[TowerTalk] Re: narrowband filters

[Jim Brown]

On Mon, 5 Jul 2004 18:00:44 -0700, Richard (Rick) Karlquist (N6RK) wrote:

>100 kHz is less than 1% BW on 20 meters.  If you can
>manage an unloaded Q of (quite difficult, but doable),
>you will lose about 1 dB per resonator.  Let's say this filter
>covers 14000 to 14100.  At 14150 (one bandpass octave out),
>you will get about 6 dB suppression per pole.

>Let's review that:  you lose 1 dB of desired signal for
>each 6 dB of undesired signal suppression, assuming a Chebyshev
>response.

>Of course, you could separate the passband and stopband
>somewhat, which would make things better at the expense
>of losing the top of the CW band and the bottom of the phone
>band.

The latter is more like what I had in mind. For most CW contests, one would be pretty happy 
with the center of the filter at 14025. The lowest frequency of interest on SSB is 14150, and 
perhaps you decide that you don't work that low, instead you move up the band a bit (where 
the QRM is less anyway). Remember, this is Field Day, not a DX contest. Now you've got a 
design problem that, given the same achievement in filter design gives you appreciably 
more rejection simply by defining the problem in a manner more appropriate to the use (or 
allows it to work with lower Q).  Now your stopband attenuation is more like 10-12 dB for 1 
dB burned in the filter.  

Applying the same logic for 40 meters, again you tune the passband to 7025 and decide 
that the SSB station will stay above 7200. Not a DX contest, but a reasonable set of 
parameters for Field Day.  On 40, the design problem is less demanding by a factor of 2:1. 

Jim K9YC