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## Re: [TowerTalk] Feedpoint Impedance

 To: "Towertalk Reflector" Re: [TowerTalk] Feedpoint Impedance "Ward Silver" Ward Silver Mon, 28 Jul 2008 23:26:39 -0700 mailto:towertalk@contesting.com>
 ```> 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 :-) Thanks. 73, Ward N0AX ----- Original Message ----- From: "Jim Lux" To: "Ward Silver" ; "Towertalk Reflector" Sent: Monday, July 28, 2008 4:16 PM Subject: Re: [TowerTalk] Feedpoint Impedance > > > -----Original Message----- >>From: Ward Silver >>Sent: Jul 28, 2008 12:46 PM >>To: Towertalk Reflector >>Subject: [TowerTalk] Feedpoint Impedance >> >>While we're on the math subject, does anyone know of an equation that >>gives >>either feedpoint impedance or radiation resistance as a function of >>position >>along a linear radiator of known electrical length? Free space would be >>fine, I can work in the effects of ground later. > > Radiation resistance should be fixed.. it's a function of the size and > shape of the radiator. Feedpoint impedance will change with position, of > course (essentially, it's some transformation of the combination of > radiation resistance, loss resistance and the reactance of the antenna) > > I don't think there's a nice analytical expression for it. One could > start with the Schelkunoff formulation (the one using sine and cosine > integrals, which are series expansions). Do you need the reactive part? > If not, you could assume that the current has a sinusoidal distribution, > and once you know the radiation resistance, the feedpoint R at the center > will be the Rrad+Rloss.. Moving away from the center, it would scale as > 1/cos(distance), becoming infinite at the very end. > > Orfanidis's online book might be a good source.. he has equations for the > current at various places along the element, and that's really what you're > looking for. http://www.ece.rutgers.edu/~orfanidi/ewa/ Chapter 16 or 22, > I suspect. > > > Practically, I've done this kind of thing with NEC.. run a series of cases > with systematic variation in the position, generate a table, and then do > interpolation. > > Jim, W6RMK _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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