Web Results 1 - 10 of about 163 for 'Microwave Theory and Applications' Stephen+F.+Adam. (0.49 seconds)
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Geez that thing has gotten expensive. That's a text book that been out for quite a number of years. My copy looks to be from 1976, bought for US$ 16 for a class at HP taught by the author.
Hope you're viewing this with a mono-spaced font. Let's define a two-port device as having four points - two incident, and two reflected:
--> a1 ----- S21 -->-- b2 --->
| ^ | | S11 S22 | | V | SWR and you would not get a 99% reflection See that diagram above? You know we can use other values of S parameters to describe other things. For example, a 'short' or 'open' would have S21 = S12 = 0, S11 = 1.0, and S22 not relevant. Now, if you measured the power going in verses the power coming out, they would be the same, is that correct? It's all reflected. Do the math, and that's '[ 1 + 1 ] / [ 1 - 1]' for a SWR of infinity.
Now let's cascade the 20 dB attenuator with this short/open, and look what we see from the left side of the attenuator. You have
Attenuator Short/Open In 0.1 0 0 0 1.0 who_cares Out 0.1 0
We can ignore the zeros in this exercise, so we see 0.1 * 1 * 0.1 BECAUSE we go through the attenuator two times - once enroute to the total reflection, and once on the way back... at least we do with every regular attenuator I've ever worked with. (Yes, I've also worked with ferrite isolators, but they're really three port devices, not two.)
Not to sure where you found that decibel value, but you are forgetting that the indicent power takes a loss going in, and the reflected power takes a loss coming back through the attenuator. The reflection would not be 1.0, but would be the product of the losses through the attenuator times the reflection of the short/open - or 0.1 * 1.0 * 0.1 - or if you want it in decibels, (-20) + (0) + (-20) [negative because they are a loss] or -40 dB. The SWR would be '[ 1 + 0.01 ] / [ 1 - 0.01 ]' or 1.02:1
The above explanation is a simplification, for conceptional use. In the real world, things get horribly complicated by the fact that S11 and S22 have values other than zero, and phase angles get involved, and there are more than two devices involved. The Adams book noted above is a good college text book that goes into a LOT more detail.
Old guy