Home |
Search |
Today's Posts |
#71
|
|||
|
|||
Dr. Slick wrote:
On second thought, i believe we are all wrong to equate S11 with SWR! Input S11 of a system will certainly never change. But the SWR is absolutely dependant on the source impedance. I give up. People will believe what they want to believe, no matter what -- it's like arguing religion. But I hope some of the lurkers have learned that SWR is independent of source impedance, even if some of the active posters just can't seem to. . . . Roy Lewallen, W7EL |
#72
|
|||
|
|||
"Ian White, G3SEK" wrote in message ...
Input S11 of a system will certainly never change. But the SWR is absolutely dependant on the source impedance. No! SWR, S11, return loss, rho, Y-parameters, Z-parameters, etc, etc are all different derived functions of the same two variables: an arbitrary complex impedance, and the system reference impedance Z0 (a constant which may or may not be defined as complex). Correct, but it doesn't have to be 50 Ohms. Only those two variables are involved, so all of these functions are locked together. If one variable changes, all the derived functions change too. Either all change, or none change; nothing else is logically possible. How about a 50 Ohm resistor, which is always 50 Ohms (impedance doesn't change), fed with 20 ohms? Or 75? Cecil is correct in saying that the SWR meter would then have to be designed for 20 or 75 ohms, but that is beside the point. SWR doesn't have to be strictly 50 ohms, and will involve TWO impedances. If your source doesn't match your reference impedance (normalized center of Smith Chart), then you won't be measuring the reflected power coming right off the source. And because most PA are not 50 ohms output, and most SWR meters are 50 Ohms, there is problem. As Roy says, the equations relating any one of these parameters to any other are all well known. NONE of them ever involves source impedance. Assuming the source impedance is 50 ohms, which it usually isn't with most PAs. If you had a network analyzer calibrated for 20 Ohms, you would certainly have reflected power and high VSWR going into 50 Ohms, and a 1:1 SWR going into 20 Ohms. This would be the same as re-normalizing the Smith Chart for 20 Ohms in the center. You certainly can do this in MIMP. I don't blame anyone for believing it's a 50-Ohm-only world! No argument about any of that... but it's a totally separate point that has no relevance whatever to your earlier statements about source impedance. it's very relevant if you consider the port on a network analyzer to be 50 ohms or not... It should be, but your PA may be quite far off. Slick |
#73
|
|||
|
|||
On Wed, 13 Aug 2003 16:13:13 -0700, Roy Lewallen
wrote: Believe me, Ian, I know how frustrating it can get. But remember all the lurkers out there who benefit from your insightful postings. Please keep it up -- it is worth while. For their sake. Roy Lewallen, W7EL Hi Roy, It is nice of you to commend Ian to continue the good effort. Why didn't you do it directly instead of posting him through me? Are the lurkers to take some lesson by this breach of netiquette? :-) 73's Richard Clark, KB7QHC |
#76
|
|||
|
|||
"Ian White, G3SEK" wrote in message ...
How would you explain what Cecil wrote? Who else but Cecil would dare attempt that? :-) I can understand your fear... Do you think the series reactance a system offers a PA may actually improve it's incident power? For example, reducing the load impedance will usually make the output device operate in a more linear way... but the efficiency drops and the greater heat dissipation and current are likely to shorten the lifetime of the device. Is that an improvement? By improve, i mean increase the incident power. I think this is possible, and i've actually measured it, if you read my original post. If you can improve the incident power (tuning) of a PA with varying the coax length, you might be able to adjust the SWR too, in certain cases... though in my case, the SWR stayed about the same. Slick |
#77
|
|||
|
|||
I apologize for the discourtesy. It did indeed set a bad example.
Roy Lewallen, W7EL Richard Clark wrote: On Wed, 13 Aug 2003 16:13:13 -0700, Roy Lewallen wrote: Believe me, Ian, I know how frustrating it can get. But remember all the lurkers out there who benefit from your insightful postings. Please keep it up -- it is worth while. For their sake. Roy Lewallen, W7EL Hi Roy, It is nice of you to commend Ian to continue the good effort. Why didn't you do it directly instead of posting him through me? Are the lurkers to take some lesson by this breach of netiquette? :-) 73's Richard Clark, KB7QHC |
#78
|
|||
|
|||
Roy Lewallen wrote:
Dr. Slick wrote: I disagree on this point. You are caught up in the 50 Ohm world, which i admit is easy to do. The SWR is based on the ratio of the forward to the reflected power. That's not correct. The SWR (more correctly VSWR) is, by definition, the ratio of the highest to lowest voltages which appear on a line long enough to have both a maximum and minimum. It can be calculated from the forward and reverse voltage waves. ISWR, the current standing wave ratio, is numerically equal to the VSWR. For Dr. Slick: Knowing the forward and reflected powers, one can use the following equation to obtain SWR. 'Sqrt' means "square root of". SWR = [Sqrt(Pfwd)+Sqrt(Pref)]/[Sqrt(Pfwd)-Sqrt(Pref)] -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 80,000 Newsgroups - 16 Different Servers! =----- |
#79
|
|||
|
|||
Dr. Slick wrote:
So we never really measure the reflected power coming right out of the PA, even if we attach the meter directly to it's output. If the forward power out of a PA is 100 watts and the reflected power into the PA is 50 watts, the PA is generating 50 watts, *by definition*. Given that definition, the implication is clear. All PA's, by definition, must re-reflect 100% of the incident reflected energy. Thus, everything you are worried about has already been defined out of existence. :-) -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 80,000 Newsgroups - 16 Different Servers! =----- |
#80
|
|||
|
|||
Ralph Mowery wrote:
I have not seen a whole lot of equations on SWR but the few I have seen never mention the source at all. When someone defined the generated power as the forward power minus the reflected power, the entire problem was defined out of existence. :-) -- 73, Cecil http://www.qsl.net/w5dxp -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 80,000 Newsgroups - 16 Different Servers! =----- |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
50 Ohms "Real Resistive" impedance a Misnomer? | Antenna | |||
The Cecilian Gambit, a variation on the Galilean Defense revisited | Antenna |