Question about using spectrum analyzer

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masonlee

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Hello,:smile:

When we use spectrum analyzer to measure antenna receive power for gain calculation, i find out that the spectrum analyzer can measure few kind of powers like channel power, average power and total power. So which kind of power should i choose?

During the measurement with the spectrum analyzer, is it a must to see a peak in the receive power vs frequency plot? I found out that it still showing some power value even the screen does not show a peak.

Thanks in advance
 

I do not understand your statement " it still showing some power value even the screen does not show a peak". What other power indication your SA has except of the screen?

Antenna measurements are always relative, so there is no real "antenna receive power". With your SA you should first measure a reference power level from a reference source (signal generator) at the same frequency, then replace the source with the antenna. The radiation pattern is relative to the maximum (on-axis) gain, again relative, in dB. Any setting "channel", "average" and "total" setting can be used.

For antenna pattern measurement, using a SA is possible but not the best option. As the test is usually done at a single frequency, or over a limited bandwidth, there are specialized receivers with a wide dynamic range for it. They also must be calibrated by a reference signal source, then, antenna maximum gain and radiation pattern is measured in dB against such reference value.
 
I mean can we record the power value when the screen of the SA just showing a noise floor signal without a peak at the center frequency?

What are the specialized receiver for radiation pattern measurement?
I quite worry about the dynamic range of SA too, it seem difficult to detect the low power signal. Normally what are the dynamic range of most of the SA?

How about the procedure to calibrate the SA by a reference signal source? Long procedure?
 


If the SA screen shows no signal above the noise floor, there is none falling to the selected frequency span window.
I know Polarad, narrow-band tunable calibrated receivers designed for antenna pattern measurement.
A good SA has up to 90 dB dynamic range, with a CW signal used to test. You can adjust a narrow frequency span to achieve it.

For antenna measurement, a standard-gain antenna is used to calibrate the receiver or SA first. Then connect the antenna under test, and all measurements are relative to the first.
As standard-gain antennas, a half-wave dipole is used up to 2-3 GHz, above it, standard-gain horns are used. For good results, either an anechoic chamber or an open test range is needed. Presence of ground and other objects affects wave propagation and should be avoided or suppressed by absorbers if necessary.

Agilent offers a white paper on antenna testing methods. J.D.Kraus: Antennas and other books are good to study on details.
 
I am a beginner in using spectrum analyzer, is it all spectrum analyzers allow to be calibrate to a reference power level?
 

masonlee said:
I am a beginner in using spectrum analyzer, is it all spectrum analyzers allow to be calibrate to a reference power level?
Click to expand...

Yes, good SAs can be calibrated by putting a reference signal to their input; then the vertical scale is calibrated by divisions of 10 dB, 2 dB, etc.
When testing antennas, the reference signal is usually taken from a reference antenna with a known gain. The tested antenna output is then measured against the reference power level.
 
In the simplest case is your reference relative a standard antenna in fare field. Calibration is performed by reading and store received level for the reference antenna.
Adjust RF-levels to find best dynamic range, covering both reference antenna and DUT, with aid of RF amplifiers or attenuators.
Replace reference antenna by DUT. Rotate it on a turntable to find total radiation or peak gain.
Ground reflections and cable radiation are typical error sources that gives false reading in a less good measurement environment. Fare field distance between RX and TX antenna is typical at least 10*lambda.
 
From the data sheet of the spectrum analyzer, i found that it allow us to switch on/off a preamp, so what is the function of this preamp?

What is ATT, VBW, SWP?
I saw these parameters from the screen of the SA, how much value i need to set them?
 

Preamplifier amplifies weak signals, should not be used if input signal is strong. Typical in this case is a strong signal something -20 dBm or more and weak if signal level is -40 dBm or less. Max allowed input level can be a signal, not visible on current selected frequency range so if unsure can it be a good idea to start with a wide frequency range to find strongest peak level.
ATTenuator opposite function of preamplifier. Reduces signal level to a range that the spectrum-analyzer can handle. ATT have often also an auto setting, where attenuation is set according to the selected reference level.
Too strong level can harm the analyzer. Max allowed input level is often written at analyzer. If I am unsure about input level do I also use an additional external attenuator to not risk expensive repairs.
VideoBandWidth - leave it wide ~3MHz for antenna measurement. In most cases is it the default setting,
SWP - Guess it is sweep time. Normally no need to adjust for antenna measurement. If SWP is related to sweep trigger, set it to auto or free run, which probably is default setting.
 


If you have these questions, PLEASE IMMEDIATELY SWITCH OFF THE SPECTRUM ANALYZER!!! YOU ARE NOT QUALIFIED TO USE IT AND MAY CAUSE AN EXPENSIVE DAMAGE!!
Start reading Instruction Manual, and ask the SA manufacturer to give you a basic course. Without being a qualified professional, do not touch such expensive sophisticated equipment!
 

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