1. X-band high-power variable RF splitter
Hao Zha, Igor Syratchev (CERN)
18/06/2015
High Gradient work shop 2015 in Tsinghua University

2. Motivation We want to design an RF power splitter:
Rectangle waveguide based, X-band (12GHz)
With 1 input port, 2 output ports;
Arbitrary split ratio and easy to adjust this ratio;
Matched (reflection <-30dB) at any split ratio;
High-power (~100 MW) use;
Large band-width (>100 MHz);
Compact size and easy machining.

3. Previous design
‘Classical’ movable short circuits. Do not work at high RF power!
Input port 1
Output port 2
Output port 3
RF contact free short circuit piston
Input port 1
Output port 2
Output port 3
Please see more details in CERN Indico page:
Igor Syratchev, CLIC RF Structure Development Meeting. April 17, 2013

4. 1-piston system Input port 1 Output port 2 Output port 3 Input port 1
Please see more details in CERN Indico page:
Igor Syratchev, CLIC RF Structure Development Meeting. April 17, 2013
Input port 1
Output port 2
Output port 3
Input port 1
Output port 2
Output port 3

5. Network of the RF splitter
Network of the splitter includes 4 port, 1 for the piston.
The reflection phase ρ determines the splitter ratio.

6. 4-port network system
Impossible to achieve matching for symmetric 4-port networks.
Asymmetric network (contain loss material) may be not good for ultra-high-power use.
RF Circulator
Ulta-High power??
Port 1
Port 2
Port 3
Port 4: To piston
Lose port 4

7. 5-port network system
Port 1
Port 2
Port 3
5-port system : with two port connected to the short circuit piston.

8. 5-port network system
Such 5-port network is matched for any split ratio, according to the transmission line theory.
Two short circuit piston should be adjusted simultaneously. 
Port 1: input
Port 2:
output
Port 3:
Port 4 & 5:
To piston
Phase = (N+0.5)*π

9. RF polarizer
RF polarizer can accommodate two orthogonal modes in the circular waveguide.
It can emerge two short pistons → one! 
Two port will be isolated, no coupling (-50dB) from each other.
Inspired by the Dual Circular Polarizer (C. Chang, S. Tantawi, et al.) and Turnstile Orthomode Reducer (Giampaolo Pisano et al.)

10. Design of RF splitter Use WR90 waveguide.
Maximum = 48MV/m
Dimensions: 16*10 cm

11. Design of RF splitter Maximum return reflection: -38 dB Bandwidth for:
Port 2: < 180 MHz
Port 3: < 270 MHz

12. Trapped-mode
The trapped-mode will affect the power transmission as well as enhance the surface field.
Frequency is determined by waveguide width.
Gif from Web

13. Towards more compact and larger band-width
The components however create some HOMs (mainly TE20) propagating in the waveguides.
Enough distances between components are needed to suppress these HOMs.  
Change waveguide dimensions:
Waveguide width ↓: suppresses HOMs and trapped-mode
Waveguide height ↑ : reduces field

14. New waveguide dimension
a↓, b ↑
b <= 12.5 mm
Final decision:
a = 20 mm, b = 12 mm

15. Design of RF splitter (v2.0)
Maximum = 50 MV/m
Dimensions: 10*8 cm

16. Design of RF splitter (v2.0)
Maximum return reflection: -44 dB
Bandwidth for:
Port 2: 370 MHz
Port 3: 280 MHz
Trapped-mode moves further away (12.03 GHz->12.17 GHz)

17. Future Machining is undergoing.
This power splitter will be future used in CLIC X-band test stands.

18. Thanks