[TenTec] Preselector for SDR

[Jim Brown]

On Fri,9/16/2016 9:28 AM, Rich Jones via TenTec wrote:
> QEX magazine May/June 2008 M0MWA presents a computer controlled turnable bandpass filter (preselector)

There is an important difference between a pre-selector and a bandpass 
filter, at least for the application that Rick has raised -- it's 
bandwidth.  The issue that he has raised, and it can be a VERY big deal 
in some parts of the world, is that there are high power broadcast 
stations either within or quite close to some ham bands. This condition 
exists in EU and AS on 40M, and in various parts of the world on 20M.  
Bandpass filters are, in general, designed to have very low SWR 
throughout the band of interest, and to reject the adjacent ham bands. 
Their alignment is fixed (adjusted on the bench with a swept analyzer, 
preferably a VNWA so that it also displays return loss). Their skirts 
are rarely very steep, and I've seen none that are steep enough to 
reject those strong BC stations adjacent to 20M. The BC stations on 40M 
are INSIDE the passband of 40M  bandpass filters.

To work in these conditions, the filter must have very high Q, and must 
be tunable, and there must be sections for each band that can be 
switched as needed. THIS is what we generally call a pre-selector.

73, Jim K9YC