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Hi Pete, thanks for the response. No, I do not have an impedance analyzer so I do not know the real and reactive components of the impedance. I agree that adding coax will change the impedance as seen by the transmitter, but if memory serves me, an ideal lossless line will move you concentric around the center of the smith chart which means no change in VSWR. A constant VSWR is a circle around the smith Chart. So while the impedance would change drastically the VSWR should remain the same. A real world transmisson line has loss but, 5 feet of loss is close to zero and in any case more loss would tend to make the VSWR better anyway. Loss makes the impedance spiral in toward the center of the chart (therefore closer to 50 ohms). I am still unable to explain what I see. Thanks again for your response. Rick > -----Original Message----- > From: Pete Smith [mailto:n4zr@contesting.com] > Sent: Sunday, June 18, 2000 9:10 AM > To: Rick Lehman > Subject: RE: [Force 12 Talk] !0 meter VSWR > > > At 07:11 AM 6/18/00 -0400, you wrote: > >I have a C19XR with a high VSWR (when measured at the > transmitter end of the > >coax) on 10 meters and do not use an LP filter. It was > suggested by force > >12 that I add about 5 feet of coax. I did that and to my amazement it > >worked. I have been trying to understand why this would work > and have not > >yet come up with a technical explanation (Force 12 cannot explain it > >either). With an ideal line, the length of coax should not make a > >difference on the VSWR seen at the transmitter. It instead would only > >affect the phase angle from a Smith Chart standpoint. From a real world > >lossy line standpoint, 5 feet of coax is not long enough to cause any > >appreciable loss or effect on the VSWR. My first thought was > that the force > >12 balun was bad as the symptoms suggest that the coax was functioning as > >part of the antenna. However, I removed the 5 feet of coax, > and replaced > >the balun with a current balun from Hygain. The results were the same as > >with the force 12 balun... Therefore I have ruled out this > possibility. My > >coax is new, so I am for the time being ruling out this possibility. The > >antenna seems to work fine on 10 meters, but I am bothered that I do not > >understand this phenomena. Any ideas? > > Hi Rick -- It seems to me that the feedpoint of the antenna is some > impedance other than 50+j0. That being the case, any length of feedline > will transform the feedpoint impedance to something else, which > may be seen > by your SWR bridge at the other end of the line as a higher or lower VSWR. > Your 5 feet is ~ 90 degrees at 10 meters, or almost the maximum > transformation (I think) that can be done this way. Probably > your previous > length was "just wrong" in terms of the impedance presented at the rig. > > This being said, I don't quite understand what the deal is - the impedance > at the feed of the C19 should be close enough to 50 ohms so this doesn't > happen. Did you happen to measure the R+/-j at the feedpoint? > > > 73, Pete Smith N4ZR > n4zr@contesting.com > > > -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force 12 Web site: http://www.qth.com/force12 Submissions: send to Force12Talk@qth.com To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk For problems with the list, contact force12@qth.com |
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