Aligning an aerial for Digital TV can be both one of the easiest and hardest things parts of an antenna replacement job. In many locations you can visually eyeball where it needs to be pointed, but in others locations it will take painstaking work or specialist tools. In this article I’ll explain the professional method of aligning an aerial. For information on do it yourself methods, see my article DIY TV Antenna Alignment Methods.
The Professional Way
Any professional TV antenna installer should use a Digital TV Field Signal Strength Meter. Even in a trivial situation where you can see the TV towers, it’s still of value to check the antenna installation with the meter because sometimes a brand new antenna could be faulty. Knowing the antenna is faulty from the start can save many hours of wasted time searching for a cabling fault that doesn’t exist!
A Digital Field Signal Strength Meters has a few different modes which are used to ensure the best quality signal. The most obvious one is the Signal Strength display. The strength of the radio signal is measured in dBµV/m, which is decibel microvolts per square metre. The Australian Standards for the Digital TV Switchover require a minimum of 45dBµV/m but recommend 60-70dBµV/m. Signal at the TV should be no more than 80dBµV/m. Anything higher can cause a new set of issues.
With Digital TV the quality of the RF signal is more important than the strength(provided strength is adequate). A strong but low quality signal will produce a worse resultant picture than a weak signal with good quality. Modulation Error Ratio(MER) is the standard for measuring quality. An MER of 25db or better indicates a good quality signal. The higher the better.
Another measure that can be used is Bit Error Rate(BER). BER indicates the total number of bit errors after the Digital TV Forward Error Correction has been used. It shows how much the TV has been able to compensate for errors in the RF signal. The BER should be as close to 0 as possible and because it’s usually a very small number it’s shown in E notation. The usual minimum should be 8E-4. The bigger the number on the right hand side of the E the better.
Finally we have the constellation display. The constellation display is effectively a graphic display of signal quality. It displays a grid of square dots, which with a good signal should be small and sharp. The lower the quality of the signal the more noisy the display will be, and this visually shows the MER and BER to the installer.
Procedure of Alignment
When aligning the antenna, the installer will typically first of all roughly mount the antenna facing the same direction as all the others on nearby buildings. Then after attaching the Field Signal Strength Meter, he’ll look at signal strength to find the strongest signal direction. After that he would check the constellation graph and BER/MER to find the best quality of signal. This may be the same direction as the strongest signal, but won’t always be so, particularly in hilly areas.
Summary of Signal Strength and Quality StandardsSignal Strength 45dBµV/m 60-70dBµV/m 80dBµV/m BER 20db 25db MER 8E-4