Introduction.
Frequency-scanned arrays are usually used in radar systems. This is because to operate a frequency-scanned array the carrier frequency must be changed to swing the array beam. As radar systems transmit and receive on the same frequency, changing the carrier frequency of the transmitter to swing the beam is followed by a corresponding change in receiver frequency, the receiver carrier frequency being dictated by the frequency of the transmitter. Both the transmitter and receiver being parts of the same radar system. If a point-to-point communication system were to be used, then because the transmitter frequency and receiver frequency of this type of system is fixed or randomly selected, a frequency-scanned array would not be feasible.
In practice, frequency scanning is often used in one plane such as the elevation plane, while phase scanning or mechanical rotation of the antenna is employed in the orthogonal azimuth plane. In one particular method of frequency scanning, known as "within-pulse" scanning, the transmitted frequency is varied from one value to another during the time of transmission of a pulse. This causes the beam to scan in the vertical plane and target returns vary in frequency according to the target elevation. These "chirp" signals are processed using surface acoustic wave filters and pulse compression techniques. Hence it is possible to transmit a pulse of long duration and then to compress it into a narrow pulse of large amplitude in the time domain, to obtain a large signal-to-noise ratio and good range resolution.
Array Scanning
In many radar applications there is a need to scan the beam in different directions. The two most common methods are mechanical scanning and electronic scanning. In mechanical scanning, the array is physically rotated through 360 degrees to give coverage in all directions, as required for search or surveillance radar systems. Should a limited sector of +/-60 degrees only be required and where the array structure is large, it is often preferable to use electronic scanning methods. The two main types of electronic scanning are phase scanning and frequency scanning. Phase scanning arrays, which is generally used with linear or planar arrays, are known as phased arrays. Although this applet was primarily designed to show the principles of frequency scanning, it has the capability built into it to demonstrate how phase scanning operates. A brief description of phase scanning is provided below and in the document "Phase scanning."
Phase Scanning
In an array of several equally spaced elements, the main beam of the array will point at an angle that is determined by the phase difference between adjacent elements. This is more fully described in the document "Phase scanning."
Frequency Scanning
In an array of several equally elements, the main beam of the array will point at an angle that is determined by varying the phase shift between the elements by changing the frequency of the carrier that feeds into the array from the transmitter. This is more fully described in the document "Frequency scanning."
The applet
Below is a screen shot of the applet. This and two addition screen shots describe the various components used to operate the applet.
Controls
Polar pattern
Starting at the top of the applet control panel are two checkboxes.
The checkbox labelled "Pattern/Phase sweep" when activated without any of the "Mode" checkboxes selected will permit a polar pattern of an array to be drawn. The scrollbars that are activated in this mode are: "No. of elements", "Element spacing in wavelengths" and "Phase between elements in degrees". The "No. of elements" scrollbar allows the number of elements in the array to be selected, the "Element spacing in wavelengths" scrollbar spaces the elements along the axis of the array in wavelengths and the "Phase between elements in degrees" allows the phase difference between each adjacent element to be selected. This phase difference can only be the same between each element. No provision is made in this applet for differing phases or differing spacings between the elements from element to element in the array. The screen shot shows a polar patten for a 10 element array with each element spaced 0.5 wavelengths from its adjacent element and the phase between each adjacent element being 50 degrees.
Phase scanning
To activate this mode, the checkbox labelled "Pattern/Phase sweep" is activated and a "Mode" checkbox selected. The scrollbars that are activated in this mode are: are "No. of elements", "Element spacing in wavelengths" and "Phase between elements in degrees". The main control scrollbar is the "Phase between elements in degrees". This scrollbar allows the polar pattern to be swung about the "zero degree" (broadside) or the "180 degree" (endfire) angles. When this phase scanning mode is activated, a yellow print out, on the bottom left hand side of the polar pattern area, indicates the angle that the pattern swings about its reference and the "Mode" that has been selected. Also, for all other modes there are printouts showing the equivalent distance in centre-frequency wavelengths of the element spacing. The phase velocity printout has no meaning in this phase scanning mode. Below is a screen printout showing the movement of the polar pattern as the "Phase between elements in degrees" scrollbar is varied. The beam movement is indicated by the normal black polar pattern being redrawn in white. In this case, the number of elements chosen is 5, the "Mode" = -1, the element spacing is 0.5 wavelengths and the phase between elements is (control scrollbar) is at 50 degrees. The white coloured polar pattern is shown by the bottom left hand side of the applet yellow printout to by 32.21 degrees and the equivalent electrical spacing d, between the elements expressed in centre frequency wavelengths l0 is 0.5. The "Mode" is shown as m = -1.
Frequency scanning
To activate this mode, the checkbox labelled "Frequency sweep" is activated and a "Mode" checkbox selected. All of the scrollbars are activated in this mode. The "Proportion of centre frequency", scrollbar is the main control that allows the frequency change to sweep the beam about the "zero degree" (broadside) or the "180 degree" (endfire) angles. The "Relative phase velocity p" scrollbar allows the ratio of the delay between the signal transmitted in vacuum between the elements of the array and that delay introduced between elements via a transmission line to be entered. The feed arrangements showing how p may be physically implemented is shown in figure 2 of the "Frequency scanning" document. When the frequency scanning mode is activated, a yellow print out, on the bottom left hand side of the polar pattern area, indicates the angle that the pattern swings about its reference and the "Mode" that has been selected. The Phase velocity, p = x, printout indicates the chosen value using the scroll bar. Also, for all other modes there are printouts showing the equivalent distance in centre-frequency wavelengths of the element spacing. In this mode, the printout may differ from that shown by the "Element spacing in wavelengths" scrollbar. Below is a screen printout showing the movement of the polar pattern as the " Proportion of centre frequency " scrollbar is varied. The beam movement is indicated by the normal black polar pattern being redrawn in yellow.
In this case, the proportion of the centre frequency (the control scrollbar) is at 1.3, the number of elements chosen is 10, the "Mode" = -1, the element spacing is 0.806 wavelengths and the phase between elements is (control scrollbar) is at 0 degrees. The relative phase velocity is p =2, meaning that there could be between each element, a transmission line that is twice the length of the direct distance between the two elements. The yellow coloured polar pattern is shown by the bottom left hand side of the applet yellow printout to by -27.025 degrees (anti-clockwise from the zero degree broadside angle) and the equivalent electrical spacing d, between the elements expressed in centre frequency wavelengths l0 is 1.048. Notice that this differs from the centre frequency element spacing of 0.806. The "Mode" is shown as m = -1.
Thanks goes to Mr. Karl L. Barrus whose code for producing the basic antenna pattern is used in this applet. His original applet may be found at: http://www.nwlink.com/~klbarrus/antenna/.
Note: When alternating between the "Pattern/Phase sweep" and "Frequency
sweep" checkboxes, double click with the mouse on the new checkbox selection
to ensure updating of the "Beam rotation, phi" angle screen printout (in
yellow on bottom left hand side of the applet screen). It is worthwhile
double clicking on these checkboxes whenever you have changed the values
of the scrollbars to ensure the correct beam rotation angle is given.