At 10/17/2007 17:49, you wrote: >It's better to target a specific usable receiver sensitivity number >that you'd like to see (after knowing what the site noise-floor looks >like) than trying to work backward into the design from the pre-amp, >but with that said... pre-amps that have been successfully used by >many people here include:
My comments on the ones I've used: >- The stock GE pre-amp (not much gain, but also not too "unhappy" in >high noise environments) The P1dB point on the UHF UHS preamp I measured was only +1.5 dBm. However, it's much narrower than any standalone preamp you can buy off the shelf, so it has some inherent out-of-band rejection. I measured a noise figure of 4.6 dB, which sounds a bit high but it's a LOT lower than the noise figure of a stock UHF GE MVP/Mastr II/Exec II RX. I just installed a VHF UHS preamp in my latest 2 meter portapeater & am very happy with its performance so far. >- Advanced Receiver Research (my favorite, but can be a little too >"hot" for the MASTR II receivers we use) No personal experience with this one, though some have questioned whether it's unconditionally stable (meaning it won't oscillate no matter how it's terminated on its input & output). >- Hamtronics (I don't like them, but others report good luck and >behavior from them, and they're cheaper than most) I'm using their LNY series preamp on a UHF remote receiver. It was very cheap at the time (~$30), but I had to package the board myself. Hamtronics claimed a NF of 0.8 dB but no way could I get it down below 2.2 dB, even when tuning it directly on an HP noise figure meter. The P1dB was reasonable for a MOSFET preamp: +7 dBm. Looking at their current offerings, I'd shop around as their enclosure doesn't look to be RF tight. Those clam-shell cases tend to seal poorly around the seams, resulting in so much leakage you'd be better off leaving the top off. >- Angle Linear (Chip's got some nice stuff there, and it won't be >cheap, but he'll also custom build some pretty nice setups if you >work with him and answer his questions about your setup and site. I >keep meaning to try out one of his PHEMPT pre-amps on one of our >systems to see if we find any reason to use them over the GaAsFET >ARR's... but haven't had any time to do it yet.) Chip used to give hams a discount (25%?) from the prices shown on his web page. If he still does, they're actually quite reasonable. And of course you get the guarantee of "unconditional stability". The packaging is beyond reproach, with hefty EMI feedthrus for power & N connectors for RF I/O. I think he uses enclosures from Compac RF, which are about as good as you can get. >Remember, if you don't bandpass filter before a pre-amp, it's going >to "stuff" a lot of off-channel extra signals (and noise) from other >nearby transmitters -- or even far away ones! -- into your receiver. >That off-frequency stuff, if strong enough is just going to make your >receiver overload and may actually perform WORSE than without the pre- >amplification. Many preamps are VERY wideband. One of Chip's 440 MHz units actually measured lower NF at 800 MHz than at 440 MHz, & was still quite usable @ 222 MHz. This is not to say anything is wrong with his preamps; they operate this way by design. It's up to the user to provide proper input filtering. Bob NO6B
