At 1/10/2009 14:01, you wrote: >A first glance and test of the BF981 indicates/measured a >relative noise figure value of <.6dB to .7dB, which is actually >a fair value better than I would have thought for an older >Dual Gate Mosfet circuit. Although a Gasfet would have a lower >(better) noise figure... would it really be practical to >retro-fit (modify) an original GLB Preamp for the difference >in noise figure performance. > >I don't yet plan to ramp up an MRF-901 preamp for noise figure >comparisons.... but I might.
The datasheet for the MRF-901 shows ~1.5 dB NF @ 450 MHz. Expect at least 2 dB NF out of the preamp, more if there's a preselector ahead of it. >Nothing wrong with all the Gasfet gain if you can keep a handle >on it (tame the tiger). One ham out here on the left (west >coast) gets away with series GasFet preamps separated only by >a 1dB pad. It's red hot, seems to be stable and works like >gang busters... not to mention the mountain top is not super >clean. So I have seen nearly 30dB of preamplifier layout behind >good enough generic T1500 type band pass filtering work fairly >well in actual applied operation. Nice to have all that gain >if you can get away with it... But there's no reason to use to use that much gain unless your RX is very deaf to begin with (worse than 0.5 µV for 12 dB SINAD). 20 dB gain from a theoretical, noiseless preamp would bring a 0.5 µV RX down to 0.05 µV, which is slightly less than kTB (lower than you can measure with a 50 ohm sig. gen.). I find that the 16 or so dB of gain afforded by the Angle Linear preamps is the perfect amount of gain for G.E. Mastr II/MVP/Exec II RXs, & probably the Micor as well. Bob NO6B
