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From: Tim Wescott on 12 Aug 2010 11:24 On 08/11/2010 09:15 PM, Vladimir Vassilevsky wrote: > > > Clay wrote: > > >> >> Hohlfeld & Cohen of course worked out the exact two requitrements for >> frequency independence. > > What is meant by "frequency independence" in this context? > I.e. what antenna parameters are frequency independent? > >> 1) You need self similarity >> >> 2) The feed point needs to be a point of symmetry of the antenna. >> >> Using just these two constraints and Maxwell's equations you can prove >> frequency independence. > > Wait a minite. If frequency independence is exact, it must subsist all > the way down to DC. How could it be for other then infinities or the > trivial cases? Lest you be called a name that ends in "ident", look up "approximation" in the dictionary. Most "frequency independent" antennas that I've seen are roughly so, over a range of frequencies, and the lower the range, the bigger the antenna. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html
From: Vladimir Vassilevsky on 12 Aug 2010 11:36 Tim Wescott wrote: > On 08/11/2010 09:15 PM, Vladimir Vassilevsky wrote: > >> >> >> Clay wrote: >> >> >>> >>> Hohlfeld & Cohen of course worked out the exact two requitrements for >>> frequency independence. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ >> >> What is meant by "frequency independence" in this context? >> I.e. what antenna parameters are frequency independent? >> >>> 1) You need self similarity >>> >>> 2) The feed point needs to be a point of symmetry of the antenna. >>> >>> Using just these two constraints and Maxwell's equations you can prove >>> frequency independence. >> >> >> Wait a minite. If frequency independence is exact, it must subsist all >> the way down to DC. How could it be for other then infinities or the >> trivial cases? > > > Lest you be called a name that ends in "ident", look up "approximation" > in the dictionary. > > Most "frequency independent" antennas that I've seen are roughly so, > over a range of frequencies, and the lower the range, the bigger the > antenna. Of course I am aware of wideband antennae, which are approximations of some kind. A common parabolic mirror would work anywhere from decimeter waves to UV. But Clay mentioned the EXACT solution, did he? Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Vladimir Vassilevsky on 12 Aug 2010 13:49
Vladimir Vassilevsky wrote: > > > Clay wrote: > > >> >> Hohlfeld & Cohen of course worked out the exact two requitrements for >> frequency independence. > > > What is meant by "frequency independence" in this context? > I.e. what antenna parameters are frequency independent? > >> 1) You need self similarity >> >> 2) The feed point needs to be a point of symmetry of the antenna. >> >> Using just these two constraints and Maxwell's equations you can prove >> frequency independence. > > > Wait a minite. If frequency independence is exact, it must subsist all > the way down to DC. How could it be for other then infinities or the > trivial cases? > > >> Details in this paper: >> SELF-SIMILARITY AND THE GEOMETRIC >> REQUIREMENTS FOR FREQUENCY >> INDEPENDENCE IN ANTENNAE >> >> ROBERT G. HOHLFELD & NATHAN COHEN >> > Could you post a link? Here is the article: http://www.mssu.edu/math/glathrom/fractal_antennae_paper.pdf They consider *infinitely long* structures which allows them to dismiss (1/s) factor on the both sides of the Maxwell equations. Devil is in the details, as usual. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com |