No doubt, making a front end better can be done. I don't remember saying it couldn't be done. Maybe I am way off base here. And, perhaps I should have said, it is difficult to increase the overall performance of a given receiver. Not impossible....just difficult.
I don't disagree for a minute your and Domenico's comments. On paper, it works great. In practice is often something totally different. Your final comment, "caveat emptor" is really what it is all about. If it works, do it. The end result, no matter what, is to somehow dig out enough signal above the noise floor so that your demodulator can have something to chew on. If overdriving or underdriving or whatever works to that end, nothing else matters.
Adding a preamp in my case is a total disaster. I could probably put a diode in my coax line and then charge batteries with the resulting DC.
The only thing I would add to your comments is that to amplify signals before an existing front, with a given antenna and feed line, can't help but have a negative impact somewhere else. Maybe that negative is no big deal. The less stages of anything before demodulation, the better chances you have at a super receive chain. And with another stage of amplification or a preamp, certainly you just have to hope that no strong signal will come up anywhere nearby in frequency or distance to what your receiver is set to. What about dynamic range....wouldn't a preamp toss that into the toilet? Again, maybe it is no big deal to the end you are after.
My advice about advice is this: Listen to others, study and learn about it yourself, and then go do whatever you want. In the world of radio, no amount of paperwork, planning and talking beats trying it to see if it works. I have several preamps of all kinds and I use them whenever possible, but I realize their limitations.
Finally, to those who sent some perty nasty comments via private mail re this thread...Gee...I am sorry. Really I am. I had no idea my comments were so flammable. I'm betting that stealing your wife or girlfriend (boyfriend?) would not be as traumatic? What can I say? I didn't mean to cause trouble. To me, this is a hobby. A fun pastime. I promise, I won't make comments again. I rarely do make comments on this board...because every time I do, there are those who take it so seriously. Geezzooo!
73!
Gary, N7BRJ/DA1BRJ
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Edward Cole Sent: Wednesday, September 19, 2007 8:19 AM To: i8cvs; Gary Memory; 'Amsat-BB' Subject: [amsat-bb] Re: New Preamp
Thanks Domenico,
I was about to insert my comments to Gary.
I would challenge his belief that manufacturers achieve better NF, as the evidence shows the contrary. If you can show me a commercial radio with 0.5 dBNF on the 150-MHz band, then I would love to know it. Almost all commercial two-way radios have a sensitivity of about 0.15 to 0.25 uV at 15-KHz BW. This will result in about MDS of > -124 dBm and a NF well over 3-dB. Commercial equipment is designed for immunity from high RF/noise urban environments and that trades off noise figure in the process (commercial radios are designed for strong signals - hams* are the crazy weak-signal nuts!) *and a few weird radio astronomers, NASA engineers, ....
My 2m eme station has a sensitivity of -147.5 dBm with 2.2 KHz SSB BW; that is a receiver temp= 58K or NF=0.79 dB. This is assuming 0.5 dB loss ahead of the preamp. A good ham radio VHF satellite receiver will be about 100K or NF=1.2 dB with a sensitivity of -145 dBm at 2.2 KHz SSB BW.
Now if you add sky noise, industrial noise, and antenna noise, the system sensitivity (Te) will suffer: Te = Tr+Tsky+Tant+Tindustrial e.g. using Tsky = 210K, Tant = 45K (very good low sidelobe eme class antenna), and no man-made noise (my situation): Te = 313K, and Pn = -140 dBm (note no antenna gain is included in this number) so my environment cost me about 7.5 dB in sensitivity. Obviously if you have several hundred degrees of industrial noise then things do get worse which will minimize the advantage of the low NF preamp. Here the use of well engineered filters may help. Preamps with better strong signal characteristics (though a bit higher NF) may also help.
Tradeoffs of NF vs gain are made in low noise amplifiers (preamps) usually in favor of low NF. As long as there is sufficient gain to overcome the higher NF of the following receiver the overall system will benefit. Usually this means a min of about 16 dB gain. This will lower the noise contribution of the following circuits by 1/40.
For the majority, a good low-noise preamp mounted at the antenna will result in significant increase in hearing ability. caveat emptor
73 Ed
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