I believe Steve is onto something critical to this discussion - here is my 'take' on the differences -- Remember that we *are* talking about the resultant SWR indications on a mis-matched line.
The way most "wattmeters" indicate SWR is to detect the forward power and the reflected power - then an SWR is computed from those values of associated detector output voltages. The result is an SWR indication based on the absolute values of those magnitudes. No consideration is provided for the phase angles (the actual forward and reflected values are complex numbers). The forward power is indicated accurately (it is proportional to the square of the forward voltage minus the square of the reflected voltage), but the computation of actual SWR is more involved. When the impedances are close to the design point (balance point) of the meter, the error is small, but the error grows as the actual impedance departs from that design impedance. Those meters that properly detect the phase as well as the magnitude of the forward and reflected powers can indicate that the SWR does not change as the meter position is moved along the line - but most do not have phase detection capability nor complex number computation capability, so for those meters, the SWR indicated will change with the meter position along the feedline. In other words, use a good VNA and you should see a constant SWR along the line, but common wattmeters are not VNAs, so some error in SWR indication is to be expected when the impedance is removed from the design point. Even the well-respected Tandem Match computes the SWR as Vf+Vr/Vf-Vr, which is the correct formula, but the detector reports only the magnitudes of Vf and Vr and does not consider the phase angle, so it is not entirely correct either - it will be entirely correct when the SWR = 1.0. 73, Don W3FPR Steve Ellington wrote: > Nope: > The 1/4 wave line transforms the high impedance to a low one and the SWR > meter reads low. It's called a transmission line transformer and is very > common. It's the reason everyone is having trouble understanding why SWR > meters read differently. The ONLY way to compare them is to swap them with > each other. Putting them in series fouls up the readings for both meters. > Steve > N4LQ > [email protected] > ----- Original Message ----- > From: "Kok Chen" <[email protected]> > To: "Elecraft Reflector" <[email protected]> > Cc: "Steve Ellington" <[email protected]> > Sent: Wednesday, November 04, 2009 7:55 PM > Subject: Re: [Elecraft] K3 SWR Accuracy - reprise > > > >> On Nov 4, 2009, at 4:41 PM, Steve Ellington wrote: >> >> >>> 4. Example: A full wave dipole center fed with 50 ohm coax. SWR reads >>> infinite at the antenna but with 1/4 wavelenth of coax, SWR reads low! >>> >> Nope -- the *impedance* at the end of a 1/4 wave transmission line >> when it is looking at a very large impedance, is close to zero, >> therefore the SWR remains close to infinite. The SWR definitely won't >> read low unless there is something wrong with the instrument. >> >> 73 >> Chen, W7AY >> >> > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
