>>The better skirts are mucho desired. > n...@... wrote: > As you so often like to state, it all depends on the > application - in many cases it simply isn't necessary.
And in many cases it's beneficial to have better skirts... > But there are other solutions: if you want a brute-force > window filter they're out there too. I have a few 5 MHz > wide 1 dB loss UHF filters sitting on the shelf here that > I acquired at the Dayton & Ft. Tuthill hamfests. ~$40 each. Seems like a good deal... but the 5MHz band-width is only (for me) usable in some system applications. Some of those applications where the 5MHz band-width would be excessive but the expected Simrex band-width would not be... > Again, the loss is low enough that in most cases a leading > preamp simply isn't needed. Leading or Post Preamp? > >Kind of makes it look like helicals are a sin doesn't it... > Ever wonder why the MVP/MastrII & Micors are so deaf > compared to more modern RXs? Nope... > I haven't measured the loss of the UHF helical assembly, but > the VHFHB front-end helicals have ~6 dB of loss. In all > those radios, their own helicals effectively are all the > post-preamp filtering you'll ever need. No it isn't... if you sweep the front end of Micor you'll find it's actually fairly wide. I seem to remember sweeping some GE Receivers and their front ends were relatively wide on the order of at least a few MHz. From Memory I seem to remember the Micor being at least 4MHz wide. > > > The GLB preselector preamp has 4 helical stages of unknown > > > (unknown to me) coupling. > > > >Depends on the Pre-selector Model and age of the box ... > >I have GLB units here with two pre-device stages and three > >post-device stages. And I have versions with a more traditional > >helical design and others with more of a lumped parts layout. > > > >A 2-Meter version I have lots of pictures of has 1 stage of > >pre-selection and four trailing stages. The active device is > >an MRF-901. > OK. MRF-901 NF @ 2M is ~1 dB, so maybe 2-3 dB NF for the unit. And that jives with my informal recorded notes for the 224 MHz GLB Pre-selector with a dual gate Mosfet. > Not bad for VHFHB, but having only 1 little resonator in front > of the active device doesn't offer it much OOB protection. > Better put a (gasp) PASS CAVITY in > front of it! ;) To quote someone who recently wrote: "As you so often like to state, it all depends on the application - in many cases it simply isn't necessary." > >However, > >In more than a few real world situations you might really > >need the filter pre-selection a lot more than the most > >optimum NF. A practical trade of pre-selection for a slightly > >higher noise figure can and does sometimes make the difference > >in a usable radio system. > OK fine. But again, we DON'T KNOW the noise figure for the > device. It's not mandatory to know the NF for every situation, only helpful for those specific situations where making a logical assumption is not allowed. > Furthermore, since the filtering distribution varies with > the model, it's very difficult to predict the dynamic range > characteristics of the unit. True along with the different active devices. > Fun to play with? Yes. Can solve some IMD/overload > problems? Certainly. But not a tool for any seriously > engineered RF system. > Bob NO6B Really depends a lot on whose money you're spending. I've seen a lot of seriously engineered RF systems that don't work very well out there in the real world. s.
