# High Frequency Carrier

Joined May 19, 2013
214
"The useful signal is modulated on a high frequency carrier in order to provide a relative narrow signal bandwidth and the needed communication capacity."

What does the author mean by "relative narrow signal bandwidth"?

For example, for amplitude modulation, the bandwidth of the transmitted signal is twice the message signal. So it is wider, not narrower.

#### Papabravo

Joined Feb 24, 2006
21,264
It means relative to the frequency of the carrier. For example an AM broadcast has an approcimate bandwidth of 10 kHz relative to the carrier frequency of 510 kHz to 1640 kHz. On the AM broadcast band the stations are spaced a minimum of 10 kHz apart. In FM broadcasting the channels are spaced 200 kHz. apart with respect to a carrier of 88 MHz. to 108 MHz. In SSB transmission the bandwidth is 3 kHz with respect to carrier frequencies from 1.8 MHz to 29.7 MHz. for the HF Amateur bands.

#### GS3

Joined Sep 21, 2007
408
I agree. The absolute bandwidth for a tuned circuit is a percentage of the center frequency so, the higher the frequency the wider the bandwidth. The more bandwidth needed for a channel, the higher the carrier frequency needs to be.