FWIW
I am of the opinion that it would be more beneficial to users of this group who come here for understanding if they could build knowledge on what they read rather than wading through long debates and choosing sides in hopes of serendipitously building upon their understanding.
Would it not be better if:
1) We first stress that the NanoVNA simply makes only one or two measurements... reflection only or reflection and transmission, also called scattering parameters that we often reference as S11 or S22 and S21 or S12 respectively?
2) Point out that Each measurement is two valued (a complex number) representing a vector as a ratio of the measured value with respect to an input or stimulus value (also a complex number)?
3) Ensure that users understand that these two parameters embody the entire definition of what a VNA measures? The quality, stability, and repeatability of the measurements are all hardware dependent. If the hardware is up to the task, it simply provides an output of high quality, stable, and repeatable complex number pairs.
4) Note that everything beyond this point is the result of computationally comparing the output complex values (the measurements) to the input values (the stimulus)? This includes the process of calibration which yields the complex values representative of the stimulus which is both normalized, offset (corrected), and scaled.
5) Refer to the complex numbers that are output by a VNA (NanoVNA) as reflection coefficients (CH0), and transmission coefficients (CH1)? These same numbers are all that is required to compute any and all of the display options offered and supported by the NanoVNA.
6) Focus this forum toward growing our understanding and utilization of the VNA as an RF measurement tool, with do and don¡¯t do along with why and why not reasoning?
Difficulty grasping the concept of loss due to the presence or absence of a sign seems to signal a lack of understanding at a more basic level. Most... possibly alll... practicing tradesmen wouldn¡¯t expect to see 100W (50 dBm) out of a 20 dB attenuator being driven by a 1 Watt (30 dBm) transmitter. Conversely; they would indeed anticipate the output of that attenuator to be 20 dB lower because of their expectation that the attenuator has 20 dB of loss.
They may even attempt to confirm this by measuring the output to confirm the power is 10 dBm as expected. If they inserted an additional 20 dB attenuator, they would confirm that the output was now reduced by another 20 dB, for a total of 40 dB of loss, by measuring the output and observing a level of -10 dBm.
The subtlety is clear to those skilled in the art, but possibly not at all clear to those who come here to learn. For their benefit we need to be clear and identify the subtleties.
Loss as well as gain is often expressed as a logarithmic ratio called decibels or dB. A positive result implies an increase or gain, and a negative result always implies a decrease or loss. When the sign is preserved, the result is always treated as a gain. By convention, the sign is often dropped, and the result expressed as either a gain (when the result is positive), or a loss (when the result is negative).
A common error is the use of the reference dBs... as in 10 dBs vs 1 dB. By definition, the unit of dB is a ratio, and thus a singular value and has no plural notation or reference. The correct use is 0.1 dB, 1 dB, 5 dB, etc.
Any extensions to dB, such as dBm, dBW, dBv, etc. are simply indicators of a fixed normalizing value that the ratio is in respect to. For example dBm is read as decibels with respect to a milliwatt. In these cases the result represents both a relative value (ratio in dB), and an absolute value; a consequence of being referenced to a fixed level or standard, (1 Watt, 1 Volt, a dipole, etc.)
Another common error is a belief that a dB is different when expressed as a voltage ratio or a power ratio. In fact they are identical, and the decibel is ALWAYS expressed as a POWER ratio. Apparent power is assumed, since input and output impedances can be vastly different, and true power can only be assumed when input and output impedances are precisely identical, or when complex voltage and current measurements can be made. The computation of dB differs when comparing voltage ratios or power ratios, in order to preserve this definition. Voltage or current measurements must be squared for their ratios to equate to a power ratio.
To summarize; the only requirement for a minus sign to ensure there is no ambiguity in the example given here, was when expressing the absolute magnitude of the power measured as a ratio with respect to 1 milliwatt (i.e. -10 dBm), and only then, because the value is below the 1 milliwatt reference level.
In the end; wouldn¡¯t we feel sufficiently rewarded by simply striving to become productive users of our NanoVNA¡¯s, and adopt a terminology convention that closely matches the industry and enables us to share our knowledge efficiently?
We really shouldn¡¯t require jargon police debates while we can be discussing how to ensure we are making measurements correctly, and how hams that have only used SWR meters in the past can interpret their NanoVNA measurement results, and increase their understanding of radio beyond simply matching their antenna.
There¡¯s some good minds here and some eager newbies also. Knowledge is power, and we have the opportunity to revive the technical prominence of our hobby.
.... or we can just let it evolve into another philosophical special interest chat room. :-)
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73
Gary, N3GO
