RF interference

[TURNBULL]

i have installed a icom h f set. When i transmit all my l e d lights glowand my own voice comes out of the engine alarm speaker. i have copper strapping through out and connected to sea cocks and engine. please help anyone.

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[IanBBA]

What type of antenna and ATU (matching unit) are you using? How have you earthed the system? Where have you connected the DC power cable into your 12V system?

There can be many reasons for this problem and some experimentation will be necessary but to start off, a little more info is required.

Ian.



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[Alex_Blackwood]

As Ian has said more info is required. However I would suspect that your aerial feeder run may be the problem, is it close to other cables especially the ones associated with the equipment the LED's glow on and the engine alarm system. Or is it close to the equipment itself? Does it cross any of these cables, If it does it should be at an angle of 90deg. Better not to cross at all! You should really have a seperation of about 50mm between the feeder and any other cables/equipment, not so easy on a small craft!
Where is the aerial positioned? is any of the equipment effected connected to any other aerials ie navaids etc. You may have to reposition the aerial to get the required seperation.


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[TURNBULL]

TTHE SET IS A ICOM 700 AND THE TUNER A ICOM AT 120. i get power direct from my main bat. bank. the cable from radio to tuner which is in the stern of the boat is thick and screened. the radio is about 5foot from control panel where all my led are. i do not have a insolated back stay but the cable from the tuner runs out through the stern of the boat where there is an insolater then about 10m copper wire another insalater with cord the rest of the way to top of mast. the ant. runs about 1foot gap between the back stay. my receptionat this stage seems good.
thanks for the help so far.
mike

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[Bergman]

Not 100% clear on the set up.

Is the "isolator" an insulator in the wire between the ATU and the 10 meter length of aerial wire?

If so this will mean that all the RF energy is being dissipated in the wire from the ATU to the "isolator". Which will probably explain the problem.

Ideally the ATU should be as close to the aerial as possible. Dunno whether the Icom gear allows remote siting of the ATU.

If it does try moving ATU to stern of boat, connecting transmitter to ATU with good quality 50 ohm coax and connecting the 10 meters of wire to output of ATU.

IF you cannot separate the transmitter and ATU then get rid of the "isolator" and connect well insulated wire to output of ATU and directly to the 10 meters of aerial wire.

Either way there needs to be a continuous electrical connection from the output of the ATU to the 10 meters of aerial.

Earthing sounds ok but perhaps worth checking that the earth from the ATU and from the transmitter both go to the same point.

Other questions are how long are the earth wires and on what frequency are you transmitting.

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[Strathglass]

Reading between the lines.

The basic positioning of the main parts or your HF tranciever sound about correct. The ATU sounds as near to the aerial take off point as practical.

One or two thinge may be causing your problems.

There should be a very good earth between the ATU and the RF earth. Check that this earth lead is a large surface area conductor.

Your actual aerial sounds to me to be to be too close to the backstay. This will have a small effect on reception as it will reduce the strength of the recieved signals.

BUT, When you are transmitting a fair percentage of the transmitted power will be absorbed by the backstay.
This will mean that effectively your whole rig (ie standing rigging and mast) will be acting as part of the transmitting aerial.
Because of this any electrical circuitry anywhere near the rigging (including chainplates) will be liable to suffer from interference. Hence your problems.

One or two guidelines.

An HF aerial must not run parallel to and be in close proximity to other conductors.

A couple of sugestions.

Use a vertical whip for your HF aerial.

Add insulators to your backstay and use it as your actual aerial.

I think it would be quite difficult to reposition your present aerial further away from the backstay, but you could try that first.

Iain

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[Birdseye]

I had exactly the same problem - the voice from the engine alarms is weird, isnt it. Does yours do it on 14mc/s only, like mine did.

the basic problem is that you are pumping RF through the boat, which being GRP is semi transparent to such radio waves. they then hit items like the engine alarm, which operates a bit like the old chrystal set radio receivers I played with as a kid.

by improving the installation, you can make sure that as much as poss radiates from the aerial as opposed to the feeders, but even then you will still get problems with some equipment.

in the case of the engine alarm, you can get round the problem by installing a small ferrite ring (maplins) round the feed wires close to the alarm itself. you can try this approach on other wires carrying just dc, such as the feed wires to the switchboard with the led s. what you are doing by this approach is blocking the wire to the passage of hf currents.

incidentally, the same "transparency" problem applies to reception ie you will pick up a lot of background noise from things like fridges unless you are very careful. you might not realise this at first since it can merge into the general noise level and simply raise the threshold. to test, operate the radio at anchor first with normal kit running and then with abs everything else switched off. see what difference it makes.

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[IanBBA]

Yep, I agree that your antenna is most likely the problem. I would guess RF is being induced into the mast head light cable and causing chaos in your control panel. You should be able to check this by disconnecting the mast cable at or below deck level. Do you have this on a connector somewhere?

Suggest you really do need to modify that backstay if optimum performance is a priority. This can be very costly on larger boats as insulators for high stress applications are not cheap; this together with modifying and reseting the rig means you have to be pretty committed to HF radio!

Other solution is to mount a whip antenna on the stern, as far as possible from any rigging. Icom make a very nice quality matching whip antenna for its auto-tuners, the AH2b. It is 2.5m of stainless steel; very nicely made but costs around £235! A cheaper option is to drop into your local ‘Good Buddy’ (CB) shop and try to find a 9-foot stainless steel ‘Tank whip’ with a heavy duty base (base normally 3-4” in diameter secured by three screws). The stainless won’t be 316L or anything like it but this will only cost around £35 and that is not a lot of money to waste in trying a whip antenna out for a season or two (before the rust marks your gelcoat!).

The major drawback of these ‘short’ whip antennas is that your AT-120 will struggle to tune such a short antenna below 4MHz. For good performance down to 2MHz you need at least 7m and that is impractical for a whip antenna.

Just done a couple of quick searches:-
Icom AH2b try Waters & Stanton at www.wsplc.com/acatalog/mobile_antennas.html
For a CB tank whip try www.4x4cb.com/public/shop_preview/products2.cfm

I have lots of info on the AT-120 in .pdf format if it is useful, including the owners manual, workshop manual and Icom paper on boat antenna earthing systems. If you let me know your email address, I will gladly forward.

Great combination the M700 & AT-120 but ... I would suggest two insulators in that backstay!

Good luck, Ian.


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[MainlySteam]

While I agree in general terms with much of what has been said, I would provide a warning about panicing in an expensive way over your antenna too much. If it is very close to and parallel to your backstay, as you say it is, that will disadvantage performance, however it may likely not be the cause of your problems.

For wire antennas mounted from stern to masthead, the best position is from one quarter, which would involve moving your tuner and the throughdeck insulator.

On a yacht, unless it is large, the antenna is always tightly coupled to the rig at marine ssb radio frequencies because in rf terms they are always very close together (usually much less than 1/2 wavelength). On our own yacht we also use a wire antenna which actually runs from the deck inside the lower shrouds and on up to the second spreader so is surrounded by parts of the rig and at no point is more than a few metres away from the aluminium mast and is within 0.3 to 1 metre of the shrouds for all of its length. While some may frown at that, the installation was tested on land against a tower and is in fact not much different to many land installations (a sloping antenna against a guyed tower, for example). It also works without doubt to the extent of impressing other vessels and we have no problem working stations around the world with it.

We have had up to 500 watt radios on board, but normally 150 watts, and we have no on board interference problems at all (TV, Computers, nav systems, auto pilot, invertor, etc). Assuming normal good practice in the radio installation in the first place, the route to follow is one of using clamp on ferrite chokes (you need to get several turns through them to be effective), 0.1 microfarad ceramic bypass capacitors (for example, across a piezo alarm input terminals will usually quiet one of those), and home made toroidal wound choke on the power supply cable and coaxial cable wound common mode choke on the input side of the antenna tuner.

If a general tidying up does not sort your problems out, and I suspect it will not unless you have made some fundamental installation error somewhere, I would suggest finding a friendly amateur radio club and asking if there is someone who really knows his stuff who can help you out. Explain that you understand that your main problems are operation of the affected on board equipment in the radiated field and common mode currents (which are normally the main culprit) - if he understands exactly what you mean, then you have your man. If he looks even slightly befuddled, and most will, then find another. An appropriate non monetary reward for services would be appropriate. Alternatively a really good marine radio technician, but that will be expensive if he has to spend time tracking issues down. Unless you approach it in a planned way, you may find yourself on a jouney to great frustration.

John
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[Birdseye]

agree entirely with what mainly steam has said, but would recommend you simply purchase a number of the ferrite rings we both suggest from somewhere like maplins and try those round (with several turns if necessary) the feed wires to troublesome equipment. they are non invasive in the sense that you dont have to cut wires, mess around with soldering irons, and it is difficult to see how you could damage anything either.

ceramic condensers will work fine (what you are doing then is shorting out the induced hf signal) but they require soldering.

but do make sure that basic installation is sound
1/ well shielded well connected high quality cable from radio to tuner
2/ tuner as near as poss to base of aerial since any wire after the tuner will radiate.
3/ nice short heavy earth wire from tuner to grounding plate. this will radiate so make sure it doesnt go near other wires
4/ antenna wire that is greater than a quarter wavelength long and is well insulated and not shorting out either at the ends or in the middle
5/ then tune in on a quiet part of the band using an am or fm signal on low power and measure the standing wave ratio.. should be between 1 and 1.5 when tuner has done its work. if its much above that, then aerial problem.

contacting your local ham radio club is an excellent idea. you will usually find helpful people with a good level of skill.

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[TURNBULL]

A BIG thank you to all of you for your help. i will try your advice.
Ian my e mail yachtselkie@lantic.net
Good sailing
mike

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