7/29/2019 Antenna Measurements http://slidepdf.com/reader/full/antenna-measurements 1/5 ELE-6100 Suurtaajuustekniikan perusmittaukset ELE-6106 Basic RF Measurements Tiiti Kellomäki 2011 This material is licensed under a creative commons licence. Use for non-commercial and educational purposes is allowed. 1 Antenna Measurements Contents 1. What do we want to measure? • R ad ia ti on p at te rn • , , 2. How do we measure it? • Spherical coordinates 3. Error sources 4. Measurement ranges Radiation Patterns – All of Same Data! dB or not dB? normalised or not? 1d/2d/3d? How to read a Radiation Pattern • Picture is actually ”gain pattern” • Copolarisation: transmitting and receiving antennas both in same polarisation (e.g. both vertical) 3 dB beamwidth about 80 degrees Largest side lobe 20 dB smaller than main lobe: side lobe level SLL = –20 dB Main lobe direction is 0 degrees Front-to-back ratio (F/B) = –20 dB How to read a Radiation Pattern dBi scale (see next slide) Gain 6 dBi - Green = co-polarisation, Blue = cross polarisation maximum –18 dBi Cross-polarisation level (XPOL LVL): –18 dBi – 6 dBi = –24 dB Cross-polarisation should be zero in the direction of the main lobe. This comes from the definition… figure out! Antenna Gain • Antennas are passive, so they cannot add power. • Imagine an ’isotropic antenna’ that radiates equally in all directions. • Compare all fields to isotropic – dBi = decibel over isotropic (sometimes just ’dB’) – dBd = dB over (half-wave) dipole, 0 dBd = 2 dBi • Similarly, if reference antenna gain is known (in dBi), then unknown antenna gain can be compared – G X = G ref P X / P ref , called gain transfer method 4 dB directive antenna has 4 dBi gain isotropic antenna radiates same power density in all directions
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Tiiti Kellomäki 2011This material is licensed under a creative commons licence.
Use for non-commercial and educational purposes is allowed.
1
Antenna
Measurements
Contents
1. What do we want to measure?
• Radiation pattern
• , ,
2. How do we measure it?
• Spherical coordinates
3. Error sources
4. Measurement ranges
Radiation Patterns –
All of Same Data!dB or not dB?
normalised or not?
1d/2d/3d?
How to read a Radiation Pattern
• Picture is actually ”gain pattern”
• Copolarisation: transmitting and receiving antennasboth in same polarisation (e.g. both vertical)
3 dB beamwidthabout 80 degrees
Largest side lobe 20 dBsmaller than main lobe: side
lobe level SLL = –20 dB
Main lobe directionis 0 degrees
Front-to-back ratio(F/B) = –20 dB
How to read a Radiation Pattern
dBi scale
(see next slide)Gain6 dBi
-
Green = co-polarisation,Blue = cross polarisation
maximum –18 dBi
Cross-polarisation
level (XPOL LVL):
–18 dBi – 6 dBi
= –24 dB
Cross-polarisation
should be zero in the
direction of the main
lobe. This comes
from the definition…
figure out!
Antenna Gain
• Antennas are passive, so they cannot add power.
• Imagine an ’isotropic antenna’ that radiates equally inall directions.
• Compare all fields to isotropic – dBi = decibel over isotropic (sometimes just ’dB’)
– dBd = dB over (half-wave) dipole, 0 dBd = 2 dBi
• Similarly, if reference antenna gain is known (in dBi),then unknown antenna gain can be compared – G X = G ref P X / P ref , called gain transfer method