At 1/11/2009 21:07, you wrote: > > My guess is that the GLB preselector consists of the filter > > followed by the preamp. Also because of the number of sections > > of filtering & the overall small size of the unit, the actual > > helical filter is probably quite lossy. > >The GLB helical layout is one helical in front of the preamp >and 4 trailing helical sections.
OK, so the NF won't be too bad. But you certainly don't get much filtering in front of that preamp. It could be easily overloaded by 152 MHz (for the 2 meter model) or 452 MHz (for the 440 model) paging. I definitely wouldn't recommend it for use on a Mastr II or Micor, since the helicals following the preamp are unnecessary ahead of the bulletproof front-end those radios have. >Max GLB stated gain is/was reported as 10-11dB Again, not quite enough for a UHF Mastr II/MVP. I tried to steal 3 dB from the output of a 15 dB gain GaAsFET preamp to feed a 2nd RX, & I can detect a slight degradation in RX performance as a result. > > If you need preselection and low noise figure, I'd look here: > > <http://anglelinear.com/custom/custom.html> > >Compare the skirt displays (pictures) for both the Angle >Network analyzer plot 440 MHz, 20 MHz spread unit and the GLB >P-450 unit... the GLB skirts appear to be notably sharper than >the Angle device. ...but the preamp within the GLB sees a much wider response, while the plots for the Angle Linear filter actually depict the frequency response ahead of the preamp. >A key question... what about selective circuits after the >preamplifier? Nothing..? direct preamp output to the receiver? Why not? Any decent RX won't need post-preamp RF filtering because it already has it. > > Pass cavities are typically 0.5 to 1 dB loss. The Angle Linear > > cans are even lower - I measured 0.25 dB on one of his 220 MHz > > filters. > > Bob NO6B > >The trade is insertion loss for selectivity... When you have >a lot of gain... tis good to trade a fair amount of it to run >at least modestly tight ship. Overengineering can be a bad thing, if it means less effective sensitivity & coverage. Bob NO6B
