> > Kind of makes it look like helicals are a sin doesn't it... > Hamtronics thinks it's a sin - that's why their receivers > have some of the best sensitivity numbers of all of the stuff > we commonly build repeater from.
Their receivers are red hot... and I like that very much because I do a lot of pre-selection and rejection before the receiver. > However, their receivers get hammered in high RF density > locations, and the result is the need of some added > filtering ahead of it. My to my surprise I haven't had any problems with their current synthesized receivers being affected by high RF Levels. Maybe I'm just really lucky in this regard... and I've got a lot of RF around the area including the obligatory 1/4kw Paging transmitters and an FM Broadcast Station very close. > This is where the GLB can and does provide an improvement. > I speak with experience here. I have used many Hamtronics > receivers and GLB preselector/preamps (that I bought new) > over the years. Preaching to the Choir again... A GLB can make all the difference to some receivers... especially those like the Midland 13-509 rigs made into repeaters. I don't have any GLB's in front of my newer Hamtronics Receivers yet. > > 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. And the coupling for this model is a slightly > > different method than some (not all) of the other models I > > have looked at. > Are you sure that's not 4 section preceding the device > and 1 following? >From memory I seem to remember there are less stages in front versus the back end, which is the way my 220 version were built. I'll go back and review the pictures for the proper numbers. The 224 MHz version I can just look at because I have one apart here in the office. > > 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. > No argument here.... If the site noise figure is worse > than the NF of the GLB, adding it won't hurt your actual > effective receiver sensitivity, and the added filtering > can be a real boost. I hope others pick up on the above statement... it's a real important item to consider. > However, I'm blessed with sites that allow me to realize > most of the sensitivity from a good preamp - with not much > ahead of it (now that I don't do much RF linking). > Kevin You are lucky guy... cheers, s.
