I have another preamp that uses an old MRF-904. Have you characterized that transistor at 1.8MHz? Is it worth replacing with a 2N5109?
Art ᐧ On Tue, Sep 22, 2015 at 3:16 PM, Richard (Rick) Karlquist < [email protected]> wrote: > On 9/22/2015 11:40 AM, Art Snapper wrote: > > The KD9SV pHEMT FET preamp is a nice addition to the shack. I was able to >> do an in/out test this morning, and it made a big improvement. This was >> the >> first time since installation during the summer, that the atmospheric >> noise >> was low enough to give it a decent test. >> >> de Art NK8X >> S.W. Michigan >> >> Just an FYI: > > After seeing this posting, I looked at various KD9SV products. > The pHEMT preamp is advertised as "Featuring components with rated noise > figures as low as 0.4 dB". Yes, but pHEMT's are not specified > at 1.8 MHz, and for good reason. They have a boatload of 1/f noise. > The NF is many dB higher at 1.8 MHz (how many dB? well it's not > specified so you have to test each one.) But it is bad. Other > KD9SV use MOSFET's like BF981. These also have high 1/f noise > at 1.8 MHz. These 1/f issues are well known in the industry, > although maybe not to the designer. It isn't clear that the > NF was actually measured. > > I spent a lot of time characterizing pHEMT's and MOSFET's in > hopes of making a low noise preamp for 160 meters. They are > non-starters, IMHO. I specifically tested the BF981. > You could do much better with bipolars like 2N5109 or JFET's. > > Rick N6RK > > > > > > _________________ > Topband Reflector Archives - http://www.contesting.com/_topband > _________________ Topband Reflector Archives - http://www.contesting.com/_topband
