Carrier frequency selection and Bandwidth

[MrSpock]

What is the relationship between carrier frequency selection and bandwidth? Does or can the selection of a carrier effect your bandwidth?

 

A1. So long as you stay away from 'zero' (as your carrier frequency), then there should be no relationship. (If you get too close to 'zero', then things might fold around and get a little bit more complicated.)

A2. Shouldn't.

 

[svermill]

Depends on your application I think. For example, in satcom applications, free space loss is directly proportional to frequency. So if your system is power limitted, the higher your carrier freq, the less power you have for higher bandwidth.

I admit that this is a fairly uncommon situation. But there may be other similar situations?

Scott

 

True but...

The higher your carrier frequency (band), the more antenna gain would typically be available at each end. This usually compensates for the path loss factor that you mentioned. This is assuming that you can optimise your antennas (for the available space) for each frequency band.

 

[svermill]

Yeah, you're right. My response was really a theoretical one without too much "real-world" application. But I couldn't tell if the question was one of theory or application so I just threw it out there. Thanks for the follow up.

Scott

 

[MrSpock]

Apprecite the inputs. I am having a problem understanding the relationship between data transfer rate ,(which I thought was the bandwidth) and cariier selection.
From discussing this with folks seems that the higher the carrier selection the higher the data transfer rate acheivable, which kinda dispells that data transfer rate is the same as bandwidth. What really is the interrelation ship of these factors. I am not interested in the range of the signal. Would appreciate any help from anyone. I thought bandwidth was the range of frequencies that a system can see and data transfer rate was the rate at which data is communicated between the transmtter and
receiver. So I am confused and beg everyones indulgance...........
Thanks
Mr Spock

 

The only relationship between bandwidth and carrier frequency is the following common sense notion:

'The higher ya go, the more room ya have.'

The higher frequencies offer 'wide open spaces' simply because with every decade increase in carrier frequency, there is ten times as much space as in the previous decade.

10 MHz to 100 MHz is 90 MHz wide.
100 MHz to 1 GHz is 900 MHz wide.

No prize for that. It is just elementary.

Practicality:

You could conceivably broadcast a 6MHz wide TV channel in the middle of the HF band (3-30 MHz), but to do so would obviously raise a chorus of complaints. And the propagation and interference would be gawd-awful.

Wideband 'Over The Horizon' radar *are* transmitted in the HF band and they *do* raise a chorus of complaints (although they are getting better at avoiding interference).

In practice, wideband signals tend to be at higher frequencies for a whole host of reasons. But these assignments are conscious decisions by the engineers and regulators - not explicit limits imposed by nature.

Antenna Limitations (percentage bandwidth):

The only technical 'fly-in-the-soup' might be percentage bandwidths for antennas. It would be difficult to design an HF antenna that would provide the required performance over a 6 MHz bandwidth centered at 10 MHz for example (~30%).

On the other hand, 6 MHz bandwidth at around 640 MHz (UHF TV Ch 42 for example) is trivial for an antenna (~1%).

Percentage bandwidth may apply to over the air propagation paths to a certain extent as well (HF propagation being an extreme example).

This percentage bandwidth detail wouldn't necessarily apply for cable connections (not involving antennas) except in extreme cases.

Carl Sagan's 'Ship of the Imagination':

If you can imagine an Upper Sideband (USB) transmitter being fed with a baseband signal (audio, video, or whatever), the transmitter is simply taking the baseband signal and sliding it from zero Hz (the baseband) to your dialed-in carrier frequency. In theory, you could dial-in any frequency you want - from DC to daylight. The baseband signal (imagine the spectrum of the baseband signal) just slides up and down the overall radio spectrum as you tune your transmitter. The baseband signal doesn't change and doesn't know what frequency it is on (so to speak).

Other modulation schemes are similar to USB except for where they are different (sounds like a Yogi Berra-ism).

Exception: There is a 'weird' effect with FM. Bessel curves and all that 'complicated stuff' relating to modulation index. Look it up.

Good luck.

J. Harvey, Ottawa
VE1BLL(at)rac.ca /VE3