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## [Towertalk] 468/f - don't bother, here's why ARRL droppedit.

 To: [Towertalk] 468/f - don't bother, here's why ARRL droppedit. ve7hcb@rac.ca (Chris BONDE) Tue, 25 Jun 2002 21:09:16 -0700
 ```So, all we are doing is using more sofisticated methods to arrive closer to what some think is the correct. (The reason that I say such is that I have read discussions on the correctness and refining of the various modelling programs.) Now, I would like to be able to do all the modelling and am trying to move in that direction. However, the formula certainly gives a very good approximation, especially if one wants to find out if there is enough room to put up the atnenna. Chris opr VE7HCB At 10:40 PM 2002-06-25 -0400, Guy Olinger, K2AV wrote: >This is just so folks know what the dipole length = 468/f business is >about. > >Note below the wild variation in resonant frequency, impedance, and >SWR limits, just by changing height. > >The formula just doesn't come real close except in a few spots, >notably 30 feet up, maybe the height of the infamous Newington alley >dipole? The table is why ARRL quit publishing the formula. > >Pick 40 meters. 468/7.150 is 65.4545 feet. EZNEC 3, High accuracy >ground. #12 wire. 15 segments. Use fixed font to view table. > >Height Z @ 7.15 SWR50 SWR75 Z @ F RES 1.8:1 SWR @ Z=75 > >20' 50.6 - j 6.2 1.13 1.50 51.4 @ 7.177 7.052-7.309 >30' 74.8 - j 9.1 1.53 1.13 75.9 @ 7.191 7.003-7.398 >40' 87.0 - j 24.9 1.94 1.41 91.5 @ 7.267 7.132-7.355 >50' 84.8 - j 41.7 2.25 1.70 90.4 @ 7.352 7.129-7.575 >60' 73.1 - j 50.0 2.42 1.94 77.2 @ 7.386 7.178-7.602 >70' 60.4 - j 47.4 2.35 2.06 63.7 @ 7.361 7.193-7.544 >80' 55.4 - j 37.2 2.01 1.90 59.2 @ 7.310 7.167-7.480 >90' 59.5 - j 27.8 1.70 1.60 63.7 @ 7.270 7.116-7.459 >100' 63.4 - j 25.4 1.70 1.44 73.1 @ 7.265 7.081-7.478 > >I am a bit mystified that some are apologetic for this formula >(468/f). > >It is only accurate at one height, 30'. The resonant center of the >antenna is either out of band or near the band edge above that height. >The 75 ohm SWR varies from near unity to over 2:1 at the design >frequency. The range of impedances at resonance is almost two to one. > >If a 40 meter Force 12 antenna had a resonance center at 7361, a SWR >of 2 at the design center, and N6BT tried to defend it, you'd be after >him with tar and feathers. I've seen hugely critical posts for a >fraction of this variance. > >This should show the danger of extrapolating small or single sets of >old data. Or for that matter trying to simplify something naturally >complex. It really takes a height table to do a cut/solder/string it >up dipole. > >And for how long did 468/f rest resplendent in the books, >unquestioned, until Joe Common Ham got his mits on antenna modeling >software? > >Arrival angles, guy wire anti-resonance lengths, beware. > >73, Guy. > >----- Original Message ----- >From: "Joe Reisert" >To: "Guy Olinger, K2AV" ; "Bill Hider (N3RR)" >; "tongaloa" ; > >Sent: Tuesday, June 25, 2002 11:18 AM >Subject: Re: [Towertalk] Guy wire resonance. Break it up or use stubs? > > > > Guy, > > > > I think if you look back a few years (perhaps 20-25), the whole >theory of > > guy resonances was first mentioned and explored by the late Jim >Lawson, > > W2PV, and I think he provided the info that is now in the ARRL >references. > > Furthermore, none of the so called WARC Bands (12, 17 and 30 meters) >were > > in existence at that time. > > > > As for the 468/F formula, that is for a half wave wire with >insulators on > > the each end. the assumption here is that the insulators shorten the >wire > > by 5%. No magic but a loose approximation for the simplest cases as >we all > > now know. > > > > > >_______________________________________________ >Towertalk mailing list >Towertalk@contesting.com >http://lists.contesting.com/mailman/listinfo/towertalk ```
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