Topband: Tunable External Preselector for better Beverage S/N?

[Joe Reisert]

Tom,

Several years ago I designed and built a few of these filters. The lowest 
frequency one I built tuned 4-20 MHz. It is complex consisting of 9 
inductors (toroids) and a 3 section 20-365 Miller type 2113 variable 
capacitor. It does work fine but mine only had a Q of about 5 (0.8 MHz 
bandwidth at 4 MHz). I think it could be scaled to 160 Meters (with a 
300-400 KHz bandwidth) but it is not for use at high power (over 50 Watts).

I'd recommend that you design a single three pole band filter, perhaps like 
the so called Cohn filter, and place a 3 section capacitor across the 
sections to tune across the 160-meter band. Since I presume you only need a 
single band, you could design a higher Q type than I did thus 
simplifying  the design. It's not a simple job but the performance can be 
quite impressive.

73,

Joe, W1JR