Saturday, 20 October 2012

Goggle Box (2)

Well in the last posting we had a look at antennas for terrestrial television reception. I remember when television changed from using 405 lines. To 625 lines on the screen to represent the black and white image. (the first time we changed to high definition) The frequencies bands used to transmit the signals were on different parts of the radio spectrum. So as you upgraded your television from the VHF band using the typical "H" configuration antenna to the UHF band you also needed to upgrade the antenna system. The simple "Yagi-Uda" antenna was the antenna of choice. 

Geek Mode On: Then with the advent of colour television people were being conned into changing their "black and white" antenna into an antenna designed for "colour reception." The same thing is happening today. People are being advised to change their antenna for one intended for "Digital Reception." This is a form of snake oil. An antenna is an antenna, it only collects the signal it can't be selective in any way other than frequency band of operation and directional. 

The grouping of UHF antenna are also sometimes listed as A, B, C/D, E, K and W. 

UHF antennas intended for television reception in the UK come in six different band versions. The spectrum of the band starts at 471 MHz and finishes at 847 MHz. Depending on where you live you would choose an antenna in one of the bands A to F. However, if you are mobile you would choose an antenna in band F, W or Black. If you have an existing antenna you can check what the band it is intended for.  Do this from the colour of the plastic cap at each end of the boom that holds the elements in place.  

Log Periodic Antenna
Antenna designs have improved over the years. They have become more compact, there have been small and in some cases significant design changes. 

One that you will see advertised is a "Logarithmic Periodic" antenna. This is often abbreviated to log periodic and is a broad band variant of the Yagi-Uda antenna.  

This antenna design is often used where a wide range of frequency coverage is needed while still having moderate gain and directionality. It can be used as a TV antenna where storage and portability are significant factors.

The next most important point in the antenna system is the coaxial feeder. (The cable that connects the antenna to the television). Television sets, coaxial feeder and antennas have a form of resistance known as Impedance. The normal impedance of the television system is 75 ohms. Purchase and use good quality 75 Ohm coaxial feeder. 

The next most important item is the external television signal amplifier. Sometimes called a booster. (You should never need a signal amplifier) The signals picked up by the antenna cannot be improved by amplifying them. You cannot replace what has been lost. Many amplifiers are not restricted to just the UHF television band and will amplify interference signals as well as the wanted signal. Your TV set has electronic circuits that control the strength of the signal as it enters the television. You boost it in and all the TV set does is to attenuate (reduce) the signal down to a level that it can work with.

Geek Mode On: More Television Interference problems (TVI) and other signal quality issues are cured by removing an in line amplifier, than any other remedy. Some amplifiers are advertised as having "special filters" to stop out of band interference. In line bandpass filters (which will only pass the TV frequencies) can be purchased for about five pounds. Remember, if you need to use a TV signal amplifier to get a picture -  then there is a fault in your antenna system.
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 and the Independent IWA members on Council) You will find the poll option on the right hand side just below the members pictures.


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