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## Re: [TowerTalk] wire antenna question

 To: TexasRF@aol.com Re: [TowerTalk] wire antenna question Blair S Balden Thu, 13 Nov 2008 14:05:46 -0500 mailto:towertalk@contesting.com>
 ```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 > > > > > In a message dated 11/13/2008 11:00:20 A.M. Central Standard Time, > > blair.balden@wmich.edu writes: > > Thanks to everyone who responded. I feel much better about what > I've been > doing. Terry, yours made such perfect sense, I thought I should > have known > it already! Because the dipole's resonant Z is usually higher > than 50 ohms, > getting minimum SWR would mean I'd cancelled out all the > reactance. If the > resonant Z were less than 50 ohms, I could get a lower SWR by > introducing a > reactive component. But, I suppose it would be rare to have a > dipole with a > resonant Z below 50 ohms. > > Thanks again to all, > 73, > Blair NP2F > > > > ----- Original Message ----- > From: Terry Conboy > Date: Sunday, November 9, 2008 10:54 pm > Subject: Re: [TowerTalk] wire antenna question > > > At 07:07 AM 2008-11-09, Blair S Balden NP2F wrote: > > >When I put up dipoles, whether they be "straight" or inverted > > >V's, I've always started out with the wire a little longer > than > > the > > >formula length of 468/f. Then I've trimmed for as close to a > 1:1 > > >match as I can get to my 50-ohm coax, as indicated by my SWR > > >meter. I can usually get a 1:1 SWR. > > > > > >These antennas usually work very well, but I realize that a > > resonant > > >dipole will not have exactly 50 ohms resistance. So, I must > be > > >cutting these antenna somewhat off resonance. I also realize > > that a > > >mismatch at the antenna (SWR of greater than 1) results in loss. > > > > > >What I'm not sure about is whether it would be better to get > the > > >antenna resonant at the desired frequency and accept the > > mismatch, > > >or whether I should continue to go for the best SWR > (minimizing > > >losses due to the mismatch). > > > > > >BTW, my rig does not have an internal antenna tuner, and I am > > >currently without an external tuner for it. So, I'm running > it > > >without a tuner (if this matters). Also, I have not been > using > > any > > >kind of matching device at the feedpoint, just direct > connection > > >from the coax connectors to the wires. > > > > > >If someone on here can give me some insight as to whether I > > should > > >try to go for resonance instead of low SWR, please respond. I > > > would > > >truly appreciate any help with the question. > > > > In your case, resonance and 1:1 SWR are exactly the same > > thing. Resonance just means that the antenna impedance is > purely > > resistive. Resonance isn't necessarily the point of highest > gain: > > many antennas actually have higher gain when they are much > longer > > than the resonant length (e.g. double-extended zepp). > > > > Indeed, a thin-wire straight (nearly) half-wave dipole in free > > space, > > or at certain heights, will have a resistive component of the > feed > > impedance of about 72 ohms. However, at your mounting height > and > > with your surrounding conductors, the resistive part of the > > antenna > > impedance is 50 ohms. This is actually pretty common, as you > > have > > discovered. (Because of the bending, V-shaped antennas > typically > > have a feed impedance under 72 ohms, even in free space.) > > > > Even if your antenna did have a feed impedance of 72 ohms at > > resonance, which would give a 1.44:1 SWR, you can only worsen > the > > SWR > > by changing the length of the wire, which mostly just adds or > > subtracts reactance. > > > > With a 50 ohm line, the ONLY way you can ever get a 1:1 SWR is > if > > the > > load Z is 50 +/- j0 ohms. (A Smith Chart would never lie.) > > > > In your situation, there really isn't anything else you can do > > with > > trimming to get higher efficiency. You already have "nirvana". > > > Life > > is good. Be happy. As they say in Seattle, "keep clam". > > > > 73, Terry N6RY > > > > > > > > _______________________________________________ > > > > > > > > _______________________________________________ > > TowerTalk mailing list > > TowerTalk@contesting.com > > http://lists.contesting.com/mailman/listinfo/towertalk > > > > _______________________________________________ > > > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk > > > **************Get the Moviefone Toolbar. Showtimes, theaters, > movie news & > more!(http://pr.atwola.com/promoclk/100000075x1212774565x1200812037/aol?redir=htt > p://toolbar.aol.com/moviefone/download.html?ncid=emlcntusdown00000001) > _______________________________________________ > > > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk > _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread [TowerTalk] wire antenna question, Blair S Balden Re: [TowerTalk] wire antenna question, Terry Conboy Re: [TowerTalk] wire antenna question, Blair S Balden Re: [TowerTalk] wire antenna question, Terry Conboy Re: [TowerTalk] wire antenna question, TexasRF Re: [TowerTalk] wire antenna question, Blair S Balden <= Re: [TowerTalk] wire antenna question, Terry Conboy Re: [TowerTalk] wire antenna question, Steve Hunt Re: [TowerTalk] wire antenna question, TexasRF Re: [TowerTalk] wire antenna question, Steve Hunt Re: [TowerTalk] wire antenna question, Steve Hunt Re: [TowerTalk] wire antenna question, Jim Brown Re: [TowerTalk] wire antenna question, Blair S Balden Re: [TowerTalk] wire antenna question, Bob Nielsen