Antenna Wave Tilt page 2

2.1) Measurement Principle

  The principle of the measurement shown in figure 2 is very simple, with the transmit antenna located on a metal ground plane (the size of which is selected to achieve optimum antenna matching at the required frequency) the receive antenna is rotated in the XY plane in order to find a null in the received signal level. It would be expected from Terman's description, that with a far field measurement, there is a singular well defined observable null in receive level when the Rx antenna is at right angles to the forward tilting wave and the polarisation is then taken to be at right angles to the null. However, this is not the case in a near field time domain measurement, because as has been proven in the previous Near Field Radiation article there exists more than one radiation from a simple monopole antenna. Therefore nulls have to be located for each individual radiating path in order to assess wave tilt at each radiation point on the Tx antenna.

Figure 2

2.2) Measurement Equipment

  Non-metallic tripods, as shown in figure 3, were made specifically for the measurement and consisted of wood, cardboard and plastic. The height of the tripod has to be adequate to ensure that the first reflection from the floor/ground or any nearby metal object occurs after all direct transmissions have occurred and the final measurements were performed outdoors with the tripod height set at 2227 mm and with a 1974 mm separation of antennas. For the receive antenna, the angle of rotation is indicated on a cardboard scale with graduations that are barely visible in the photograph. The antenna is rotated through 360° in steps of 15° and held in place by a plastic clip attached to the cardboard scale.

Figure 3

  As the antenna cable can easily cause measurement errors when placed in front of the Rx antenna, the antenna is mounted with a right angle adaptor and a rotating joint. This ensures that the associated cable does not move, is at right angles to the Rx antenna and causes the minimum measurement errors. The rear view of the Rx antenna shown in figure 4 displays how the coaxial cable is mounted at the rear of the graduated scale.

Figure 4

  The view of the Tx antenna, shown in figure 5, displays the aluminium foil ground plane which is adjusted in size to obtain the optimum return loss. The bridge/coupler used to monitor reflected power was specifically manufactured in house for this application and had a low insertion loss up to 3 GHz (Yes, we can supply these if requested). This bridge/coupler is mounted directly below the Transmit Antenna.

Figure 5

  The test equipment used is shown in figure 6 and includes a VNA that has a time domain facility, a computer to download the S2P Touchstone files from the VNA and an ultra wide band amplifier with power supply to bring the transmit level to +10 dBm, which was well above the ambient radio noise level. The VNA was set to the widest sweep from 300 KHz to 3 GHz in order to obtain the maximum resolution of peaks in the time domain display.

Figure 6

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