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From: Jerry Avins on 12 Aug 2010 00:10 On 8/11/2010 11:13 PM, fisico32 wrote: ... > Could anyone please define self-similarity requirement? > I know what a fractal is. It is self-similar: its small scales look the > same as the large scales... > > But a wire antenna is a straight wire made of small straight > wires...however it is not self-similar....Why? The divisions between the straight segments is only imaginary, not salient. The EM waves can't figure out where one segment ends and the next begins. Jerry -- Engineering is the art of making what you want from things you can get.
From: Vladimir Vassilevsky on 12 Aug 2010 00:15 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? Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: glen herrmannsfeldt on 12 Aug 2010 01:24 Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: (snip) > 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? No need to go below about 7Hz, the resonant frequency of the earth. (and commonly used to communicate with submarines (very slowly)) On the upper end, there is VUV, vacuum ultraviolet, optical frequencies that won't go through air. -- glen
From: Steve Pope on 12 Aug 2010 02:42 glen herrmannsfeldt <gah(a)ugcs.caltech.edu> wrote: >A good source of detailed information about antenna design >as the ARRL handbook, available in many public libraries. I don't remember them covering log periodic designs, at least not in depth because they are too broadband to be of much advantage to amateurs. I did however see some log periodics up on top of the hill in Vatican City, along with a number of other of large antenna types, many of which I did not recognize their design. Imagine an infinite budget for long-range antennas for the purpose of assisting shadowy fringe groups globally... that's the Vatican antenna farm. Steve
From: Vladimir Vassilevsky on 12 Aug 2010 10:50
glen herrmannsfeldt wrote: > Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > (snip) > > >>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? > > No need to go below about 7Hz, the resonant frequency of the earth. > (and commonly used to communicate with submarines (very slowly)) If an analytical function is exactly constant at an interval, it should be constant everywhere. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com |