EVLA Front-End CDR

1
EVLA Front-End CDR

• Octave Bandwidth OMT Development in the 1-8 GHz
  Range

2
Frequency Ranges

• L 1- 2 GHz 21 (Octave)
• S 2 4 GHz 21 (Octave)
• C 4 8 GHz 21 (Octave)
• X 8 12 GHz 1.51
• Ku 12 18 GHz 1.51
• K 18 26 GHz 1.441
• Ka 26 40 GHz 1.541
• Q 40 50 GHz 1.251

3
Why Quad-Ridge?

• Septum, Boifot types 1.51 maximum
• cf. Q-R horns 2-18 GHz
• Low impedance structure

4
Good Mode
fco 0.68 Z 50 W
fco .91 Z 500W
fco .74
5
Bad Mode
fco .69
fco .88
fco 1.28
6
fco vs. distance
7
Quad-Ridge

8
OMT Components
thermal gap
mode suppression

Window
Circular-to-Square
Square-to-QR
QR-to-coax
9
Sliding Load

10
(No Transcript)
11
Return Loss
12
vs. ERA OMT
13
Isolation
14
90-degree hybrid

Hybrid
In
Coup.
RCP
Iso.
Thru
Nothing
15
Mis-matched90-degree hybrid

Hybrid
In
Coup.
Less RCP
Gr
Gx
Gy
Gl
Iso.
Thru
Some RCP
16
Math
17
Axial Ratio (AmplitudesVoltages)
18
For alt 1 dB

• (With some assumptions) we can specify that the
  return losses of a pair of ports must sum to
  better than 34 dB
• This sort of tradeoff applies to any polarizer.
• With perfect amplitude phase, the sum would be
  25 dB.

19
Next to do

• Reduce mass of OMT.
• Improve Cooling.
• Better window material.
• Investigate manufacturability.

20
Conclusions

• Prototypes have been built
• Return loss is acceptable across band
• Isolation is good across band
• Suck-out suppression is effective