How do I design the feed network for a large phased array antenna?
Feed Network Architecture
The feed network is often the most critical and challenging component of a phased array. For a 1024-element array with a corporate feed: the network has 10 levels of power dividers (2^10 = 1024), and the signal travels through all 10 levels. Each level adds insertion loss from the transmission lines and dividers. Typical loss: 0.2-0.5 dB per level at X-band, giving 2-5 dB total feed loss. At Ka-band (higher loss per unit length): feed loss can exceed 6-8 dB.
To manage feed loss in large arrays, the tile architecture is used: the array is divided into tiles (sub-arrays of 16-64 elements), each tile has its own short feed network (2-3 levels, low loss), and the tiles are combined digitally or through a lower-loss waveguide distribution network. This modular approach also simplifies manufacturing and testing.
For transmit arrays: the feed network must handle high RF power without arcing or excessive heating. Waveguide feed networks handle higher power than microstrip but are heavier and more expensive. Stripline offers a compromise (higher power handling than microstrip, lighter than waveguide). The power handling scales with the network's cross-sectional dimensions and the dielectric breakdown strength.
Frequently Asked Questions
How do I minimize feed losses?
Use low-loss substrates (e.g., Rogers RT5880: tan δ = 0.0009), thick substrates (lower conductor loss for microstrip), waveguide for long distribution runs, and keep the network as short as possible. Tile architecture with distributed T/R modules eliminates the long feed runs.
What about Wilkinson vs T-junction dividers?
Wilkinson dividers provide matched, isolated ports (good for receiver arrays where port isolation prevents element coupling through the feed). T-junction dividers are simpler and more compact but provide no isolation. For transmit arrays: T-junctions are often acceptable because the PA isolates the feed from element impedance variations.
How do I test the feed network?
Measure the S-parameters of the complete feed network: S11 (input match), S1n (insertion loss and amplitude balance to each output port), and the phase of each output relative to the input. A vector network analyzer with a multiport test set can measure all ports simultaneously.