What is a snake feed?
Figure 1:Principle of a serpentine feed or snake feed
A serpentine feed (or snake feed) exists when the lines of a linear phased array antenna are fed serially via detour lines. The beam steering in one dimension has been implemented by changing the transmitter's frequency.
The ratio of the distance between the radiator groups D to the length of the detour line L acts as value for a slewing of the antenna pattern by the angle ±θ1. The detour line between the radiator groups causes, depending on its length, a phase shift φ of:
|φ =||2 πL||(1)|
Where λh is the wavelength in the waveguide. For steering by angle ±θ1, a change in the wavelength of the transmitted frequency is required by:
|Δλ = 2 λ0||D||sin θ1||(2)|
with λ0 being the wavelength at the center frequency of the transmitter.
The detour lines usually have a length that should be longer than the multiple of the wavelength. This extended length is necessary because a slighter frequency difference causes the same phase shift for steering the direction of the pattern then. To achieve steering angles of up to ± 45°, the antenna's bandwidth would have to be about 30% for a ratio of D/L=5, which is hardly feasible. With the ratio D/L=20, 7% is sufficient.
However, this frequency change also affects the phase shift within each antenna row. Thus, the resulting antenna pattern steers not only in elevation angle but also in azimuth. The steering plane is therefore oblique in the room. Older radars (Russian naval radars mostly) rotated the entire antenna to compensate for this azimuth shift. American radars of similar design have corrected this lateral angle error by software. The main disadvantage of this type of phased array antenna is the limitation to narrowband transmit signals. For wideband wave forms, parts of the transmit signal would be steered in another direction.
Picture gallery for serpentine feeding
MR-750 “Top Plate”
with antenna rotated in the opposite direction to correct twisted azimuths,
(© 2015 Jörg Waterstraat www.kriegsschiffe.net)
Figure 3: AN/SPS-48E with software correction of the azimuth angle.
- Merrill Skolnik: Introduction to Radar Systems McGraw-Hill Education, 2002, ISBN 978-0072881387, P. 582ff