[amsat-bb] Re: Thanks for all the ...

Andre sats at pe1rdw.demon.nl
Sat Jan 21 14:27:23 PST 2012

Op 21-1-2012 22:06, i8cvs schreef:
> ----- Original Message -----
> From: "Andre"<sats at pe1rdw.demon.nl>
> To:<amsat-bb at amsat.org>
> Sent: Saturday, January 21, 2012 8:12 PM
> Subject: [amsat-bb] Re: Thanks for all the ...
>> Good point, a preamp is to cancel out the cable losses, anymore and you
>> are just boosting the background noise and not improving the reception.
>> So if your cable is very short there is not point in using a preamp.
>> 73 Andre PE1RDW
> Hi Andre, PE1RDW
> It depends on the noise figure NF of your receiver.
> Lets suppose the NF of your commercial receiver is as usually in the order
> of 6 dB, then a preamplifier with a NF = 0.5 dB and 15 to 20 dB gain will
> improuve your (S+N)/N ratio even if the lenght of your coax cable between
> the antenna and receiver is very short.
> 73" de
> i8CVS Domenico
true you can add a 5,5 dB improvement that way but you are talking EME 
class LNA, about 60 usd per dB is far more then most hams are willing to 
a more reasonable priced preamp is more likely to be around 1.5 dB NF 
and would probably cost around 35 usd per dB improvement.
However you still run the risk over overloading your reciever frontend 
with strong signals because you are not only boosting the satelite by 15 
to 20 dB but also the athmospheric noise and other inband signals while 
you only needed 6 dB to overcome the NF of the reciever.
by the way my ts790 is measured by DJ5IR at 4.5 dB NF so a typical 
preamp would only improve by 3 dB at the cost of lowering my IP3 from 
-14.5 dBm to -29.5 dBm, granted not everyone would do a portable 
opperation with a ts790 but I do.
a more typical portable reciever is probably a ft817 and is measured at 
5.4 dB NF by DK2DB.


BTW I know that the real IP3 stays the same but the offending signals 
are boosted giving a virtual worsening of the IP3
73 Andre PE1RDW

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