So much good information here. Thanks to all who responded with useful 411 and not being drunk behaving like children and cussing at people.
Anyway ..
Tallman the specs you referenced are for the RX filter not the TX bandwidth ?.. i must have not been clear. At least thats what I think looking at the specs.
I believe the spec refers to the shape factor of the filter
That, according to many intellectual books on radio, definition indicates the degree to which the real filter differs from the ideal". - Shape Factor
To calculate the S.F. just divide the bigger number by the smaller. the closer to 1 the better.
2980 Shape Factor Calculation
4/2 = 2
148
2.3/1.1=2.09
The 2980 spec starts at -5db and the 148 at -6db. It was unpublished on the Ranger site the spec at -6db. Makes me wonder why Ranger would vary away from the "Normal" spec's. My guess, is the shape factor is more than 2.
Again, according to real radio books, a shape factor of 2 or more is a the very bottom end of performance.
Referring to the BW of the filter with can be calculated using the formula:
Bw=Fr/q
Where
BW = bandwidth at -3db ponts
Fr =resonant frequency
Q= selectivity factor
2980
Audio bandwidth - 300hz - 2500hz
Upper and Lower sideband separated by 600 (2x300(lowest audio frequency passed))
IF Frequency 10.695
Q=Fr/BW
10.695mhz/600hz=17825
The problem is the -60db point on the 2980 is +/- 4khz - thats wide enough to pass 2 ssb conversions!! Far to wide to provide good adjacent channel rejection which appears as poor receiver selectivity.
148
7.8mhz/600 = 13000
It may seem, at first, the rci with a filter Q of 17,825 is better but not so
The -60 db is 2.3 khz which is only wide enough to fit 1 channel - this makes the 148 filter superior to the RCI export and appears as good selectivity and adjacent channel rejection.
The 148-GTL DX has even better performance
-6db 2.1khz
-60 db 3.3 khz
Filter Q = 17825 - Same as the RCI 2980
3.3/2.1 = 1.57
These are nice specs.
One can use this information to determine how cheap the manufactures of CB type radios are. As an example:
Uniden Washington (8719 pll)
The spec
-6db 4.2 -60db 7.9
7.9/4.2 = 1.88
Q= 7.8/600=13000
Its obvious that this a single conversion receiver that gets it's selectivity from the IF filter - shared by SSB and AM - Thats wide enough to pass a full DSB signal + a bit more.
When I see -6 db at more than 3.0 khz if makes me worry that the receiver is compromise - and in this case it is. What other reason would there need to be to have a filter wider than the incoming signal? To me thats like trying to filter dust with chicken wire fencing.
Thanks to ALL , I hope I got this right.
After all this investigation - A nice Kenwood does seem like a viable option.
