edited April 2019 in Installation, Antennas and Interference
The admin config tab has a place to calibrate the S-meter. What are people using there with various antennas?
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S-meter calibration (dB) -16
Waterfall calibration (dB) -15
However in most cases I don't think you can rely upon the off-air readings as everyone's antenna (and distribution systems) vary so much.
Martin - G8JNJ
I'm not sure how you would calibrate for an antenna.
Antennas used for measurement purposes generally have a calibration curve showing the Antenna Factor (AF) vs. Frequency.
The gain depends upon a number of factors including antenna pattern, height above ground, polarisation and frequency, so unless your measuring receiver can accept a calibration file to compensate for these variables you can't get a direct readout.
Most folks have to read the level from a calibrated receiver and then correct the values using a lookup table, chart or some other external software.
Martin - G8JNJ
But you really need to calibrate both antenna and amplifier together.
The loop response is not flat, but over a small frequency range you may be able to get close, if you know the field strength of a particular signal.
Martin - G8JNJ
The Kiwi is different: Between the normal roll-off at the high end (due to sampling, the LPF, etc.) and the "limited attenuation" high-pass filter (e.g. attenuation goes from about 0 to 12dB between 12 and 7 MHz) to minimized overload from low-frequency HF signals, I've had to give up on making the Kiwi's S-meter read anything useful.
I suppose that a useful addition would be to have a number of S-meter and waterfall calibration points with frequencies, interpolation being done between. For example, "5000,-8; 7000,-9;10000,-12;12000,-17" (e.g. frequency in kHz, calibration value) to establish an approximated curve: 16 of those would likely be "good enough".
Certainly an "edge case" but it would be useful, nonetheless.
I haven't measured very many spot frequencies over the Kiwi's 0-30 MHz range but on 5 different Kiwi's, putting "-15" into the S-meter calibration seems to cause the S meter to agree very well with calibrated signal generators at every point I have measured with every Kiwi I've measured. I don't think I've noticed anything like the 12 dB variation Clint mentions.
Adding to this, the S-meter in the kiwi seems to detect and report rms noise pretty well. If one uses the indicated bandwidth along with the measured no-signal noise floor and compares this with the results of a Y-factor noise figure measurement from a calibrated noise source the results seems to support the S-meter being calibrated with the above value - at least within the +-1 dB or so accuracy of the reporting. Not all noise detectors do this well.
I'll try to make a more thorough measurement with more points and post the results, when I'm next able to. I've been needing an automated way of measuring known signals, noise and interm-odulation distortion levels anyway.
This all keeps getting more fun all the time !!
This measurement indicates that I was a bit low with my previous estimate (in this thread, above) of the filter loss, by as much as 2 dB. A filter I fabricated with the same values but larger (1210) higher Q inductors understandably had less insertion loss.
As part of generating a nominal calibration table for Kiwis I'm now trying to understand the contributors to the amplitude errors a KiwiSDR makes vs. frequency. Clearly this LPF unflatness is one of them but I don't think it entirely explains what we measure.
At one time, somewhere on this forum that I can no longer discover, John (I think) mentioned that the algorithm in DSP added some correction to the reported amplitude. This may have been to correct a sin(x)/x artifact of the sampling process - or it may have been something else. I'm foggy on this.
Can John or someone else point me to something that describes that issue so that it can be added to the effects that make a Kiwi's measurement less than flat vs. frequency over the .01-30 MHz range? Understanding this may help in generating a correction table which will be useful in making absolute noise (antenna temperature) and signal (e-field) measurements.