Hi Peter, I believe your statement that SWR is affected by the generator's impedance is a common misconception. Actually, SWR, as is typically defined, is independent of generator (i.e. transmitter, in our case) impedance. It is only a function of the impedance of the transmission line and the load impedance. In other words, if the transmission line Z is 50 ohms and the load is 50 ohms (resistive), the SWR should be 1:1, irrespective of the transmitter's impedance.
In your example of a transmitter with an output Z of 100 ohms, indeed, you are correct that it isn't matched to transmission line, but this will have no effect on the SWR (it only affects the power that the transmitter is able to provide to the transmission line): if the load is matched to the line, there are no reflections from the load end, and thus there are no standing waves, and therefore the result is an swr of 1:1. Best regards, - Jeff, K6JCA -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of [EMAIL PROTECTED] Sent: Sunday, January 28, 2007 7:23 AM To: Jim Lux; k5nu; FlexRadio@flex-radio.biz Subject: Re: [Flexradio] sdr swr Jim, see in text 73 (sorry no spell checker on this system.......) groeten Peter petervn(a)hetnet.nl <mailto:[EMAIL PROTECTED]> ; pa0pvn(a)hetnet.nl <mailto:[EMAIL PROTECTED]> ; pa0pvn(a)gmail.com ; pa0pvn(a)amsat.org . ________________________________ From: Jim Lux [mailto:[EMAIL PROTECTED] Sent: Sun 28-1-2007 15:55 To: [EMAIL PROTECTED]; k5nu; FlexRadio@flex-radio.biz Subject: Re: [Flexradio] sdr swr At 03:08 AM 1/28/2007, [EMAIL PROTECTED] wrote: >Mike > > > >What is still wondering me the most is that when the SWR is close to >1, the transmitter impedance > >should not have any influence on the SWR. Actually, no.. think about this... what if the Transmitter output Z were 100 ohms... You'd still see a mismatch. In the simplistic sort of view, a 50 ohm line hooked to a 50 ohm load presents a 50 ohm impedance at the other end of the line, so you'd think that the mismatch localized at that point. BUT, say you broke that line half way, and put the meter there. Looking back towards the transmitter, you see 100 ohms, with some length of 50 ohm line (i.e. probably something other than 100 ohms, and almost certainly NOT 50 ohms), looking towards the load, you see 50 ohms. So the mismatch is apparent no matter where along the line you look. If there in no power coming back then you do not see the impedance of the transmitter. Most SWR meters, also, make the assumption that they are in a 50 ohm line. That is, they don't actually measure the voltage along the line at several points and calculate the standing wave ratio in a "Z independent" way. (they're not a slotted line with a probe...) Therefore, while the can measure SWR (which is pretty much the same everywhere in the line) they can't measure absolute Z very accurately (that is, they can't tell you WHERE the mismatch is) nor can they accurately measure the forward and reflected power separately.. As far as I know most SWR meters are in fact 2 directional couplers, and are working pretty well. To measure without bothering to much over the tranmitter impedance one can put a -3 or -6 dB Power attenuator between the Transmitter and the SWR meter. I found that this is verry usefull if you are mesuring in high SWR conditions. Then the tramsmitter impedance is working with the reflected power. I always think of an endless coax in an properly terminated system. >All the energy coming from the transmitter is absorbed > >in the antenna system, nothing comes back. The transmitter sees 50 ohm. > > > > 50 ohm ->-- coax-----SWRmeter---------coax-----------antenna(or > dummy), all 50 ohm's > > > >If the transmitter does not like this 50 ohms does not matter for >the SWR, might however cook > >the transmitter. The transmitter of the SDR is build for a 50 ohm load..... > > > >Keep in mind that the output impedance of a transmitter is almost >never 50 ohm, but the > >transmitter is build to operate efficiently with a 50 ohm load. An excellent point Peter makes here. Say the transmitter had a zero output impedance (i.e. it was a stiff voltage source, like a high quality audio amplifier). It would drive the 50 ohm load just fine, although the transmitter "efficiency" might vary quite a bit, depending on the design. But, in such a system, an external power or VSWR meter is not going to give results that look the same as in a system with 50 ohm outs and ins. All the Thevenin match criteria says is that if source and load Z are conjugates, you get maximum *net* power transfer. (consider a source with 50+50j Z and a load with 50-50j... complex conjugates.. the 50j on the source cancels the -50j on the load..... so this transfers power from generator to sink most efficiently.. but there's "reactive power" circulating back and forth between the two.. While it's theoretically the same 100% efficiency as a 50+0j to 50+0j, in a real system with IR losses in the connection, the reactive termination is worse) Good point Jim, reactive part of the impedance can put quite a bit of sand in the system. Half of the working of antenna tuners is to campensate for that. Idially the tuner should be at the arial connection, now you get 1/4 labda and 1/2 labda effects of the coax on the reactive parts as well. Jim, W6RMK -------------- next part -------------- An HTML attachment was scrubbed... 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