Joe wrote: > Hello Bob, > > I agree that the GLB is a space saving device, but don't you think that > the multistage helical coil stages in the preselector with beat a single > cavity in skirts and out-of-band rejection?
Curves for the GLB are available here: http://www.repeater-builder.com/glb/glb-preselector-order-form.pdf While the skirts and OBR of the GLB might beat a single cavity, many times it isn't necessary - especially if the receiver that follows is able to cope with high out of band signals and the preamp that follows the cavity has a high overload point. The problem is, like any receiver that has several helicals in cascade before the first active stage, the loss that precedes the active stage has a majority role in the overall NF of the system that follows. It matters little what the quality of the active stage is, because the loss has already determined (for the most part) the system Noise Figure. The GLB preselector preamp has 4 helical stages of unknown (unknown to me) coupling. If they are over-coupled (and I believe they are not looking at the response curves) only a few dB of loss will occur before the active stage. If they are lightly coupled (which I believe is true) then several dB of loss occurs ahead of the active stage. Every dB of loss ahead of the first active stage ADDS to the system NF - period. This loss can NEVER be recovered no matter how good the preamp is that follows. Kevin
