Wednesday, 11 June 2014

Car Radio on a Boat.

There is no reason why a radio as used in a car should not work on your boat. However, because boats are filled with electrical items. There is a likelihood that you might suffer from one or more different forms of interference. Typically interference comes in two different flavours - natural and man made. You can take measures to reduce and in some instances even eliminate many kinds of interference. It also comes in wideband and narrow band as well. If the interference is narrow band it may effect a small number of harmonically related frequencies. You may find that one station can't be received - but another station a little further down the band is ok. However, if the interference is wide band, it may make the whole band unusable and may also spread across onto other bands. Covering say the FM and DAB frequencies.

Typically natural or man made interference enters a radio via two different routes. Through the antenna or through the power supply cables. The noise may also be entering by both routes at the same time. Electromagnetic noise can be generated by electronic devices many of which can be found on a boat. The type and frequency of the noise can vary greatly. Every time you bring an electronic device on a boat, you are importing sources of interference at the same time.

The most important part of any radio system is the antenna. If you can't get a good clean signal to the radio. No matter how good the radio is, it can't produce a silk purse from a sows ear!
VHF radio transmissions do not bend to any significant degree and generally follow a line of sight path from the transmitting antenna to the receiving antenna. It must be remembered the signal gets weaker the further one gets from the transmitter. The strength of these signals on the average becomes quite weak at distances greater than 35 to 45 miles. As far as possible, install the antenna on the boat away (proximity issues) from any nearby potential sources of interference. Pumps and power supplies as used for computers and charging mobile phones are common culprits. Internal antennas are no good. The antenna needs to be mounted outside on the roof of the boat. 

The medium and long wave bands have more back ground noise associated with the wanted signal. This noise is often a mixture of man made and natural interference. The problem here is that interference on these bands can carry for many hundreds of miles. Amplitude Modulation does not carry a good broadcast quality (Fidelity) signal. The medium wave and long wave bands are also subject to signal fading, fluttering and even dropping out all together. These bands are also subject to weather effects such as the flash of lightning which can propagate for thousands of miles. As a result many broadcast stations have migrated away from these bands. There are plans in place to remove all broadcast stations from these bands and assign the frequency range to other purposes. One problem with amplitude modulation is that there are two or more stations broadcasting on one frequency you will hear a mixture made up of all the stations. (Aircraft still use AM because of the ability for two or more signals on the same frequency to be heard at the same time by a ground station. It is possible to transmit a distress signal even if another aircraft is transmitting.)

The VHF bands are used for what we call line of sight communications. We have become more accustomed over the years, to what we call broadcast quality which is being provided by a frequency modulated signal. The signal can also carry additional information such as stereo, station identification and many other things. One of the phenomena is called 'capture effect' When two stations are broadcasting using FM on a given frequency.
(co-channel) The strongest station will be captured by your radio and you will only hear the strongest station. The same frequency therefore can be reused again and again in different but non-adjacent areas because the coverage is essentially intended to be a strong local broadcast station.

However, before you install any radio equipment you need to check the following items.

  • Are the batteries in good condition - replace if in poor condition.
  • Are the battery connections good - wire brush clean and include a smear of petroleum jelly on the connections afterwards.
  • Is the earth strap in good condition - wire brush the connections clean.

So now you have installed the radio into your boat. You are using a good quality coaxial cable to connect the radio to the antenna. You turn on your radio and select the FM band and find a local broadcast signal. If the signal is OK, your good to go.

However, if you find that there are only a poor signals available due to interference. You will have to do a few simple steps to locate the source. Go to you boats electrical control panel and turn off all the 12 volt switches other than the one controlling the power to the radio. Turn off the power from the landline. If you are using an inverter turn the inverter off close the boat down electrically as far as possible. If the interference has gone away - one or more of the sources of interference could be on your boat. If the interference is still persisting then it is from an external source. You can move your boat away from your neighbours boats where the interference my originate, and run the test again.

To test the boat start to power up the boat one circuit at a time. If the interference returns turn that circuit off again and continue powering the boat up by adding one circuit at a time. Then when all the 12 volt circuits have been checked. Turn on the mains again so that other items such as phone charges and laptops get powered up again. Last of all turn on the inverter.

Tracing the source of intermittent interference problems can be difficult. Items such as water pumps only run infrequently so you need to open a tap to make the pump work. Its the same with central heating pumps and bilge pumps. If the source of interference is intermittent then the chances are that it could be a pump of one kind or another. It could even be the compressor pump in your fridge.

Now start up the engine to check the alternator (if the hash frequency is high-pitched and changes with engine speed) Decoupling capacitors that will reduce the interference are available for certain kinds of alternators. Many alternators have the capacitors built in.

Now you should have a list of any circuits that are causing a problem. But what we don't know is which way the interference is arriving.

Disconnect the antenna from the radio and listen for the noise:

  • If the noise is gone or significantly reduced then the noise was coming down the antenna
  • If the noise is still there and at the same sort of level then the noise is coming down the supply cable.

Supply Cable: Connect a choke in series with the supply line. If needed connect a decoupling capacitor from the radio side of the choke to the radio case.  Always put the choke as close to the radio as is possible. You can buy the choke at Maplins or Halfords stores. This is a choke and fuse holder that is available on the internet for a few pounds.


Now for the hardest part of solving the issues. If you have electrical items that are radiating interference there is little that can be done to mitigate that at the radio. You need to sort out the individual sources of interference. The most common source of problems are switch mode power supplies. These come with not only with LED lighting systems but also with fluorescent lights, computers, remote controllers, baby monitors, radio receivers, electronic locks, wireless security systems and mobile phone chargers. 

Unfortunately, the market is flooded with cheap, non-branded, LED lamps which do not comply with the essential requirements of the EMC Directive. In the current environment of negligible market surveillance, the CE/kite mark is next to meaningless. Especially when domestic radios and switch mode power supplies seem to operate on or at the same frequencies, producing interference in the range 30 - 300Mhz. As well as FM on 88-108 Mhz DAB in the UK is roughly in the 210-230 Mhz band. Easy to see why the problem happens. LED lighting is rapidly becoming the most serious of the EMC problems encountered. Either the LED lamps are generating radio frequency interference, the power supplies  or both. Europe continues to monitor, these issues, with the amendment of standards covering lighting product emissions, such as EN 55015:2006. 

If you wish to retro-fit LEDs to existing lamps, ensure they are resistor-limited, as these will not cause interference to your radios. Some of the well-known manufacturers are following the rules and producing EMC compliant lamps.

One problem with LED lights even the ones with the CE mark. Different types give off more interference than others.  You need to buy a few different samples until you find the one that operates without creating the problem. So the cheaper LED lighting will potentially be the most problematic in terms of interference, but they will also be the preferred choice for the average consumer, and therefore the most prevalent.

On Rosie I overcame the problem by only using the cheap and cheerful user replaceable LED lights. The ones without a built in voltage regulator. I then used a separate voltage regulator that converts DC voltages between 10 and 30 volts to a constant 12 volt DC. So no matter the voltage at the battery. The regulator controls the voltage to all the lighting circuits on the boat. The regulator has high quality filtering built in and more than meets the EMC regulations.

The consumer Magazine Which? has been investigating the radio interference produced by some LED light bulbs People have been finding their DAB and FM radios and even their TV will no longer receive stations when LED lights are turned on. Their article appears in the April 2014 edition of Which? magazine. 

When it comes to mains power supply for charging. I created a 12 volt car socket (Cigarette Lighter) and I use the 12 volt charger cable. This cuts out the need for a switch mode power supply for the laptop and smartphone. You can also use clamp-on ferrites, positioned close to the source of the emissions can help to suppress or reduce the amount of trash that is radiated by the cables. Maplin do a good range of clamp-on/clip-on ferrites.

So what can be done at the receiver. In reality very little. You can use good quality connectors, good quality cable, good quality antenna designed for the band you are wanting to receive signal on. More than anything else - do not use a signal amplifier (sometimes called a pre-amplifier) between the antenna and the radio.   

Let me make that a bit more understandable - DO NOT use a signal amplifier between the antenna and the radio.  If a signal is already poor an amplifier will not improve matters. The amplifier will boost the wanted signal and it will also boost the unwanted signal. In fact the amplifier will usually make the situation much worse. 

Now, there are simple omnidirectional antennas and there are multi element directional antennas.  Let us suppose that the radio station north of your boat is the one that you want to receive. South of your boat is a signal that is causing interference. A vertical omnidirectional antenna will receive both signals equally. However a directional antenna can be 'pointed' at the wanted signal. The antenna at the same time will be pointing away from the interference. The wanted signal will take advantage of the antenna gain. The interfering signal will be rejected by the front to back ratio of the antenna. If the antenna gain is say 3db, the received signal will be improved. If the front to back ratio is 3db the interference signal will be halved in strength.  

No comments:

Post a Comment

Please put your name to your comment. Comments without a name may automatically be treated as spam and might not be included.

If you do not wish your comment to be published say so in your comment. If you have a tip or sensitive information you’d prefer to share anonymously, you may do so. I will delete the comment after reading.