![]() The use of a compressed Smith Chart therefore allows the designer to visualize device parameters over the complete frequency range, where both positive and negative resistance behavior may be exhibited. From 6 to 10 GHz, the pole lies inside the Γ 3 = 1 boundary of the Smith Chart in Figure 4.10, indicating that negative resistance can be generated in this device using passive shunt feedback. An example of the use of a compressed Smith Chart to plot the negative resistance behavior of an MHG9000 GaAs MESFET, in terms of shunt feedback pole locations (as defined in Chapter 8) on the Γ 3 plane from 2 to 18 GHz is shown in Figure 4.10. In order to represent negative resistances we need to compress the conventional Smith Chart to be a subset of a larger chart, which typically has a radius of |Γ| = 3.16, this value being chosen to represent 10 dB return gain. The sign of the log(rho) takes care of this, mathematically, since the return is always =< the incident signal.Negative resistance values plotted on a Smith Chart lie outside the |Γ| = 1 boundary of the conventional Smith Chart. I think the author you reference merely added the (-) in front anecdotally, not mathematically, given that RL is a negative entity. I think the equation would be correctly written as RL=20*log(rho), as 20 is just a scalar value and should not be signed. It's not unlike pulling a vacuum, once you reached -1 atmosphere the rest is just an empty void the size of which is irrelevant. If your device (Spectrum Analyzer for example) has a noise floor of -120dBm than you cannot measure any return loss accurately beyond that floor. Perhaps more subtly, discussing the return as a negative value indicates that the limit of measurement is determined by the sensitivity and noise floor of the devices measuring the value. ![]() Mainly because it is a level returned which is less than the reference used to generate the return signal. Return Loss, as I know and use it, has always been a negative quantity. Here are some of the slides of my 80m antennas: Inverted V center fed and 1/4 wave vertical With appreciation for all of the useful comments I have received off-list. Here is a Smith Chart plot with the locus of points of every impedance (resistance and reactance) that results in a mag reflection coefficient of 0.33, and an SWR of 2:1 I chose to originally plot RL as negative just to match the shapes of the reflection coefficient and SWR curves. I did note that the NanoVNA save software allows the user to plot RETURN LOSS as either a positive quantity or a negative quantity. I am still learning a lot about these Nano VNA's and how to interpret their graphs. That's tough to cover the entire band without an antenna matching device somewhere. So on 80m, my favorite parts of the band are at the low end (CW and Digi modes around 3580 kHz) and the very top end with PHONE nets around 3990 kHz. Of course, hams do not operate on a single frequency, but QSY on a band. My 9th grade understanding of SWR and Smith Charts tells me that everyone can "see" the Bull's Eye on a target as the goal in darts, and in antenna matching, the goal is a perfect match to a 50-ohm feed line at some frequency. RETURN LOSS (as a positive number, in dB) was not a term I was familiar with before playing with this Nano VNA. The larger the radius (centered on 1.0 as 50 ohms) the higher the absolute value of reflection coefficient and the higher the SWR. ![]() I was hoping to get some feedback on is this portrayal of |reflection coefficient| or rho as the radius of a circle on a Smith Chart. ![]()
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