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## Re: [TowerTalk] wire antenna question

 To: Blair S Balden Re: [TowerTalk] wire antenna question Terry Conboy Thu, 13 Nov 2008 16:01:35 -0800 mailto:towertalk@contesting.com>
 ```Blair is right that a 40+j10 ohm load has an impedance magnitude is 41.23 ohms, but it has a vector angle of 14 degrees. The SWR is only the simple ratio of the impedances when the vector angle is 0 degrees, i.e. the impedance is purely resistive. With normal RF transmission lines, you can never have a 1:1 SWR when the load Z is reactive. No kidding. The formulas for the calculation when the load Z is reactive are a pain due to the imaginary numbers involved. This page explains it all: http://www.microwaves101.com/encyclopedia/vswr.cfm (The good stuff is at the bottom.) However, it's much simpler to use one of the programs like ARRL's TLW by N6BV or this online calculator from W9CF: http://fermi.la.asu.edu/w9cf/tran/ 73, Terry N6RY At 11:05 AM 2008-11-13, Blair S Balden wrote: >Hi Gerald, >I guess this is something I'm not clear on. By my calculations, an >antenna having an impedance of 40+j10 or 40-j10 would have 41.23 ohms. >If SWR is the ratio of the two impedances (coax and load), then 50 >ohms / 41.23 ohms would give an SWR of 1.21. It would take 30 ohms >of reactance combined with 40 ohms of resistance (40+j30 or 40-j30) >to give 50 ohms. So, it would seem to me that the >40-ohms-at-resonance antenna would need to have 30 ohms of reactance >combined with it in order to achieve an SWR of 1. >At least, that is what I would expect. Am I missing something in my thinking? > >Thanks, >Blair, NP2F > > > >----- Original Message ----- >From: TexasRF@aol.com >Date: Thursday, November 13, 2008 12:33 pm >Subject: Re: [TowerTalk] wire antenna question > > > Blair, that is not quite right. Using a 50 ohm swr indicator, the > > lowest swr > > is shown when the reactance is zero. > > > > Using a transmission line program, 40 ohm load 40+j0; swr=1.25. > > 40+j10=1.37; > > 40-j10=1.37. > > > > So, when you tweak your dipole for the lowest vswr at a given > > frequency, you > > are in essence resonating the antenna at that frequency. > > > > 73, > > Gerald K5GW _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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