.

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[Robin]

Thanks Steve

Both the old and replacement Raymarine GPS heads were fed into Navcentre 600 plotter, the plotter has a choke on it's power/Seatalk lead I'm sure, but there isn't a choke on the GPS Aerial lead where it plugs into the Navcentre though. The Navman combined GPS/plotter also has a choke on it's power lead as standard.

Robin

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[Robin]

The irony is that we had delayed fitting a DSC set because there was an on going problem on a friend's boat where his new NASA DSC VHF affected the GPS. In his case the GPS aerial was on the wheelhouse (outside) above the VHF (inside). He was told I believe by NASA to ground the VHF (that wasn't in the instruction manual) which he did but it made no difference. However as best I know he eventually solved the problem by putting aluminium foil under the wheelhouse headlining. I never really imagined we might have a problem since our GPS aerials are nowhere near the VHF set or aerial, but was told NASA were probably making some changes and decided to wait for the Mk2 version.

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[alan]

Looking at your answer I'd guess that the problem lies in your VHS set. You have replaced the antenna nd feeder recently so it shouldn't be that - however, don't discount it completely, 'cause it could be faulty. Holding a prtable VHF close to the antennas does not seem to re-create the problem.

As a quick and cheap test, I would borrow a VHF from somebody (one that is known to work on their boat without interfering!!!!) and try that on your permanent antenna.

I bet it works OK..............

Good luck.

Alan.



<hr width=100% size=1>Alan Cloke

 

[Robin]

A week ago if a friend asked me if my set interfered and could he borrow it I would have set No an Yes! It was only by chance I found the problem so I suspect I might not be alone, leastways amongst those of us using older VHFs. It seems to me that in recent years every bit of electronics comes with a health warning leaflet on EMI or whatever which probably means the problem is widespread, they want to make the excuses before the complaint comes maybe?

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[Birdseye]

to answer your question above, EMI (despite the acronym) is just radio waves, and the received signal strength decays just like any other radiation. so if, by mischance, you were radiating 24w of EMI up your aerial, it would be heard just as far away (ignoring frequency effects) as 24w of vhf. as an example, i regularly get EMI on the 3.5 mcs ham band from one of my neighbours central heating thermostat.

obviously, seperating the aerials as much as poss has to help because it reduces the field strength of any radiated EMI (but not of anything coming up through the 12v supply) but it is unlikely that that alone will be the solution. the real answer is to have a VHF radio which is not putting out more EMI than the gps sets can cope with

at the risk of being boring, go through the logical sequence as i outlined since this will eliminate the routes one by one. if you can stop the problem with simple ferrites on the power leads, then this has to be cheaper than replacing the vhf. ditto to curing antenna problems.

we have enforced EC standards governing electromagnetic intereference on all electrical equipment because it is a major and growing problem. it can be generated by any electrical machinery, and if there were no standards enforced, you wouldnt be able to receive any radio or tv signal because of interference.

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[Robin]

Thanks again.

That seems to suggest that it comes out of the aerial rather than the set, or is in the power supply. I might do some playing around this weekend with the spare aerials etc, also maybe borrowing a choke from the Yeoman lead to try.

Otherwise it's plan B, carry on as before, leave the VHF in 'Solent' mode (off) or transmit in <10 secs bursts only which is probably pretty much in keeping with most VHF conversations. Reading the other post on DSC noise I'm not sure I want to change set yet either, it would soon get switched off if I couldn't turn it to an acceptable level.

Robin

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[Birdseye]

Only one further point - you dont know whether prolonged use of the vhf (which cannot always be avoided) might damage the gps aerial. If it were my problem, the one option I wouldnt consider is the one you plan - I would want to solve it.

You dont have to have the full DSC set - you only have to have one that is DSC compatible. ie can be converted to full DSC with an add on.

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[Robin]

Sorry, just back from the weekend sailing. I was being a bit flippant with plan B, I do really want to solve it! I was hoping to borrow a couple of snap on chokes from the Yeoman to try but they aren't apparently removable. The Plotter used for the 2 Ratheon GPSs has chokes on the Seatalk feed and the power in feed, but the lead to the GPSs which carries the power to them does not have a choke. The Navman combined plotter/GPS does not have a choke like I thought either. Unfortunately I wasn't able to try the aerial swaps this weekend the sailing was too good to miss, but that will be the next step and in all probability I will buy a new NASA DSC as well so then I can ring the changes all round. The separate DSC sets seem to cost as much as the combined DSC/VHF set, not to mention requiring 2 lots of space, 2 power leads etc... so a combined NASA it will be, however if the alarm noise is like people say it will go off just like it does in the Solent in summer.

Thanks again for all the helpful ideas.

Robin

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[Strathglass]

Just a thought.

I have a new XM DSC and until the number is input the DSC part doesn't function. (You only get one chance to do that however)

The only pain is that it must have a GPS connected otherwise it bleats at you every 5 minutes until you manually put in a position.

They are quite low cost, certainly better than buying one of the 'convertible' VHF sets.

Iain



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[Robin]

Update To GPS Interference From VHF

We have now fitted a new VHF (NASA DSC) and our previous problem has vanished, no problem with the GPS signals when transmitting at 25W. So it would seem that it was the old VHF set itself that was causing the problem.

All I have to do now is find out why the Lat/Long is not getting from GPS to VHF, its back to the manuals again.

Anyway, thanks to all who replied with helpful suggestions

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[duncan]

Re: Update To GPS Interference From VHF

thanks for closing of the post Robin - it really helps and many more read the thread than contribute visibly!

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