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Re: [TowerTalk] Feedpoint Impedance

 To: "Towertalk Reflector" Re: [TowerTalk] Feedpoint Impedance "Ward Silver" Ward Silver Tue, 29 Jul 2008 21:24:22 -0700 mailto:towertalk@contesting.com>
 ```Elliptic integrals would be preferable to watching the Seattle Mariners blow another ninth-inning lead :-) Thanks, Jim - this is going to get me close enough! 73, Ward N0AX ----- Original Message ----- From: "jimlux" To: "Ward Silver" Cc: "Towertalk Reflector" Sent: Tuesday, July 29, 2008 10:58 AM Subject: Re: [TowerTalk] Feedpoint Impedance > Ward Silver wrote: >>>>> Radiation resistance should be fixed.. >>>> Oh, right...duh. >>>> >>>> The current equation will do - but the radiator (I may have confused >>>> things by referring to it as a "dipole") may be anywhere between >>>> one-half and one wavelength long, so the current won't be a simple >>>> cosine function. It *will* be zero at the ends :-) >>> >>> I think Orfanidis's book has the equation you're looking for.. actually >>> several approximations, of varying fidelity.. >>> >>> I'm pretty sure there's some standard assumptions of the current >>> distribution shape (ranging from uniform for very short, to triangular >>> to sinusoidal to something else), possibly broken up into segments. >>> >>> What sort of accuracy are you looking for? >> >> 20% ought to do it. There will be significant variation depending on >> proximity of ground, type of ground, and so forth. At this point, I'm >> just doing a feasibility study. >> >> 73, Ward N0AX > Here you go... > > Current on a thin wire of length 2*h > k is propagation constant (i.e. pi gives you a half wavelength dipole) > z is the distance from the center > I(0) is the current at the center > > I(z) = I(0) * sin( k *(h-abs(z)))/sin(k*h) > > (Eq 21.4.2 from Orfanidis's book) > > Seems to match (by eye) what I got from a series of NEC models.. > > I haven't looked into the off center fed aspect...This is just the current > distribution on a center fed wire. > > Later pages in Chapter 21 of the book talk about King's three term > approximation (which works up to about 1.25 lambda). Figure 21.6.1 shows > a comparison between sinusoidal, King and numerical integration. > > If you want real pain, he goes on to work out an exact kernel for the > solution of Pocklington's equation using, why yes, elliptic integrals. > > (I'd just use his canned Matlab routines....) > > > Jim, W6RMK _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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