I'm not sure, it is kind of like which came first the chicken or the egg.
  Many early radios were designed in such a way that the 80 and 40 meter 
bands were derived by the difference frequencies between the vfo and 
mixer and the 20 15 and ten meter bands used the sum of the vfo plus 
oscillator.  Hope that makes sense.  If you remember theory, when you mix 
frequencies, such as a vfo plus oscillator you get on the output, both 
original frequencies plus sum and difference.  In my national ncx 200 
which was my first radio, the if as I recall was 5.2020 and the vfo was 
somewhere around 9 mhz.  Anyhow, when your mixing a signal and one of 
them is ssb, if you use the sum, the sideband stays the same, and if you 
use the difference such as my national did for 80 and 40, you get the 
opposite side band.
73
Butch Bussen
wa0vjr
open Node 3148
Las Vegas


On Wed, 13 Oct 2010, Phil Scovell wrote:

> I think I heard once but I can't remember so does anyone know why the =
> various bands are used for upper and lower side band?  Just curious.  =
> There's got to be a reason why it is different based upon the band you =
> are using.
>
> Phil.
> K0NX
>
>