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Re: (no subject)



----- Original Message -----
From: "Clint Bradford" <clintbrad4d@earthlink.net>
To: <amsat-bb@amsat.org>
Sent: Tuesday, February 12, 2008 7:24 PM
Subject: [amsat-bb] (no subject)

> Is everyone satisfied with this exam question?
>
> E2A08 What is the primary reason for satellite users to limit their
> transmit ERP?
> A. For RF exposure safety
> B. Because the satellite transmitter output power is limited
> C. To avoid limiting the signal of the other users
> D. To avoid interfering with terrestrial QSOs
>
> I just find its Answer B "incomplete" ...
>
>
> Clint Bradford, K6LCS / KAF3359
> 909-241-7666

Hi Clint, K6LCS

In my opinion question B is not incomplete but it is useful for an exam
because it is deceitful to decide in favour of answere C or not and infact:

The primary reason for satellite users to limit their transmit EIRP is not
because the satellite transmitter output power is limited (B) but only to
avoid limiting the signal for the other users (C)

Lets work an example considering a tipical linear transponder for CW and
SSB like those used for many OSCAR satellites.

AGC threshold   = -77 dBm
AGC dinamic range = 50 dB
Output level above the AGC threshold  = + 30 dBm = 1 watt

Now suppose that you are the only user with an EIRP of 1000 watt arriving
at the satellite receiver with a signal  level of just - 77 dBm i.e. the
signal level at wich the AGC start to be operational.

In this condition the transponder output power will be 1 watt only for you.

Now suppose that I come up with you at the same time arriving at the
satellite receiver with the same level of - 77 dBm  and we both send a
stady carrier

In this condition and since the transponder is linear you will get 0.5 watt
and I also will get 0.5 watt i.e. 1 watt in total because the total
available power from the satellite linear transponder will be splitted in
equal parts between you and me.

Now suppose that you only increase your EIRP by 10 dB i.e. you transmit
with 10.000 watt EIRP

In this condition your signal level at the satellite receiver will be
- 67 dBm and so you will activate the AGC going into it's dinamic range
decreasing the satellite sensitivity by 10 dB for me.

Since I cannot increase 10 time my EIRP my signal level will be now barely
audible into the noise but you will get about the total power from the
satellite transponder i.e. about 1 watt only for you and a few mW only for
me

In any case your increase of EIRP x 10 time in power has not created the
condition to increase the total transponder output power wich stand the same
1 watt but you have only reduced the satellite receiver sensitivity by 10 db
for me and for the other users.

For the above mentioned reason the answere C ( To avoid limiting the signal
of the other users ) stand correct.

73" de

i8CVS Domenico


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