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I think also there's a confusion engendered by the term "efficiency".
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Is that the efficiency inside the Tx? Or the "efficiency" of power transfer to the antenna (i.e. Pradiated/Psupplied)? (you'll see "antenna efficiency" as a term used to relate, for instance, the fraction of power received by a preamplifier from that incident on the antenna's Effective Area - It's not a correct usage, but there it is, and we're stuck with it) And in any "real" system (i.e. not perfectly matched), some of the power reflected back by the antenna also gets reflected from the source back towards the antenna again. This is why systems with very low loss lines (open wire) work adequately well - especially back in the day when the amplifier had a tuning network as part of the output circuitry. 1.5:1 is -14 dB is 4% of the power is "reflected" back. (a 0.2 dB drop in the power received, compared to a 1:1) - I venture that most folks don't know or can control their system losses to a few tenths of a dB. Unless you're building radiometers or measurement systems (I've done both). And in any case this whole "forward" and "reflected" thing is just a convenient mathematical model for analysis. One can also consider it as reactive power (which is what they do for AC line power transmission and distribution). The other thing is that those "line loss" numbers are for an integer number of wavelengths. For HF, most of the loss is the IR losses, not dielectric losses, so it's more about the RMS current at any given point. -----Original Message-----
From: <[email protected]> Sent: Jan 21, 2025 8:25 AM To: <[email protected]> Subject: Re: [nanovna-users] Measuring R,X Resonant Frequency You are right to ask yourself. There is one fundamental thing that you forget, that the TX -> line -> Antenna system is only adapted in one direction. The antenna adaptation devices (box for example) only serve to bring 50 ohms to the TX terminals. Fortunately, the Tx does not have 50 ohms of internal impedance but only one or two ohms, otherwise the efficiency would be catastrophic. If when looking back at your antenna you see 50 ohms, your antenna looking towards the TX does not see 50 ohms. On the other hand, once your agreement is complete with your coupling box, if you turn the coupling box over, that is to say place 50 ohms in place of the TX and look, with a NanoVNA, through the antenna base, you will see the same impedance as that of your antenna. This is a way to preset a remote adapter. Keep in mind that if the generator impedance is fixed you will get the maximum power out of it in a combined load (50% efficiency). But if you are left free to choose the internal impedance of the generator, whatever the load, the maximum power will come out of the generator if its internal impedance is zero (100% efficiency). -- F1AMM François |
