Saturday, 13 October 2012

Goggle Box (1)

Watching a bit of television of an evening can be one of the ways of spending a more relaxed part of your cruising day. Add to this a cold beer or a glass of wine and your enjoyment is increased no end. Getting television onto and into your boat is not such a difficult thing. Modern flat screen technology has made it as easy as hanging a picture on the wall.

But as you cruise around the system you are bound to cross through good, bad and indifferent coverage areas. After a while you get your eye in for choosing the better places to moor up. For me, it tends to be away from trees and tall buildings, with a vista of around a mile in the direction of the transmitter site.

Depending on your particular viewing interests, either satellite or terrestrial television may fulfil your watching needs. We have all had a go at messing around with television aerials and satellite dishes trying to improve the quality picture and sound. I will start with a look at terrestrial television.

A television aerial can be made of almost anything. I remember in my student days a wire coat hanger stuffed into the antenna socket was enough to get a watchable picture. However, since the advent of digital television broadcasts. The quality of the received signal needs to be a bit stronger than was needed before. 

To get a decent signal for the new digital services you need to have three things. The first one is to know the direction in which to point the antenna. The next one is the antenna polarity.  The third one is an aerial with a reasonable figure for gain. Gain is a measure of the antenna performance. Usually expressed in Db however, the "gain" figures are meaningless unless you know what the antenna is measured against. The simplest antenna is a dipole and antennas get more mechanically complex in design to increase the gain figure. 

Geek mode on: As antennas are often designed on a computer system using specialist software. There is also a notional form of antenna known as Isotropic. So when you look at the packaging, if it makes a claim figure of any number of db the figure is meaningless. The figures should be dbd (db gain over a dipole) or dbi (db gain over an isotropic benchmark)

5 element antenna

A "Yagi" antenna (which is in reality a "Uda" antenna) is one of the most popular of television antenna designs. It is of very simple construction and has a high gain figure for its size. You are probably familiar with this type of antenna, as they sit on a small pole on top of chimneys on just about every house in the country.

All television signals are radiated in either vertical or horizontal polarity. If your antenna is in the wrong polarity, say vertical when it should be horizontal for instance. The signal will be reduced by 3db or it will be half the strength when compared to a signal received with the correct polarity. In the diagram above, the antenna is in vertical polarity. If the antenna was turned and the elements laid flat, it would be in horizontal polarity.

The antenna is directional, in other words you have to point it into the correct direction. The role of the television antenna is two fold. First is to improve the wanted signal and its second job is to reject unwanted signals. It does this by reducing the strength of any transmitted signal arriving from any direction but the front. As the antenna gets longer and more elements are added two things occur. The gain figure is increased slightly and the antenna becomes much more directional. 

Geek Mode On: The longer an antenna becomes due to the number of elements added. This can create a beamwidth problem. As more elements are added the beamwidth of the antenna decreases.  The antenna becomes more difficult to keep pointed in the correct direction. If the beamwidth is very narrow and your boat is moving around on its moorings. This can be enough to cause the signal strength to fade in and out.

Pointing the antenna in the direction to the nearest transmitter site can be a problem. There may be a range of hills between you and the transmitter. To all intents and purposes you need to be in line of sight of the transmitter. In populated localities where reception is difficult, there may be alternative local transmitters intended to fill in weak signal areas. 

Note: If you need to point your antenna to a different transmitter site - you will almost certainly need to retune your television to pick up the signals as the frequencies used, will change between transmitter sites.

To be continued. 

I have created a poll that will run between now and the 1st of January 2013. It's not very scientific. Its a simple question "Will CART Succeed". You have three choices Yes, Unsure and No. Now you can vote for one of the three choices. If you change your mind before the poll closing date. You can come back and change your vote. (I wish we could do that for politicians) You will find the poll option on the right hand side just below the members pictures.


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