VHF problem

[Andrew_Fanner]

Last weekend, fitting my nice new DSC radio I noticed a problem. I have a Navman 900 plotter which connected its NMEA output quite happily. However, the radio wouldn't squelch properly. We assumed that the NMEA link was a bit noisy as it was a temporary lashup and the screens were not joined. However, the noise only stopped when the plotter was actually turned off. Disconnecting the plotter-radio link made no difference.

Testing the aerial we found that there was some connection through either the aerial or the cable.

The questions are:

Is the dodgy aerial likely to be the problem with picking up noise from the plotter?

Can I safely shorten the coax lead from the aerial to the radio and reattach the plug? (OK, es, I could buy a new aerial but I'm loath to do so if there is no need)

What, in a plotter, might be making rf noise and how might I screen it? The plotter and radio are about 4 feet apart and don't share a live power feed or anything like that. The boat is diesel so no ignition noise either.

Any radio experts out there? I ought to know but its 20 odd years since doing the ham radio stuff and I've forgotten almost all of it.

Two beers please, my friend is paying.

 

[BarryH]

Its more than likely the NMEA signal thats causing the interferance. Try shielding the VHF coax on a copper sleeve or braid. It might also be an idea to put a choke or filter in the powe cable of the VHF. You can get them from RS or Maplin's.

OK, to hell with it. Unbolt it and we'll use it as an anchor!

 

[byron]

Plotters are notorious for interfering with VHF. You can also try aluminium kitchen foil as a shield.

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[oldgit]

First thing yes you can shorten cable.Either reuse PL259 plug or buy new one about a quid from anywhere.
Next we need to know if RF is getting to radio via power or aerial cable.
Remove ant from back of radio does noise go or stay?
waste of time putting chokes in 12V cable if interference is coming thru aerial.

Just hold tight dear it will not be so rough when we get round the corner,trust me.

 

[BarryH]

I put chokes in the power cable of my VHF, it stopped all manner of pops, clicks and hisses. Didn't stop the crap from the local fisherman tho!!

OK, to hell with it. Unbolt it and we'll use it as an anchor!

 

[Andrew_Fanner]

Thanks all. If SWMBO lets me down to the yard this w/e I'll give the suggestions a try, after a trip to Maplins!

Two beers please, my friend is paying.

 

[Strathglass]

Go to Maplin and get some ferite rings. Like big Polos.
Feed both the power leads for each unit through one ring and make up to ten turns through the centre. Do the same with the coax cable from the VHF to the aerial.
That should get rid of most of your problems
Iain

 

[Geoffs]

Agree with ferrite rings on power leads, but not co-ax cable. Never a good idea to coil a radio co-ax, be it VHF, broadcast radio or TV. This will change the characteristic impedance of the transmission line, increasing the standing wave ratio (SWR) on a transmitter. This will decrease transmitted power, and in extreme cases, could even damage the radio.

Keep any spare co-ax looped in as large a radius as possible.

Old Chinese proverb 'Man who sail boat into rice field, soon get into paddy'

 

[Strathglass]

I have passed coax through ferite rings many times with power levels above 100 W RMS at VHF without problem.In some instances coiling coax may introduce a secondary resonance which could change the VSWR but it will not chance the impedance of the coax cable which is determined the relative diameters of the inner and outer conductors and the dielectric constant of the insulating material between them.
Ferite beads are certainly much easier to use than wrapping units up with foil especially as regards to ventilation.
Iain

 

[Piers]

The large polo is a ferrite core (or split toroid as it is sometimes known), and should always be as close to the kit as possible, with the cable wrapped around the core, once only.

As far as shortening the VHF aerial cable, yes, you can do it safely without causing a problem, provided you leave at least 3'. The reason is that around 3', resonance occurs in the cable, attenuating the transmitting signal strength considerably. Typically, from 25 watts to 16 or so.

So, use the ferrite cores on both products as close to the kit as poss, with one turn only. Shorten the VHF aerial lead to around 4' to 5' minimum. Ensure the aerial connector is made absolutely correctly. Separate the two bits of kit as much as poss.

Of course, all this assumes the aerial is up to scratch. If more than a few years old, try fitting a new one - I have always used Shakespeare aerials. More expensive, but superb performance and good shielding of the circuit board withing the aerial.

And no, I'm nothing to do with Shakespeare....

Piers du Pré
www.playdeau.com

 

[Piers]

One other thought. The aerial cable should be continuous without connectors (other then the one to connect to the radio.

An in-line connector adds about 3dB loss to the system. And if the connectors are not made to the manufacturers spec (ie., some need to be soldered), the aerial can suffer even more.

Piers du Pré
www.playdeau.com

 

[Andrew_Fanner]

Well, the net result of several hours work last wekend was:

Plotter and radio power leads have said ferrite rings on each, and indeed a noise filter from the car radio shop as well.
The antenna lead has been shortened slightly and the antenna checked (its fine)
Screens etc in the NMEA and power leads are earthed.

Result?

Still some white noise and occasional digital clicks that won't squelch out. The noise is pretty weak as it vanisges if a proper transmission is received. The noise occurs on all channels I tried. If I move the antenna to the pulpit rail the effect is reduced, of course if I unplug the antenna the effect goes away

Sticking the radar reflector over the plotter screen made no difference so I guess its in the wiring somewhere.

I'll keep fiddling.

Two beers please, my friend is paying.

 

[oldgit]

if the noise goes when you unplug aerial the interference is via antenna,therefore something is radiating RF.
Moving antenna a foot or two can sometimes stop noise as the harmonics(the noise )tend not to travel very far. ie only few feet from source. So moving aerial outside RF field.

Just hold tight dear it will not be so rough when we get round the corner,trust me.

 

[Strathglass]

Andrew look at one of my previous sugestions and just slip a couple of ferite rings over the coax near the back of the set. Even without looping through the rings it should make the required difference.

Iain

 

[Andrew_Fanner]

Sorry, my bad on the ferrite rings, both live feeds are wound around a ring (not the same one)

Two beers please, my friend is paying.

 

[Piers]

Right.

Am I right when I say the Chart Plotter is the culprit? In other words, if you turn it off the problem goes?

If so, then QED, the CP is radiating, using something as its aerial. To combat this further, try the following,

1. Beefy( and I mean beefy) ferrite rings on every cable associated with the CP, even the GPS lead etc. (Only one turn of the cable, though, and close to the CP). This should help eliminate the cables being used as aerials in their own right.

2. If there is an earthing terminal on the CP, ensure it is directly connected to the Ship's earth, or at the least to the negative terminal of the battery.

3. Shielding the main CP box in BacoFoil.

Piers du Pré
www.playdeau.com