Well, I have decided to buy and try a Simrex Pre-Selector,experiment with it, test it.......whatever. At the very least I will learn something from my experiences and it may end up being useful to me, anyway. I certainly appreciate the different points of view here and have learn't of alternative methods and the reasoning behind them. For me, all of it is good information :-)
73, Jack. VK4JRC Sent from my Apple iPad Tablet PC On Jul 31, 2010, at 8:53 AM, "skipp025" <skipp...@yahoo.com> wrote: > > > Re: Simrex - GLB Pre-Selector Pre-Amplifier > > > Kevin Custer <kug...@...> wrote: > > What you are missing is that ANY losses ahead of the > > first active stage add to the noise figure of the > > system - directly. > > What we have here is... failure to communicate... (a line > from a famous movie). > > Actually I was trying to high-lite the active device > Noise Figure comparison and the lower spec'd gain value > of (at least) the GasFET GLB was mostly from the insertion > loss of the Pre and Post Filtering. > > > So, while filtering exists in the GLB device, so does > > loss, and this loss is more than what is experienced > > when using a quality large diameter cavity. > > Sure... but again we are comparing a box to another box > and they are not the same device. Please allow me to paste > some of the text from a recent post (by me). > > [pasted text] > > "The Simrex (aka GLB) units are actually amplified > pre-selector assemblies, not just plain wide-band > Receive Pre-Amplifiers. > > [end of pasted text] > > > Many times Skipp you tell us there is no free lunch, > > and the same applies to the comparison of selectivity > > and loss between the GLB and a quality cavity followed > > by a good active stage. > > Correct... and a Simrex GLB Pre-Selector should really not > really be directly compared to a cavity followed by a good > active stage. The Simrex GLB box is more of a true Pre-Selector > layout and contains post amplifier filtering. To better > equate a similar layout would have you add at least one > or more cavities after the active device. And yes we should > clearly acknowledge the hopefully obvious lower loss through > a higher Q Quality Cavity. > > > Real world test. Take a Hamtronics receiver (no preamp) > > and do a basic bench sensitivity test to obtain a baseline. > > If you find something around -123 dBm your in the right > > ballpark. Now install a bi-polar GLB preselector/preamp > > in front and measure the sensitivity again - you'll > > find you have lost several dB of bench sensitivity - at > > least 3 or 4 dB. Take the same receiver and add a quality > > 1/4 bottle with a good preamp (your choice - something > > with 1.5 dB NF or less) and do the test again. Now, the > > receiver hears at -123 to -127 dBm (dependent mainly > > upon the quality of the preamp that follows) because the > > filter hasn't severely ruined the system NF ahead of the > > first active stage. Even though the GLB has gain, the > > noise figure of the design has already determined the > > sensitivity that will be realized by the receiver that > > follows. > > The primary land mine in the above comparison is the "Now > install a bi-polar GLB preselector/preamp". Remember Simrex > and GLB offer or did offer a GasFet version of their Pre- > selector box. > > Separate the above in the proper context and the focus > should be on the filtering in front of the same type of > active device. No one here should discount the higher Q > cavity will be the better spec. But again even a high Q > cavity with a same or similar active trailing device is > still not the same box as the Simrex GLB Pre-selector. > You should account for the Simrex GLB integrated post > active device filtering. > > > The GLB preselector/preamp should not be considered > > for adding basic sensitivity, > > Did anyone make that claim? > > > because it's possible (depending on how good the receiver > > is to begin with) the opposite will happen - > > You are correct. > > > however, it will protect a receiver that lacks good > > front-end filtering, like the Hamtronics. Like > > everything, the situation helps to dictate what > > equipment will give the best results. > > We agree and it's a Friday... Mark today on your calendar. > What more could anyone ask for? > > > Kevin > > cheers, > skipp > >
