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From: Tim Wescott on 11 Aug 2010 19:10 On 08/11/2010 01:34 PM, Jerry Avins wrote: > On 8/11/2010 4:18 PM, fisico32 wrote: >> Hello Forum, >> >> a common wire antenna works the best when its size is equal to half the >> incident radiation wavelength. >> That means that the physical size is a constraint in terms of the >> operating >> bandwidth and performance. >> >> There are frequency independent antennas that have a finite size but can >> work on a much larger bandwidth...How can they do that? >> Those antennas are self-scaling....in what sense? >> The flat spiral antenna is an example. >> >> Does it means that by simple rotating the antenna we get the smaller and >> larger scale of the antenna itself? So what? >> >> A wire antenna that is 1 meter long is also made of smaller straight wire >> sections which are smaller versions of the antenna, but that is not a >> frequency independent antenna.... >> thanks! > > Broadband antennas are self-similar on different scales, much like > fractal patterns. One example is the log-periodic antenna, described in > http://www.radio-electronics.com/info/antennas/log_p/log_periodic.php > A long bow-tie is broadband also. You can see that when you realize that > the part of the antenna beyond where most of the energy has radiated > away doesn't count. In effect, these antennas adjust their effective > length to match the RF wavelength. I believe that the quoted log spiral falls into that "self-similar" set, as does the discone antenna (which is basically a dizzy bowtie). -- 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: Tim Wescott on 11 Aug 2010 19:11 On 08/11/2010 02:04 PM, glen herrmannsfeldt wrote: > Jerry Avins<jya(a)ieee.org> wrote: >> On 8/11/2010 4:18 PM, fisico32 wrote: > >>> a common wire antenna works the best when its size is equal to >>> half the incident radiation wavelength. > (snip) >>> There are frequency independent antennas that have a finite >>> size but can work on a much larger bandwidth...How can they do that? > (snip) > >> Broadband antennas are self-similar on different scales, much like >> fractal patterns. One example is the log-periodic antenna, described in >> http://www.radio-electronics.com/info/antennas/log_p/log_periodic.php >> A long bow-tie is broadband also. You can see that when you realize that >> the part of the antenna beyond where most of the energy has radiated >> away doesn't count. In effect, these antennas adjust their effective >> length to match the RF wavelength. > > I hadn't thought of it that way before. Though the range of the > whole UHF TV band now (with higher channels removed) is less than > a factor of 2:1. The VHF TV channels cover a range from 54MHz > to 216MHz, a factor of 4:1. > > A low-Q UHF antenna near the middle of the band won't be so far > off on either end. A log-periodic array will have much more consistent directional properties, though. -- 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: Tim Wescott on 11 Aug 2010 19:12 On 08/11/2010 02:41 PM, Clay wrote: > On Aug 11, 4:18 pm, "fisico32"<marcoscipioni1(a)n_o_s_p_a_m.gmail.com> > wrote: >> Hello Forum, >> >> a common wire antenna works the best when its size is equal to half the >> incident radiation wavelength. >> That means that the physical size is a constraint in terms of the operating >> bandwidth and performance. >> >> There are frequency independent antennas that have a finite size but can >> work on a much larger bandwidth...How can they do that? >> Those antennas are self-scaling....in what sense? >> The flat spiral antenna is an example. >> >> Does it means that by simple rotating the antenna we get the smaller and >> larger scale of the antenna itself? So what? >> >> A wire antenna that is 1 meter long is also made of smaller straight wire >> sections which are smaller versions of the antenna, but that is not a >> frequency independent antenna.... >> thanks! >> fisico32 > > Hohlfeld& Cohen of course worked out the exact two requitrements for > frequency independence. > > 1) You need self similarity > > 2) The feed point needs to be a point of symmetry of the antenna. Is that "can only be if 1 and 2 are satisfied", or is that "is true if and only if 1 and 2 are satisfied"? -- 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: glen herrmannsfeldt on 11 Aug 2010 19:32 Jerry Avins <jya(a)ieee.org> wrote: (snip, I wrote) >> Also, you mention the effect on a transmitting antenna. >> The same should be true for a receiving antenna, but it isn't >> so obvious. > I agree not so obvious on its face, but the reciprocity theorem saves > the day. (The reciprocity theorem follows from the second law of > thermodynamics. I believe it.) I remember in a frosh class someone trying an example of a parially reflecting ellipsoid such that it would violate reciprocity, focussing more light on one side than the other. -- glen
From: glen herrmannsfeldt on 11 Aug 2010 19:35
Tim Wescott <tim(a)seemywebsite.com> wrote: (snip, I wrote) >> I hadn't thought of it that way before. Though the range of the >> whole UHF TV band now (with higher channels removed) is less than >> a factor of 2:1. The VHF TV channels cover a range from 54MHz >> to 216MHz, a factor of 4:1. >> A low-Q UHF antenna near the middle of the band won't be so far >> off on either end. > A log-periodic array will have much more consistent directional > properties, though. How about a two-dimensional array of bow-ties? Usually only two across, but one could do more than that. I believe I used to know some that were sold with optional cross-coupler such that you could connect them in parallel. (The spacing carefully chosen to impedance match the combination.) -- glen |