Prestige

Digital Television and Boats

by Ian Fletcher

Television, mobile phones and mobile internet use much the same technology to get information to you on the move.  They all suffer the same problems caused by distance from the transmitter, reflections from other objects and noise and interfering sources.  So, many considerations of TV apply also to these other technologies.

Analog TV is disappearing soon, so its only advantage, which was to give a picture and sound, albeit noisy, when the signal was very weak, is now irrelevant.  Digital television is good for boats, mostly because it doesn’t suffer from ghost effects caused by reflections arriving from behind the antenna.

Digital TV sets need a signal which is stronger than the noise by about 20 dB (10 times the voltage).  Less than that and there’s no picture or sound.  Digital TV is particularly susceptible to sudden bursts of noise (impulse noise) because if the set loses synchronisation with the signal it takes several seconds to reacquire it.  The picture either freezes or gets blocks of pixels which are stuck and obviously wrong (pixellation).  The sound stops and may spit alarmingly.  On boats you can get impulse noise from anything that switches on and off.  The usual culprits are 230V inverters, refrigerators, electronics and chargers, but even LED lamps can do it.

The trick is to ensure the signal to noise ratio is always high enough, by both getting a good signal to start with, and treating noise sources to reduce them.

Antennas

Antennas for digital TV are the same as analog ones.  There are only a couple of main types on boats.  The common mushroom or dish types have an almost omnidirectional response, responding to signals coming from all directions.  I can report that the cheap versions don’t seem to be as effective or reliable as the big name ones.  They all respond best to horizontal polarization, used in most major population areas.  In many regional areas the polarisation is vertical, meaning your antenna may not work too well (often it will be fine).  In that case use a VHF marine antenna, build your own antenna or just use a coat hanger.

My homebrew antenna swings from a spinnaker halyard in the foretriangle on our Jeanneau SO 42 DS.    The cable just goes through a forehatch.  The plywood formers catch the wind and flip over, but the crossed loops do work for horizontal polarisation.  It needs further work.

The best antenna is the diversity type, using two or more antennas separated by a couple of metres and electronics to get the signal from the better of them, changing over in timescales shorter than seconds.  They are really worth the money if you go to places where you anchor behind big trees or near big cliffs, they sniff out whatever signal there is.  They are immune to reflection effects but may have weak output in horizontal polarisation areas.  In that case, set them up with some slope in the antennas, not necessarily 45 degrees.

Height of the antenna is important only if you anchor close to obstructing hills or trees or buildings.  On the open ocean there is very little difference between the signal received 2 metres above deck and at 20 metres.  However, putting the antenna at the masthead does avoid the obstructing effect of the rigging.  I had a problem once with severe directional effects in an antenna, which turned out to be caused by reflections from the spreaders which were at the same level and metre or so away.  Moving the antenna up or down by a metre or so solved it.  If you have a problem where the picture fails as the boat swings at anchor, it’s either reflections from rigging or rails within a couple of metres of the antenna or the antenna is no good.

Antenna Problems

The most common problem by far is failure of the cable, followed by loss of power to the antenna preamplifier.  Cable failure is usually due to corrosion, but mechanical failure due to movement can be the problem.  Here’s how to check it:

Is the TV working properly?  In a strong signal area it should work with just a piece of wire in the antenna socket.

Is the 12V power supply from the battery getting to the bottom of the cable?  Unplug it and check the socket with a voltmeter.

Are the connectors at each end of the cable in good condition?  I bet the top or outside one is covered by, and full of, powdery corrosion.  You must be able to measure the 12V supply voltage from the centre to the outside of the top connector.  If there is corrosion clean everything well, taking the connector off if necessary.  I’ve used methylated spirit, WD spray, proprietary electrical cleaners and even water.  But avoid getting dampness inside the cable or connectors.  If you do, dry it as best you can and leave it in the sun to dry completely before reassembling.

Most antennas use F type connectors, which are a screw fit barrel which connects the outside shield of the cable to the outside of the socket.  The solid centre conductor of the cable extends to the front of the connector and makes contact directly with the socket centre.  F connectors are perfect in houses.  They don’t like water or salt.  But, even if the corrosion on yours looks like a ball of cotton wool, their simplicity means you may be able to clean yours up if you don’t have a replacement on board.

With the top connector off, check the cable for corrosion.  If it doesn’t look perfectly clean and shiny, carefully cut off a centimeter or two of the plastic cover to see if the metal and braid shield is corroded.  Repeat until it looks OK.  Then reform the shape of the end the way it was, screw the connector back on, and clip off the end of the centre conductor almost flush with the face of the connector.  You can polish off minor corrosion from the centre conductor.  Scraping with a blade is usually enough.

Now you can measure the 12V DC supply voltage across the connector, so put it all together again after cleaning up the socket as well.

It should now work again, test it.  If it does, cover any exposed connectors with self-amalgamating tape to keep water out.  Use cable ties to hold the cable against any possible movement.

If it doesn’t, the antenna itself is faulty and you’ll have to take it down.  You may have to spend money to fix or replace it.

Oh, I forgot, most of this happens above your head at the top of the mast.

Interference

If you are in a weak signal area and the picture pixellates, it could be due to some directionality in the antenna.  See if it correlates with the direction of the boat as it swings at anchor.  It could also be due to interference from devices switching on the boat.  Interference from external sources is very unlikely, it’s on the boat.  To find the guilty device arrange to turn everything off except the TV.  Then turn all electrical devices on one at a time until the problem starts.  This may be a slow process if you have to wait for very intermittent devices like fridges and bilge pumps to actually switch.  When you find it, the usual solution is to add a filter on the power leads of the offending device to stop the electrical noise getting out from it.  A Jaycar Cat. No. AA3074 filter should work.  I’ve pulled filters from old computers or better ones from old 12V mobile phone chargers.

Which TV?

Digital TVs have a picture which is 16 units wide and 9 high, whereas analog TV used 4:3.  If you replace an old one keep this in mind so it fits and looks good in the saloon.  LED models are much lighter than plasma.  With many boats having 230V inverters, you may not be restricted to marine models.  However, 12V televisions generally have lower power consumption, and this may be critical for you.  Try to avoid losing the instructions unless you carry suitable crew (younger than 18).

A personal video recorder built into the TV, or separate, allows you to avoid missing programs while the radio sked is on, or the fish are biting.  You may also want a DVD player.  This brings up the issue of picture quality; TVs do vary markedly in this and it’s all because of the display, not the tuner.  If you are used to a monster screen and surround sound at home a few extra dollars to get a sharp picture with good color may stop later regrets.  Color is adjustable but you can see real differences in sharpness in the shops.

Last but not least, how heavy is it?  Can it fit on your bulkhead?  Does any swivelling wall mount have a lock to stop it moving?  Can you see it around the mast?  Can you hide the power and signal cables?  You’ll think of more questions.

Here’s a nice Jeanneau Merry Fisher with its TV antenna discreetly mounted  above the roof.  If such a mounting gives varying performance swinging at anchor another 30 or 50 cm higher could fix it, but spoil the appearance.  If radio reception were a problem I’d put a matching whip on the port side.

Other Technologies

Satellite communications can suffer the same problems with impulse noise but are generally robust.  The rigging can obstruct the signals.  Remember that the satellites are somewhere towards the equator and the elevation of reception may be quite low unless you are in the tropics.

Mobile internet and mobile phones have low signal strength as their main problem.  Get one which will accept an external antenna and put it as high as you can.  Active amplifiers sold as antenna boosters are illegal in Australia, because they severely reduce the range of the base station for other users.  The Carriers can also sue you for using one.  Otherwise, like us, you’ll just have to go on deck to get some signal, if there is any.  This is a good idea anyway, since mobile phone signals are very variable because of both designed in and natural factors.  the more signal you have to start with the less likely you are to drop off in the holes.

I haven’t mentioned broadcast radio, possibly because when Jeanneau put a car radio in our boat, they forgot that the battery gets turned completely off on boats, so it needs reprogramming every time.  Needless to say, it doesn’t get used much.  I could put a separate fused power feed from the battery but that defeats the purpose of turning the batteries off.  Most radios have a yellow power wire which just keeps the memory alive. If so, put a 0.1 A fuse right at the battery in the line.  On all production boats including Jeanneau, the antenna is stuck on the hull, which is fine in strong signal areas.  Away from cities a splitter on the VHF antenna is a much better idea, or even a longer separate whip.  Noise affects MF-AM and VHF-FM radio differently but the method of finding the culprit and curing the problem is much the same.  Digital radio is very similar in performance to digital television.

Enjoy your cruising.

Ian was a communications engineer and discovered fishing and sailing after retirement.  His wife and he own a Jeanneau Sun Odyssey 42 DS in which they cruise the waters of southeast Tasmania, especially in winter. One day they might sail to Australia, but having lived up north there, where it’s too hot and there’s nowhere to cruise to, they understand why people go racing.