> -----Original Message----- > From: Repeater-Builder@yahoogroups.com [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Bob Dengler > Sent: Thursday, August 30, 2007 12:53 PM > To: Repeater-Builder@yahoogroups.com > Subject: RE: [Repeater-Builder] Duplexers > > At 8/29/2007 06:40 PM, you wrote: > > > > >If you have two watt meters and an antenna matching device you can > put > > > one > > > >wattmeter between the transmitter and the matching device and tune it > for > > > >minimum reflected power on the first meter. Then with a second meter > > > >between the tuner and the mismatched load you can see the second > > > wattmeter > > > >that is reading the reflected power. The second wattmeter will have a > > > >higher forward power reading than the first due to the added re- > reflected > > > >power. > > > > > > This doesn't sound right either, as there should be no reflected power > at > > > the antenna if it's been matched further down the line. The tuner > would > > > be > > > adjusted so as to create a conjugate impedance of the antenna at the > end > > > of > > > the feeding coax, thus eliminating the mismatch. > > > > > > My guess is that the higher power reading on the wattmeter is due to > the > > > weird impedances it's seeing on both its input & output. > > > > > > Bob NO6B > > > > > > >Hi Bob, > > > >Please read again what I wrote. I am not sure that you are following how > the > >meters are in the circuit. Remember that whatever you do at the > transmitter > >end of a transmission line has no affect on what is going on in the line > >itself. The only thing that will change the swr on the line is what you > do > >at the load. > > > >73 > >Gary K4FMX > > OK, after talking to a senior RF engineer at lunch here at work I think I > understand what's going on. The part that threw me was having the > matching > circuit in the middle of the feedline & the fact that any reflected power > from the load MUST be totally re-reflected back by the matching circuit, > otherwise there would be power reflected back to the TX, which by > definition does not occur in this example. Because of the multiple > re-reflections between the matching circuit & load resulting in multiple > waves back & forth within that coax section, typical single-wave thinking > doesn't apply. > > I guess it's a useful way to illustrate why coax gets lossier if you use a > tuner far from the antenna. > > Bob NO6B
Yes coax is lossier with reflected power on it. The part of the power that gets reflected from the antenna back toward the transmitter gets attenuated a second time by whatever lose the cable has to begin with. Then when that portion of the power gets re-reflected at the transmitter end and is on its way back to the antenna again it suffers attenuation a 3rd time by the coax so all the re-reflected power does not make it back to the antenna. Then a portion of the re-reflected power gets reflected again back toward the transmitter along with the new wave of power. This repeats itself again and again adding to the loss but after a few round trips of bouncing up and down the feed line most of it gets radiated and some has gotten attenuated to a miniscule amount. Of course this process is continuously repeated as power is constantly applied from the transmitter. But my reason for using the first wattmeter and the tuner was to have a nearly perfect flat load on the transmitter so that one could see the true power coming out of the transmitter. The second wattmeter after the tuner will then show the higher power which would be the sum of the forward and re-reflected power so it could be seen that reflected power does indeed get re-reflected at the transmitter (in this case at the tuner)and makes its way back to the antenna. In common applications with just a single wattmeter and no tuner involved where there is reflected power on the line the wattmeter in the forward position will show the forward power plus the re-reflected power. To find power out of the transmitter you would subtract the reflected power shown in the reverse position from the indicated forward power on the meter. This works over a wide range of impedances with a bird wattmeter. 73 Gary K4FMX
