Hi,

A system which operates in the frequency range of 800MHz to 900MHz, and comprises channels of 30KHz bandwidth, means that:
1. Each channel can receive signals with no attenuation over a passband of 30KHz?
2. Each two adjacent channels are 30KHz apart from each other?

Thank you.

 

Hello,

With a 30 kHz bandwidth you are using 30 kHz of the RF spectrum.
In an ideal case the channel seperation can be 30 kHz.
This is only possible with VERY sharp filters.

Bertus

 

Hi,
Thank you.

- So adjacent channels aren't 30KHz apart from each other, but more than that? (As there're no ideal filters?).

- Why is it that the lower the bandwidth, the higher the possibility of interference?

Thank you very much

 

Since there are no ideal filters and real filters have rolloff on the sides, in the US, channel bandwidths are defined with a filter mask. The transmitted signal must stay within the mask. These masks are roughly trapazoidal in shape. Because two adjacent trapazoidal masks will always overlap at some point, adjacent channels must be geographically separated in order to avoid interference. To get a license for most two-way radio frequencies, you must go through a frequency coordinator who calculates, based on your ERP and antenna height, whether you will cause or receive interference to or from other nearby stations. It is his job to find a frequency that will do neither.

Until now, in the VHF Hi band, channels were 25 kHz wide but spaced 15 kHz apart. This only meant that adjacent channel stations had to be spaced farther apart geographically.

 

- Why is it that the lower the bandwidth, the higher the possibility of interference?

I don't know that this is true. What was your source of information? One would normally think that the narrower the band, the more channels are available and the less overlap there would be.

Narrower bandwidths ARE more susceptible to multipath fading because the signal cancellation typically occurs at one frequency at a location. With wideband, some parts of the signal may still get through.