AS 1417:2015 Receiving antennas for radio and television in the VHF and UHF broadcast bands—Design, manufacture and performance of outdoor terrestrial television antennas
A3.2 Test set-up
The main criteria to be considered in locating a suitable area for an antenna test range are—
(a) selecting an area free from objects likely to cause multipath reflections; and
(b) ensuring that there is provision for both antennas to be mounted sufficiently high above the ground to ensure that the ground-reflected signal is sufficiently small.
Techniques can be employed to allow the height to be lowered without impact upon measurement accuracy, such as placing an absorbent material, such as carbonized cones, on the surface between the antennas.
When both the main path signal and a reflected signal are received, the output signal level increases or decreases as the phase of the two signals varies with both path length differences and frequency. Antennas that have differing directional characteristics will result in differing generation of and reception of multipath or reflections and will cause measurement uncertainty.
Multipath signals may also vary with time due to temperature and other effects, therefore it is important to minimize, as far as possible, any multipath reception, both from the ground and other reflecting objects (including the test equipment and operator). If multipath signals are at least 26 dB below the main signal, then the error due to this effect should not exceed 0.5 dB.
The test range layout as depicted in Figure A1 is based on keeping causes of multipath reflections sufficiently distant from the transmit and receive antennas to ensure that any multipath signals are minimized. For many of the tests it also assumes that the transmit (Tx) antenna has good F/B ratio and sidelobe directivity characteristics.
A3.3 Minimizing multipath reflections
Methods of reducing the amplitude of multipath at the receive (Rx) antenna position due to ground reflections include the following:
(a) Mounting both the Tx and Rx antennas as high as possible above the ground surface.This works mainly by increasing the angle of both the transmitted and received multipath signals so that the directional characteristics of both antennas can reduce the multipath signal. However, in a non-permanent installation it may be difficult to safely and accurately support some antennas if the height exceeds 4 m.
(b) Minimizing the separation between the Tx and Rx antennas as much as possible without causing significant near field or mutual coupling problems. This decreases the effect of external interference and reflections and also ground reflections by increasing the reflected signal angles.