High-Frequency (K-Band) Antenna Feed Network with Waveguide

I designed these feeder network for high-frequency (K-Band) waveguide fed beam forming networks for microstrip antenna . At higher frequencies and for high power application microstrip feed line is not a suitable option due to low power handling capability and high feeder losses. This was designed at 20.9 GHz to feed an array of 2 x 2 element of microstrip patch.

Basic element of this design is waveguide. I selected waveguide WR42 which has operational bandwidth from 18 GHz to 26 GHz. I realized whole network feed network by dividing it into subsection

• Realization of E-plane Tee
• Realization of H-plane Tee
• 90 degree bend
• Waveguide to microstrip patch transition network
• 1:2 way power divider
• 1:4 way flower type power divider

E-Plane and H-Plane Power Divider

In a typical application in RF or microwave systems, a divider/combiner used as a power divider to divides an input signal into a plurality of equiphase, equi-amplitude signal outputs, when used as a power combiner, combines the outputs of several powers. It is desired that the power divider/combiner exhibits a low overall power loss, is symmetrical in configuration to avoid phase and amplitude imbalances, and provides sufficient isolation between its output ports (or input ports). Furthermore, the device must provide sufficient impedance matching between its input and output ports

For 50Ω input and output, the transmission line impedance is about 70Ω.Usually, at high frequencies, the transmission lines must be separated for some distances in order to reduce parasitic coupling. Designs using matching posts and irises had limited bandwidths (~12%), but a power divider with a thin absorbing septum between two of the waveguides achieved a return loss > 30 dB across the whole of band.

Main designing task in realization of wave guide Tee, is impedance matching at junction. Due to sudden discontinuity at junction higher order modes are generated in waveguide. Generation of higher order modes is responsible for storing energy near junction. This increases reactive part of impedance. To cancel this reactive part of impedance I designed a septum that load waveguide with opposite reactive part as it is at junction. User iris in side arms to transform resistive part of side arm waveguide to main waveguide.   Verified my design using Finite element method and optimized for best performance result.

As shown here H-Plane Tee is realized using a septum at center of Tee. Length and position of septum was optimized  for better return loss and power division. Transformer section, to match input and output port impedances, was designed by narrowing the broader wall of input port at junction.

Similarly an E-Plane tee was designed at k-band with center frequency 20.9 GHz. E-plane power divider design is somewhat different from H-plane power divider regarding to septum and transformer design.

Simulation result at C-Band

Waveguide to microstrip patch transition network

The desire to reduce the amount of power loss, waveguide Power divider required in microstrip antenna feed system. Normally Waveguide has large output impedance, (greater than impedance of free space) so it is required to low down the Waveguide impedance to match the impedance of coaxial cable (50 ohms). The standing wave that result from a mismatch causes a power loss, a reduction in power handling capability and an increase in frequency sensitivity. The waveguide has the advantage of being rectangular which greatly facilitates the attachment of the coaxial cable connectors.

As shown here simulated return loss performance is better than –20 dB over the desired band. Insertion loss is less than 0.025 dB. Resonance is occurring exactly at 20.9 GHz. Insertion loss is less than 0.025 dB. Its 10 dB bandwidth is around 18.18%. Structure was fabricated using CNC milling machine at Fabricated Hardware is shown in Figure-3. It was fabricated in 3 pieces.  This hardware was designed to feed a microstrip patch antenna directly without using any separate connector. It has a pin of diameter 1.3mm (50 ohm coaxial line) at output port that has to be soldered at patch. Due to connector connection at output port it is difficult to test this hardware individually on VNA for S-parameters measurement. It can test along with patch antenna.

Measured Results
Divider Type	Return Loss (dB) (Over the Band)	Insertion Loss (dB)	Inter-Port Isolation (dB)	Power Division (dB)	Amplitude Imbalance (dB)	Phase Imbalance (Deg.)
1:2	Better than         -24 dB	-0.10	-25 .2	-3.10	± 0.12	± 2.0
1:4	Better than         -20 dB	-0.20	-23.3	-6.20	± 0.16	± 2.0
1:8	Better than         -20 dB	-0.35	-23.0	-9.35	± 0.19	± 3.0

Transition

S-parameters				Bandwidth (15 dB)
S11	S12	S21	S22
-27.5	-0.08	-0.08	-32.184	11%