In layman's terms it's a way of filtering out unwanted frequencies.
For example, if on BBC2 you keep getting interference from another station from a different transmitter on top of the picture you are watching, that's when you would fit a banded filter to filter out the unwanted signals.
Picture of a filter Picture of a television screen
mounted on a pole with interference
As they all have different channel numbers on them which one do you use use?
I will try to explain about the UHF Frequency spectrum.
The UHF band we use for UK TV works from 471.25 MHz to 847.25 MHz but we also convert our frequency table into channel numbers using 8MHz separated blocks as follows
471.25 MHz = channel 21
479.25 MHz = channel 22
and so on all the way up to 847.25 MHz = channel 68.
Are you still with me?
My local transmitter works on the channel numbers of
If I wanted to fit an aerial filter that would cut out all channels above channel 28 this would stop all unwanted signals above that channel and would stop all channels that could interfere with my channel numbers.
But, you will find that its impossible to get a filter box for the channels you want to filter so you will have to get one to the nearest channels you want to filter out.
On the front of a filter box it might say ( B filter ch 39-53 )
This would mean that if you fitted this aerial filter to your aerial it would cut any channel before channel 39 and all channels above 53.
So as you can see this filter box would be no good as its cutting out channels 21 to 39 witch would cut out my local transmitter signals .
Another filter box might as ( A , 21 TO 34 .If I use this filter box all the channels above 34 would be cut out so this is the best filter box to use.
Hope I have not confused you even more but this gives you an idea of how aerial filters are used!