At 2/13/2007 06:28, you wrote: > > Your math looks sound, Jeff, except I thought 1/2" Heliax > > wasn't quite so > > lossy @ 450 MHz (~1.5 dB/100 ft IIRC?). > > > >Scenario 1: Preamp = 0.5 dB NF, antenna feeding 200' of > > 1/2" Heliax (3 > > >dB loss @ 450 MHz @ 1:1 VSWR) > >1.5 dB per 100 ft versus 3 dB for 200'. I think we're in agreement on this >one... :-) > > > Realistically > > though, not many > > people connect the input of their preamp directly to 200 ft. of > > line. > >When I wrote up the scenario I was thinking of a typical weak-signal home >station. Joe Ham's weak signal station at home could easily have 3 dB of >feedline loss. Joe Ham isn't running inch and five-eigths up their tower. >30 feet or so in the house/shack, maybe 100' buried between the house and >the tower, 70 feet up the tower, probably not an unreasonable scenario.
If "Joe Ham" were serious about weak signal reception, he'd mount the preamp at the antenna. Otherwise, 3 dB is already a lot of loss to tolerate for weak signal work; what's another dB? >Without knowing, or having a means of measuring, return loss of the preamp, >one might assume that the best preamp for their weak signal station is the >one with the lowest advertised noise figure, and I think the math shows that >is not always the case. Which is why I originally asked the question - is >it really fair to judge the merits of two preamps based soley on noise >figure without knowing how good of a match they present? My answer would be >no. OK, but I submit that the input match is only of secondary concern. Drop the feedline loss to 1 dB & the extra loss becomes only 0.5 dB. > > Usually there's a filter ahead of it that has loss & > > can be tuned to > > somewhat tune out the mismatch. > >For a repeater installation, maybe, but not for weak signal work where you >typically don't have anything between the antenna and preamp except for >feedline to avoid any additional losses that would directly impact the NF. Again, for any halfway-serious weak signal work, either your feedline loss will be < 1 dB or your preamp will be at the antenna. >I'd like to get my hands on a modern NF meter and, combined with VNA >measurements, compare a few off-the-shelf preamps. One of these days... > > --- Jeff Only 94 days 'till Dayton... Bob NO6B
