At 6/27/2007 21:30, you wrote: >Hi Ken, > >Nothing wrong with Angle Linear stuff... I use a lot of both brands. >But Chip Angle took a long time to come around to phempt (fet) preamps >for the vhf band and I didn't like him for a long time telling me (and >everyone else) we were full of $#%(* for wanting to use phempt preamps >in the vhf range.
Well, he's (mostly) right. Around here the environmental noise temperature @ 146 MHz is ~3000 K. At that level another 150 or even 300 K contribution from a preamp isn't going to be noticed. I've noticed that rural areas appear to have much lower noise temperatures, but I haven't rigorously measured those areas. What data I've found from other sources suggest it's still at least 300 K in those areas. > Now Chip sells vhf phempt preamps... go figure. I figure he caved in to the extra sales potential ;) >For many years my friend Tom H. brought the lastest and greatest >Agilent gain and Noise Figure Instrument to the VHF and above (Dayton >Hamvention) Dinner (held each year at the Holiday Inn). Many people >brought both homebrew and commercial preamplifiers to be measured. It >was a fairly rare chance for an average person to see exactly how a >specific preamplifier performed. ...looking into a 50 + j0 source. Problem is once you put that duplexer or bandpass cavity in front of it, most of the preamp's input bandpass is now terminated in a pure reactance. Who knows what effect that off-frequency reactance is going to have on your preamp unless the noise figure measurement is done with a load-pull system, or the noise figure is measured "in situ" by placing the noise source on the RX input of the system (antenna port of duplexer - TX disabled of course!). Granted, most of the time there's no serious degradation (that you can notice just by using the system), but there's no guarantee. At least with Chip's preamps you get his guarantee of "unconditional stability". Having said all this, I use Hamtronics & Janel preamps at home & my easy-to-reach sites. For the mountaintop & other limited-access locations, only Angle Linear here. Bob NO6B
