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Re: Preamps, NF and Bandwith, the dilemma..



You will not find a wideband LNA with nearly the same NF capability as a
narrowband LNA.
What makes an LNA, an LNA, is the impedance matching in the input stage.
The transistor is matched from 50 ohms to the (conjugate) of the optimum NF
impedance, which is often very far from 50 ohms.
A broadband LNA has to make a broadband match of a variable (with
frequency) and often high VSWR (with respect to 50 ohms).  You cannot do
that without significant compromise of the match and/or loss in the
matching network--either one adds up to a higher noise figure.
Just like you don't see anyone (successfully) selling broadband-matched
3-30MHz HF antennas.  They're either matched and lossy--or just lossy.
Go for the real thing--everyone wins!


At 12:18 AM 5/18/01 +0200, Jens H. Jensen wrote:
>Hi
>
>I would like some advice from anyone here experienced in weaksignal 
>satellite work.
>
>I'm about to buy or build this all-important preamp, but there is a choice 
>to be made:
>
>Narrowband or Wideband.
>
>I would like to use my wideband receiver, an AR5000, as the downlink from 
>AO-40.
>But then again, this receiver is capable of a lot more than that (10Khz to 
>2.6Ghz), so the thought springs to mind that i might buy a wideband preamp, 
>to be able to listen to other interesting signals above 30Mhz.
>
>My question:
>
>How much, if any, degradation is my reception of 2401Mhz likely to suffer, 
>if i choose this wideband preamp, rather than a narrowband one.
>This of course assumes that specs of the two candidates are roughly the 
>same, that means NF and Gain.
>
>I know that local conditions are hard to guess about, but are there any 
>fundamental rules regarding this that i should be aware of?
>
>I can get a narrowband DB6NT for 248DM, but the wideband brother of it 
>would increase the utility of the RX immensely :-)
>
>73 de OZ1LRG
>
>----
>Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
>To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>
------------
Scott Townley		
nx7u@arrl.net
Gilbert, AZ  DM43
     
HF/VHF/UHF/Satellite MOBILE station
http://www.primenet.com/~nx7u

------------
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