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Re: Question from a newcomer


My suggestions are included in the text below, right after each question.

hasan schiers, N0AN

or use amsat.org or arrl.net remailer
----- Original Message -----
From: "EA4BFK - Alex Fernandez" <ea4bfk@retemail.es>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, November 21, 2001 4:09 PM
Subject: [amsat-bb] Question from a newcomer

> Hello
> This is to ask for some help in using the Downlink calculator from Gene
> .
> The questions are the followings:
> 1.- How  can I calculate the NF for my recceiver knowing that is as TM255
> with the following specs:
>      Sensitivity 0.13 uV in SSB at 10 db (S+N/N)? Selectivity 2.1 Khz

Don't worry about the NF for the receiver...it contributes very little to
the overall sensitivity of your satellite station. The sensitivity of your
system is set out at the first amplifier after the antenna. If you have a
preamp, use it's noise figure and gain.

At the receiver in the shack, I would use somewhere between 8 and 12 dB for
noise figure. Try both values and you will see, it doesn't change much,
because the major contributor to receive sensitivity is at the first RF amp
at the antenna. This can be a separate preamp, or the downconverter itself.
Don't forget to include the coax cable and connector loss between the
antenna and the preamp...at least a couple of tenths of a of a dB at 2.4

> 2.- What Sky N temperature and Ant N Temperature I need to put knowing I'm
> going to use a 60 cm offset Dish with a helix of 5.25 turns?

Just use the program defaults, they are in the ballpark, and you can't
"guess" them any better, nor does it sound like you have any way to measure
them...just don't change them, or you will get yourself totally confused.

Basically, enter the following:

Antenna gain (make sure to specify linear or circular with L or C)
Coax/Connector loss between antenna and first RF Amp
NF of Preamp or DownConverter (DC)
Gain of Preamp or DC
Coax Loss from DC to Radio
NF of Radio (8 to 12 dB)
Squint Angle (0 at apogee), investigate other cases by changing only this
Distance (55,000 is a good far value, but play with other values based on
your tracking software)

If there are extra blocks between your antenna and the radio, put in 0 NF
and 0 Gain.

Have fun

> The balance of the data is clear for me.
> Thanks to every body
> Alex - EA4BFK
> ea4bfk@eresmas.net
> ----
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