What power you need for 1296MHz-EME-QSO?
Tested by Peter Cerveny, EA6ADW (DF5JJ)
with real echo tests (5.5m-dish, power range 1W to 1000W)
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Many echo tests with calibrated AGC-voltage (S-Meter) gave following results. 
All power values are effective at feed! Power range of tests were 1W up to 1000W, antenna 5.5m-dish,  0.30dB-LNA, 250Hz-bandwith. 

Gain      Dish                                     ERP           Ptx  ---  for 3dB-echo in apogee 
dBd       D (m)                                  (kW)          (W)      nf=0.3dB, (250Hz-BW) 

40.8       12     ------------------------       16           1.3 
40.0       11     ------------------------       19           1.9 
39.1       10     ------------------------       23           2.8 
38.1         9     ------------------------       29           4.5 
37.2         8     ------------------------       36           7.0 
36.1         7     ------------------------       46          11 
34.7         6     ------------------------       64          22 
34.0         5.5  ------------------------       75          30         Antenna EA6ADW(DF5JJ) 
33.1         5     ------------------------       92          45 
31.1         4     ------------------------     146        113 
28.5         3     ------------------------     266        375 
25.1         2     (4x55el)  -------------     582      1800         Below 2m-dishes the gain includes loss by blocking from feed 
18.3         1     (2x26el)  -------------   2786    41000 

examples: 
5.5m-dish:  own echo (3dB)  with 30W = 75Werp 
5.5m-dish:  QSO with  2m-dish,  Gain 34dB - 25dB = 9dB , consequently Ptx=30W+9dB=240W for both stations 
2.0m-dish: own echos with 1800W!
For QSO´s  of  yagi-stations with circular-stations you have to add 3dB power for same signal strenght. EA6ADW is able to run linear and also circular mode. 
In further echo tests below of   0 dB S/N EA6ADW got following results: 
measured down to 10 Watt, 
PWR     measured                             definition of quality                         report 
30W    3.0 dB S+N/N =   0dB S/N good readable for random QSOs    "O" 
20W    2.2 dB S+N/N =  -2dB S/N good readable for sked QSOs        "M" 
15W    1.8 dB S+N/N =  -3dB S/N readable for sked                              "M" 
10W    1.3 dB S+N/N =  -5dB S/N TMO readable, callsign difficult    "T-M" 
 5W                                  -8dB S/N detectable                                          "T" 
 3W                                -10dB S/N temporarily detectable                     "T" 

max.1000W  16 dB S+N/N(250Hz)  good for  SSB without headphone 
 
Minimal 15-30 kW ERP (real! at feed) are good to work in sked with EA6ADW. I also can switch to linear polarisation ( LR = linear rotatable) , which allowed QSO's with optimal quadro-yagi-groups with elevation-drive. 
Smallest worked station was 3m-dish, 70W (40kW erp) with 5dB S+N/N, also a 2x72el.yagi, 1200W with 6dB S+N/N,
smallest reception was 2m-dish with 100W (but bad RX-performance of partner, QSO NC). 
My RX-ODX are  Cassiopeia  1.0 dB,   Cycnus  1.0 dB,   Saggitarius 1.5 dB,   Moon  0.7 dB 
 
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