> n...@... wrote: > >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.
There's more to the story... Consider a hopefully linear preamp seeing high level signals approaching and passing an example 1dB compression point. As the preamplifier operates above/out it's linear range (IMD) trouble can quickly appear. No trailing close-in preamp filtering means the buckshot (IMD) gets thrown directly into/at the receiver front end. The GLB trailing helicals by reasonable engineering do fairly well address the (excessive high-level non-linear operation above the classic preamp 1dB compression point) adjacent close-in buck-shot potential (generation). The bare front end of the GE Master II, MVP and MSR-2000 receivers are noticeably wider relative to the GLB trailing helical section. So high level and overloaded non-linear preamp compression should be better dealt with by the tighter GLB Preamp versus dumping a bare preamp output straight into one of the above mentioned or similar type receiver models. Most people tend to stare only at the front protection parked in front of a preamp. In an application where you're having to deal with unwanted high level signals blowing the the preamp into non linear operation... the back end behind the active really starts to matter. > >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. Park a tight selective trailing filter network or series cavities with cranked up insertion loss behind the Gasfet example above and try again. Blocking the close in unwanted adjacent stuff from the preamp output should help. > > > 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. Yep, but the notable difference is the trailing GLB 4 section helicals versus nada (zero) trailing the Angle Linear Preamp. Depends on what camp you align with... but I'm trying to emphasize what happens after the preamp very often seriously effects what people perceive to be lack of filtering out in front. > >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. In the example case... the receivers greater band-width front end might not be enough filtering of the right type. > > > 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. > Over engineering can be a bad thing, if it means less > effective sensitivity & coverage. > Bob NO6B Smart engineering should mean the best possible effective sensitivity, which is a good thing. cheers, s.
