VHF Interference

PatrickB02

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Following the recent threads on handheld VHFs, I bought myself a Standard Horizon HX270S, which I took out on the boat for the first time last evening. It works fine, but I am getting interference on some channels. The interference takes the form of a short burst of static every second or so, and is worse on some channels than others - 16 is particularly bad, but 12 (and others) are fine.

I discovered that if I turned off my nav instrument, a B&G Network Quad, then the interference disappears, and I am pretty sure that the interference is cause by he depth sounder part of the Quad. When the Quad is working, you can hear a quiet click every second or so, which, according to the B&G website, is..

“…perfectly normal and indicates that the unit is functioning correctly. Within the display, there is a powerful transformer that increases the transducer voltage to the levels necessary to fire a depth pulse. It is this particular component that you can hear.”

I am guessing that this pulse is generating a radio signal, which the HX270S picks up as static, but that it only happens on those channels of the HX270S which are operating on a frequency close to that of the radio signal generated.

Am I on the right lines? And if so, what can I do to prevent this interference?

Thanks very much
 

sarabande

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Any big radar installation nearby ? Often a regular ticking sound, especially from Grey Funnel Line.

(I know, they're "ticking" all the time !)
 

mikefleetwood

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I would have thought the sounder was quite a likely contender, especially after you switched it off and stopped the interference.

Most sounders operate at quite high frequencies and the output pulse quite likely produces harmonics in the VHF range. It's just unlucky one seems to land on channel 16!

As to how to fix it, that is more difficult. If you go to a TV aerial specialist, you should be able to get hold of ferrite rings for interference suppression. Thread the transducer lead from the sounder a couple of turns through a ring, placing it as close to the sounder (not the transducer end) as possible. Do the same with the power cable and any other leads from the sounder. Hopefully this will reduce the interference. You may want to check the Aerial cable for any breaks in the outer braid, too. And make sure the aerial and it's cable are sited as far from the sounder as possible.

I hope this helps.
 

PatrickB02

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Thanks for your input, Mike, I will certainly follow up on that.

I'm guessing that this must be a fairly uncommon occurrence. Maybe it's just a faulty VHF and I should take it back?
 

mikefleetwood

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I wouldn't have thought the VHF was faulty - unless it shows other symptoms.

Try the filtering. It would be interesting to see if others have had this problem - and how they cured it.

Another thought - do the cables for the sounder transducer and the VHF aerial run side-by-side at all? If so, move one of them.
 

lenseman

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Radio Frequency Interference (RFI) is a fairly common problem and there are a few simple tests that can be conducted, for free, by anybody without any formal radio knowledge.

There are basically two main routes that interference gets into a VHF radio, via the aerial socket or via the power leads. Sometimes interference can enter via speaker leads if you have an extension loudspeaker located elsewhere on your boat.

More rarely, interference can enter via slot radiation through the VHF radio case but in marine applications, I have yet to find this as a problem as they are constructed in a fairly watertight box.

In your particular case, I would like you to turn on your VHF radio and also any other pieces of equipment which you would normally have turned on whilst at sea.

Do you have interference? If so, turn off each piece of equipment in turn and note if the interference stops or diminishes. Note all the equipment that when you turn it off causes the interference to disappear.

Now, with the possible (probable) interfering equipment turned back on, DISCONNECT the antenna from the VHF marine radio which is suffering the interference. Turn on your VHF marine radio.

During this test, DO NOT TRANSMIT OR PRESS THE TRANSMIT KEY WHILST DISCONNECTED.

Now listen to the radio with the volume at normal listening levels with the mute (squelch) control set correctly and note whether the interference is gone or diminished. If it is gone, then the interference is getting into your VHF radio via the aerial socket, if it is only diminished, then SOME of the interfering signal is entering via the aerial and you will have to do other tests.

If it is gone, your first consideration is to obtain a Radio Frequency Band-Pass Filter (BPF) which operates at 156MHz and fit this immediately at the input socket of your VHF radio. Then connect the aerial plug to the other end of the RF BPF.

Now repeat the interference test but leave the band-pass filter AND aerial connected. The problem should have ceased.

If during the original test and with the aerial disconnected the interference was still there and at the same volume, then the interference is via the power leads and this is where any ferrite beads or Power lead Chokes should be fitted. The same applies if you have extension loudspeaker leads.

I hope this helps?
 

PatrickB02

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David, thank you very much for the detailed reply. I will do the test as you describe to determine the source of the interference. Just one more question (for now, at least!) - my VHF is a handheld; is it possible to fit a BPF to a handheld?

Thanks again.
 

lenseman

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Although you said "following on from other VHF handhelds" I did not fully appreciate that yours was also a handheld, but the principles are still the same.

I have just read the pdf manual on your particular model and although I have been heavily involved with testing, inspecting and prosecuting under MPT 1251 / MPT 1252 over a number of years, I did not recognize that model.

Handheld VHF radios of all sorts are a compromise and are built to a price and the RF standards to which they have to comply are less rigorous than the built in VHF radios nevertheless they have to be fit for purpose. I doubt yours is faulty but unfortunately you are using it in a area where it is susceptible to an interfering signal.

The removal of the antenna is a valid test and as it is a portable VHF radio you can conduct a few further test which I did not mention earlier. Your antenna is a screw on type probably a TNC thread or even a a mini-UHF with a courser thread.

Turn the Mute (squelch) control to the far end of its track, such that only the very strongest signals would be allowed to enter the radio. This is the position of the control that is furthest away from normal operation. Now move about your boat and locate all places where you get the interfering signal.

If necessary, do this test with the antenna removed and be mindful of what I said previously about transmitting or squeezing the PTT accidentally!! /forums/images/graemlins/frown.gif I would hold it with finger and thumb on the front and back of the case only and not be tempted to hold normally as you might squeeze the PTT /forums/images/graemlins/shocked.gif

Having moved around your boat, both inside and outside you should now have a 'map' of the interfering signal and are better placed to locate the actual source. Even though the interfering signal is known to emanate from a piece of kit, this does not mean that it is actually coming from the box itself bit might be a random signal from any part of any lead coming from the particular source. For example, the source could be from the instrument mounted in the wheelhouse (cockpit) or it could be a transducer located low down at the bilge area forward. Hence using the mute (squelch) control to eliminate strong signals.

Get back to me or phone if you want any further advice?
 

wotayottie

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lensman sounds as if he knows what he is talking about, but there are a couple of extra suggestions. the first is to borrow another portable VHF of another make and try that aboard. the second is to try your vhf aboard someone else's boat.

in the first case, if the other vhf doesnt sufferfrom the same problem then whatever the cause, it seems to me that your vhf isnt fit for purpose. techical fault and legal fault arent necessarily the same thing. if the other vhf does suffer the problem then its a fair indication that that its not a vhf problem but a depth sounder of whatever problem.

if your radio works on someone elses boat then it isnt the radio. etc etc.

it should not be the case that your depth sounder affects your radio - one or the other is faulty from a practical point of view
 

PatrickB02

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OK, I have done the tests that you described, and the results are as follows:

Firstly, I double-checked that it is the B&G Network Quad that is the source of the interference. Sure enough, the interference is mainly suffered when the VHF is held within a few feet of the display head (either inside or outside of the cabin). I tried holding the VHF near the cabling to/from the transducer, and near the transducer itself, but neither of these caused any interference.

I also listened to the timing of the clicks from the depth sounder, and these correlated exactly with the timing of the bursts of static interference on the VHF. When I turn off the Quad, there is no interference. So, I am confident that I know the source of the interference.

I then tried all of the channels from 1 to 28, because it was apparent that not all channels were being equally affected by the interference. A couple of channels (16 and 23) suffered interference wherever the VHF was in the boat, most suffered interference if the VHF was held within a few feet of the Network Quad display head, and a few (10,13,14,21) suffered no interference at all.

With the antenna removed, there was no interference at all.

Finally, the squelch control – this runs on a scale from 1 to 15, and all of the tests above were done on setting 1. Increasing the squelch one step at a time, I found that on a setting of 8 or 9 the interference became intermittent, and on 10 it was eliminated.

So, what next? For the time being, I will just use the VHF with the squelch control turned up to eliminate the interference. I’m aware that this will mean I don’t receive weaker transmissions, but I can live with that for the time being. I have sent an e-mail to B&G tech support, so I will see what they say. And I will test my VHF on someone else’s boat, and vice-versa, as Wotayottie suggests.

Any other thoughts? Thanks again for all your help with this,

Patrick
 

mikefleetwood

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It sounds as if your problem is RF interference radiated from the display head itself, rather than via any of it's cables (as I expected). In this case, I think your first action should be to talk to the manufacturer - explain that his equipment is interfering with "emergency channel reception" and see what happens.

I am not familair with this particular unit, but you could experiment with some additional shielding. Try wrapping the display with kitchen foil - being careful not to cover any ventilation slots. Usually any electrically "noisy" circuits will be enclosed in metal boxes and/or the inside of the casing (if plastic) sprayed with a metallic coating.

I note, your VHF is new - did you have one before and did it suffer the same problem?
 

lenseman

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Hiya Patrick, sorry I could not reply sooner. Well done with the tests so far. /forums/images/graemlins/cool.gif

Now there are a further set of tests that can be conducted.

You say that the interference is totally eliminated with the antenna removed, is this with the squelch at its most sensitive position or with it backed right off, I wonder?

Anyway try this test, with the antenna removed find a very small piece of solder and when I say small, I mean about ½"-1" long. The thin solder is best. If you can't get thin solder then by pulling the thicker type it will stretch and form a waist before it breaks.

It is the thin part I would like you to gently insert into the TNC socket of the VHF hand-portable to form a µ-aerial. This (in the trade is called a 'gimmick' and with this as the new receiving antenna you can close in even further to the interferer to discover exactly where the problem is coming from. As you fine tune the location of the source of interference, you can cut down the 'antenna even shorter so that possibly only a millimetre or two are protruding from the TNC socket on the VHF handheld.

Use the squelch control on the VHF radio to attenuate the received interfering signal from the B&G Quad. You might even find that you can conduct the above tests with no antenna (not even a gimmick) at all but with the TNC socket pointing towards, and very close to the Quad?

With this method, you should be able to go right to the source of the problem. If you cannot find any solder, a snipped down paper clip or even a bit of tin-foil from a chocolate bar might do BUT if using a paper clip, be very aware of not damaging the TNC socket on the VHF radio /forums/images/graemlins/shocked.gif

You will probably find that the source of the problem is the very front display of the B&G Quad and that possibly one of the RF decoupling capacitors is not working and the display clock (refresh) is generating RFI.

If you discover a specific source, you might try using tinfoil or a frying pan against the Quad to see what effect it has, as to whether it weakens the interferer?

If you can locate at which side of the B&G quad the problem lays, it goes a long way when e-mailing the manufacturer and he/she can see you have done some work in trying to locate the source and would be more willing to respond. Early display panels were notorious for generating RFI and as this piece of equipment is no longer made, I hope you can get it repaired.

If the quad was on a bench, it might be possible to delve inside with the 'gimmick' to locate the exact location. As I have said, it is possible a clock (refresh) chip which is causing the problem and possibly only a few pence in capacitor(s) to repair. /forums/images/graemlins/confused.gif

Is the boat located in Saint Peter Port or is it on the mainland of the UK?
 

PatrickB02

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Hi David,

My turn to apologise, for not responding sooner to your brillaint post. Thank you so much for taking the time to set this all out.

I'm busy tonight, but will certainly have a go at the "gimmick" test over the weekend and let you know how I get on.

I have also e-mailed tech support at B&G, although with no response as yet. As you say, they will hopefully be more responsive if I can tie down more exactly the source of the problem.

Thanks again for all your help,

Patrick
 

PatrickB02

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Just as an update on this, I have now received a reply from B&G technical support. They confirm that the interference I am experiencing is indeed caused by the depth sounder circuitry in the Network Quad. They comment that this is perfectly normal but that the effects are more pronounced on certain VHF units. They go on to recommend that I move further away from the unit when I wish to use my VHF.

Is it just me, or is that a not particularly impressive reply?!

I have replied as follows:

“I'm sorry, but that is not a satisfactory solution.

On some channels (particularly channel 16) the interference is still present even at the bows and stern of the boat, as far away from the Network Quad as it is possible to get. The boat is 21 feet long, and the Network Quad is mounted on the central bulkhead, so it is not possible to get more than about 10 feet away from it.

Obviously, clear reception on channel 16 is important from a safety perspective, and I am surprised therefore that you admit that it is perfectly normal for your product to interfere with this. There is certainly no mention of this in the Network Quad user manual, indeed it says, "The latest technology and screened cables throughout the Network System ensure the ultimate protection from interference between units and other systems."

Please can you reply with a more satisfactory solution to this problem.

I would also note that I have raised this problem on the Yachting and Boating World Practical Boat Owner's Reader to Reader Forum and will report your comments there, as I am sure they will be of interest to other owners of Network Quad equipment”.

Lets see if that generates something more satisfactory!

Patrick
 
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