[amsat-bb] AO-40 Beacon and Transponder noise calculations

[Charles Suckling <charles.suckling@xxxxxxxxxxxx>]

I have completed a first draft of a spreadsheet which calculates the level 
of the beacon above cold sky, the level of transponder noise above cold sky 
and the advantage to be gained in beacon S/N ratio (including the effect of 
transponder noise) by increasing dish size (dB relative to a 2ft (60cm) dish).

I would like to share some of the predictions, to see if they apply 
generally. They are based on measurements made at my QTH with two antennas, 
a 5ft dish and a 10ft dish. The levels measured on the 10ft dish fed into 
the spreadsheet  predicted the levels seen on the 5ft dish within 1dB for 
both beacon S/N and transponder noise, as shown below.

The main reason for doing this exercise was to quantify the improvements 
that might be expected in system performance in going to a larger downlink 
antenna. As the results show, it is always worth using as  large a dish as 
possible, although there is a diminishing return as the antenna gets 
larger. The diminishing return kicks in more rapidly for systems with a low 
noise figure, as the transponder noise starts to dominate the receiver's 
own noise more quickly.  For systems with a higher noise figure, more 
benefit is obtained with a larger dish.

The predictions are all based on data taken on 16/12/01 at MA=101, where 
the squint was 16 degrees. At lower squint angles, a larger dish will win 
you less than the improvements shown, as the transponder noise will 
dominate more quickly. The reverse is true at larger squint angles, where 
larger dishes will have a greater payoff.

Squint = 16 deg, RX NF = 0.65dB, Tsys=75K, RX BW=2.5KHz

Dish Dia 	Beacon/Sky 	Transponder/Sky 	dB ref 2ft dish
(ft) 		(dB S/N) 	   dB (S+N/N)
2 		20 			1.6			 0
3		 23.5 			3.1 			2.1
4 		26 			4.5 			3.1
5 		28 			5.8 			3.7
{5} 		{27} 			{ 6}
6 		29.5 			7.1			 4.1
8 		32.0			 9.2 			4.5
{10} 		{34.0}			 {10.9}			 4.7
{} is measured data

Squint = 16 deg, RX NF = 2dB, Tsys=200K, RX BW=2.5KHz
Dish Dia 	Beacon/Sky 	Transponder/Sky 	dB ref 2ft dish
(ft) 		(dB S/N) 	   dB (S+N/N)
2 		15.5			 0.6 			0
3		 19			 1.3			 2.8
4		 21.5 			 2.2			 4.5
5 		 23.4			 3.0			 5.6
6 		 25.0 			 3.9			 6.3
8		 27.5			 5.5			 7.2
10 		 29.4 			 7.0			 7.6

Squint = 16 deg, RX NF = 470K, Tsys=470K, RX BW=2.5KHz
Dish Dia 	Beacon/Sky 	Transponder/Sky 	dB ref 2ft dish
(ft) 		(dB S/N) 	   dB (S+N/N)
2		 11.8			 0.3 			0
3 		 15.3			 0.6 			3.2
4		 17.8 			 1.1			 5.3
5		 19.7			 1.5 			6.7
6		 21.3 			 2.1			 7.8
8 		 23.8			 3.2			 9.1
10 		 25.7 			 4.3 			10.0

73

Charlie

PS Hope the formatting of the tables worked!



----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org