Reflective array antenna
In
Reflective array antennas usually have a number of identical driven elements, fed
The driven elements are fed by a network of transmission lines, which divide the power from the RF source equally between the elements. This often has the circuit geometry of a tree structure.
Basic concepts
Radio signals
When a radio signal passes a conductor, it induces an electrical current in it. Since the radio signal fills space, and the conductor has a finite size, the induced currents add up or cancel out as they move along the conductor. A basic goal of antenna design is to make the currents add up to a maximum at the point where the energy is tapped off. To do this, the antenna elements are sized in relation to the wavelength of the radio signal, with the aim of setting up standing waves of current that are maximized at the feed point.
This means that an antenna designed to receive a particular wavelength has a natural size. To improve reception, one cannot simply make the antenna larger; this will improve the amount of signal intercepted by the antenna, which is largely a function of area, but will lower the efficiency of the reception (at a given wavelength). Thus, in order to improve reception, antenna designers often use multiple elements, combining them together so their signals add up. These are known as
Array phasing
In order for the signals to add together, they need to arrive
If the antenna is properly aligned with the signal, at any given instant in time, all of the elements in an array will receive the same signal and be in-phase. However, the output from each element has to be gathered up at a single feed point, and as the signals travel across the antenna to that point, their phase is changing. In a two-element array this is not a problem because the feed point can be placed between them; any phase shift taking place in the transmission lines is equal for both elements. However, if one extends this to a four-element array, this approach no longer works, as the signal from the outer pair has to travel further and will thus be at a different phase than the inner pair when it reaches the center. To ensure that they all arrive with the same phase, it is common to see additional transmission wire inserted in the signal path, or for the transmission line to be crossed over to reverse the phase if the difference is greater than 1⁄2 a wavelength.
Reflectors
The gain can be further improved through the addition of a reflector. Generally any conductor in a flat sheet will act in a mirror-like fashion for radio signals, but this also holds true for non-continuous surfaces as long as the gaps between the conductors are less than about 1⁄10 of the target wavelength.[2] This means that wire mesh or even parallel wires or metal bars can be used, which is especially useful both for reducing the total amount of material as well as reducing wind loads.
Due to the change in signal propagation direction on reflection, the signal undergoes a reversal of phase. In order for the reflector to add to the output signal, it has to reach the elements in-phase. Generally this would require the reflector to be placed at 1⁄4 of a wavelength behind the elements, and this can be seen in many common reflector arrays like television antennas. However, there are a number of factors that can change this distance, and actual reflector positioning varies.
Reflectors also have the advantage of reducing the signal received from the back of the antenna. Signals received from the rear and re-broadcast from the reflector have not undergone a change of phase, and do not add to the signal from the front. This greatly improves the
Gain limits
As more elements are added to an array, the
The gain of practical array antennas is limited to about 25–30 dB. Two half wave elements spaced a half wave apart and a quarter wave from a reflecting screen have been used as a standard gain antenna with about 9.8 dBi at its design frequency.[4] Common 4-bay television antennas have gains around 10 to 12 dB,[5] and 8-bay designs might increase this to 12 to 16 dB.[6] The 32-element SCR-270 had a gain around 19.8 dB.[7] Some very large reflective arrays have been constructed, notably the Soviet Duga radars which are hundreds of meters across and contain hundreds of elements. Active array antennas, in which groups of elements are driven by separate RF amplifiers, can have much higher gain, but are prohibitively expensive.
Since the 1980s, versions for use at microwave frequencies have been made with patch antenna elements mounted in front of a metal surface.[8]
Radiation pattern and beam steering
When driven in phase, the
The main lobe of the antenna can be steered electronically within a limited angle by
Another option for steering the beam is mounting the entire antenna structure on a pivoting platform and rotating it mechanically.
See also
- Mars Cube One (2018 spacecraft design with Reflective array antenna)
References
- ISBN 1-329-66770-0.
- ^ "The Effects of Earth's Upper Atmosphere on Radio Signals". NASA.
- S2CID 42085218.
- ^ "Standard Gain Antennas".
- ^ "ULTRAtenna 60". Channel Master.
- ^ "EXTREMEtenna 80". Channel Master.
- ISBN 978-1-4832-5854-6.
- ^ Huang, john. Reflectarray antennas.
This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).