If you want to measure the Gain of an antenna, a Standard Gain Horn (SGH) is necessary.
If you were to order a SGH, it typically would have a table listing the Gain vs. Frequency. But if you want to verify the Gain at each Frequency or if you want to make your own SGH then this is the procedure to make that table.
What kind of Antenna should I use for my SGH?
It must have a polarization that is compatible with the Antenna you eventually intend to test (AUT).
Polarization types:
Most commercially available SGH's are Vertically polarized (and Horizontally polarized if you rotate them 90degrees). They are also made of sturdy aluminum and extruded from rectangular waveguide to form the Horn shape. This is a pretty robust construction that will not change much over time; It can be nicked and handled frequently without changing the Gain... to a certain extant. Dents WILL change the antenna radiation pattern.
Test Setup:
The Network Analyzer could be replaced by a Signal Generator and Power Sensor if you prefer; it will make calibration harder, in my opinion. An amplifier may be necessary to bring your signal above the noise floor of the Network/Spectrum Analyzer.
Calibration Procedure:
If you were to order a SGH, it typically would have a table listing the Gain vs. Frequency. But if you want to verify the Gain at each Frequency or if you want to make your own SGH then this is the procedure to make that table.
What kind of Antenna should I use for my SGH?
It must have a polarization that is compatible with the Antenna you eventually intend to test (AUT).
Polarization types:
- Horizontal
- Vertical
- RHCP (right hand circular)
- LHCP (left hand circular)
Most commercially available SGH's are Vertically polarized (and Horizontally polarized if you rotate them 90degrees). They are also made of sturdy aluminum and extruded from rectangular waveguide to form the Horn shape. This is a pretty robust construction that will not change much over time; It can be nicked and handled frequently without changing the Gain... to a certain extant. Dents WILL change the antenna radiation pattern.
Test Setup:
The Network Analyzer could be replaced by a Signal Generator and Power Sensor if you prefer; it will make calibration harder, in my opinion. An amplifier may be necessary to bring your signal above the noise floor of the Network/Spectrum Analyzer.
Calibration Procedure:
- Find an Anechoic chamber or antenna test range that is quiet at your frequencies of interest (FOI).
- There are no background signals (i.e. cell phone tower) large enough to be picked up by your SGH.
- Acquire two identical SGH's.
- Space the horns apart from one another so that we measure the Far-Field Gain of the SGH's.
- Rule of thumb: distance between antennas, R, should be >> wavelength at FOI. (R > 100λ)
- Align the horns as best as possible so that they both have the same polarization and are facing each other.
- Calculate the freespace loss (Lf) from Friis Equation: Measure the distance R between the horns with a measuring tape, aperture-to-aperture.
- Calculator is here.
- Set the Transmission Power (Pt): Adjust the power of the Network Analyzer to overcome the Freespace loss minus the expected Gains of the two horns; Signal should be well above the noise floor of the Network/Spectrum Analyzer.
- Pt = Lf - Gt - Gr + Pr, where Gt and Gr are the Gains of transmit, receive horns.
- Set Pr, in this equation, to an appropriate level above the noise floor of your receiver (~ -20 dBm for most Network Analyzers)
- Remove the SGH's and connect the RF cabling together. Conduct an S21 THRU calibration on a Network Analyzer at your FOI. S21 should now read 0dB.
The Measurement:
- Align the horns.
- Re-attach them to the Network Analyzer cables.
- Make fine adjustments (up/down, left/right, swivel) to each horn until you see a maximum amplitude for S21.
- Determine Gain of each Horn:
- Note the S21 amplitude on the Network Analyzer (dB)
- Add the freespace loss (dB)
- Divide by 2 to yield the Gain of each identical horn.
- Gtr = (S21+ Lf ) / 2 , where is the Gain of each identical transmit or receive horn.
- It is a good idea to repeat this measurement for several values of R to ensure you get accurate, repeatable values for Gtr.
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