This is a description of the measurement range for antennas we have in Ekenäs 25 km south of Kalmar. The measurement equipment has been improved continuously since it was built in 1990, and this is what it looks like today.
We have designed the entire system ourselves. The software for controlling the system and communication with a network analyser is written in C++. The data is presented using Matlab and PolPlot.
A Vector Network Analyser (VNA) is used to collect measurement data. We mainly use HP8753D (6 GHz) and Advantest (8 GHz). The Antenna Under Test (AUT) is usually connected to receiver port of the network analyser and we therefore measure S21, but we can also let the signal go the other way. The network analyser communicates with a PC through GPIB. When we need a stronger signal, we can boost the signal with a power amplifier.
The turntable is designed with a motor with a double worm gear for minimal backlash and maximum torque. On the gearbox there is also an absolute angle encoder with, as of now, 14 bit Grey-code for 4096 positions (0.088 degrees). For new systems we use a similar one with BCD-code and 3600 positions, ie a resolution of 0.1 degree. The motor is controlled through a frequency inverter. We usually use three speeds, approx 10 deg/s for rough positioning and 1 or 3 deg/s when doing the actual measurements. An I/O card in the PC handles all communication with these external units.
The system is a slant range, ie the transmit antenna is positioned 2 m above ground, and the Antenna Under Test is positioned on top of a more than 10 m tall tower. The distance between the transmit antenna and the antenna under test is about 20 m which is far field in 2.4 GHz and higher frequencies.
We have a variety of transmit antennas, on top there are parabolic dishes for 3.5 GHz, in the middle to the right a parabolic dish for 5 GHz, in the middle to the left a mount for AEL horn antenna for 0.5-6 or 1-12 GHz and below horn antennas for 1.7-2.7 GHz. All sets have both horizontal and vertical polarisation. The desired antenna is selected with a coaxial relay that is positioned in the cabinet behind the antennas. The antenna type and position have been chosen to illuminate the ground as little as possible.
Well, it looks like the antennas points out of direction, but it is the wide-angle perspective that fools the eye.
The measurement mast is a 8 m tall Wibe tower with a "cart" that runs outside the mast. On the cart is mounted a double worm gear with a motor and an absolute angel encoder. The antenna under test is mounted on a glass fibre tube that is parallel with the transmit antennas. The entire mast is also rotatable, so in the future we will be able to make even more advanced measurements. The motor and worm gear are mounted on an axle and can therefore be rotated for desired elevation angle. Here we make measurements on a panel antenna for 3.5 GHz.
The cables are of type Heliax 1/2" and Andrew C2FCP. The cable housed in the tower is a measurement cable from Rosenberger Carlberg, and is built to withstand continual bends and stress. The measurement system has slowly been developed, and we now have a set of functions that we find practical: - make measurements "on the fly" or while standing still. In the first case we make measurements whilst rotating the antenna under test, which saves a lot of time. We choose rotation speed and number of measurement points, so the antenna under test turns no more than 0.2 degrees while taking the measurement data. For more exact measurements we stop the turntable in each measurement position. - measuring at many frequencies at the same time, we usually make measurements with 5-10 MHz intervals within the desired frequency band, and a number of points outside, in total usually about 20 frequencies. We always measure both amplitude and phase. - fully automated, the measuring does not require an operator once started. A quick measurement (20 frequencies and 360 degrees, with a 2-degree interval) only takes 3 minutes. An accurate reference measurement takes 30-60 minutes. - open format data files, all data files are ASCII-format which can be read by many programs. You are welcome to visit us in Ekenäs if you want more information! Update 041103: As of November 2004, we now have a measurement range in Kalmar, with similar performance with the above described. |
