![]() This is important, to accurately map our tunnels, we need to know the exact details of what we yelled into the the entrances. Now, the astute, might have noticed that I casually dropped in the word relative before. The whole set of all possible combinations of shouting and listening at each entrance is what we call the S-Parameters of the tunnel. So, for example, we may have measurement of S32 to indicate that this is a measurement of what we heard at entrance 3 while someone yelled into entrance 2. We will number each entrance from 1 to n and record details of what was heard at each entrance, again using the nomenclature we used before. We can get a group of friends to stand at each entrance and take it in turns to shout and listen. Imagine a tunnel network with many entrances. We can now start to make this as complex as we wish. i.e a measurement of what you heard at port 1, when you shouted into port 1. Using the same nomenclature as before, we can call this S11. Congratulations, you have just measured the reflection of the tunnel. If hear the echo, back at the same volume, you might reasonably that none of the sound could have reached the other end. Perhaps there is a blockage, somewhere in the tunnel that is reflecting your voice. ![]() Now, you do the same, but this time listen to the echo of your shout reflect back to you. “Dude, Some moron built a wall inside this tunnel!!” “Yeah, I think the author was trying to illustrate a point about reflections” “Lovely brickwork on those entrances though” In VNA parlance, we will call this a S21 measurement, meaning we have made a measurement of a signal at port 2(your friend) of a signal injected at port 1(your beautiful voice). You have just measured the ‘thru’ characteristics of this tunnel. Maybe there is some material in there that is absorbing the high pitch noises, or closed service tunnel that has created a resonance. Perhaps, we can detect a difference between the high and low pitch sounds. We might be able to work out the length from the volume and delay(you were careful to synchronise watches) of the received sound. You have a great idea! What happens if I shout into the tunnel and have a friend listen at the other end. We will assume that you don’t want to go in, because, well, your scared of the dark and don’t want to be hit by a train. Say you are standing at on end of a dark train tunnel and want to know something about what is inside. For an alternate analogy take a look at KirbyMicrowaves site. That sounds pretty complex, but it’s really not. “Atrévete a Entrar…”by Hadrian Fernandez is licensed under CC BY-NC-SA 2.0 It doesn’t hold a candle to the performance of a instrument grade piece of kit, but for the price, and used with care, it’s not too bad.Ī VNA is designed to inject a known frequency and amplitude source signal into a RF port and measure the relative amplitude and phase of this signal when received at a number at one, or more of it’s ports. Luckily, there are cheap alternatives now, such as the NanoVNA.
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