----- 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