Measurement and calibration
NPL has developed a range of facilities and measurement techniques for calibrating microwave standard gain horns, near-field scanning probes, reflectors and base station antennas.
The gain of standard gain horns is measured inside a temperature controlled anechoic chamber using the three antenna extrapolation technique. This involves measuring the power transmitted between the three pairs of antennas as a function of distance and then extrapolating these results to obtain the true far-field values for each antenna. The complex reflection coefficients of the antennas are also measured to correct for mismatches. An uncertainty of ± 0.05 dB is typical for most standard gain horn antennas.
For some antennas, in particular probes to be used for near-field scanning applications, it is also necessary to know the polarisation parameters, port-to-port phase, isolation and probe pattern components. The axial ratio, tilt and sense of polarisation are measured using three antenna techniques, which avoid the need for a separate polarisation standard. Nominally linearly or circularly polarised antennas can be measured.
Full characterisation of near-field scanning probes is provided, with the normalised probe receiving coefficients provided in data files consistent with the most widely used near-field scanning software.
NPL also offers a lower cost service for the calibration of wideband EMC horn antennas in a dedicated indoor anechoic test chamber. These horns, usually double ridged waveguide antennas, cover the frequency ranges 1 GHz to 18 GHz and 18 GHz to 40 GHz, however antennas operating up to 110 GHz can be measured. These antennas are typically measured at a distance of 1m or 3m, with apparent gain uncertainties of ± 0.7 dB for waveguide fed antennas and ± 0.8 dB for coaxial.
Antenna properties measured include:
- infinite range gain
- finite range gain
- antenna factor
- complex reflection coefficients
- axial ratio
- tilt angle
- sense of polarisation
- pattern components
- port-to-port phase
- port-to-port isolation
Types of antenna measured include:
- standard gain horns
- near-field scanning probes
- broadband ridged guide horns
- conical log spirals (left or right polarisation)
- array antennas
- base station antennas
Feed types accommodated include:
- even waveguide sizes from WG8 (IEC-R 22) to WG26 (IEC-R 740)
- WG11A, WG15, WG17, WG23, WG25 and WG27 (IEC-R 40, 84, 120, 400, 620, 900)
- precision type-N
- 2.92 mm
- 2.4 mm
- SMA (additional uncertainty may apply)
NPL also offers a consultancy service for range design and validation.
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