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From: fisico32 on 11 Aug 2010 16:18 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
From: Jerry Avins on 11 Aug 2010 16:34 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. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: glen herrmannsfeldt on 11 Aug 2010 17:04 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. Also, you mention the effect on a transmitting antenna. The same should be true for a receiving antenna, but it isn't so obvious. -- glen
From: Jerry Avins on 11 Aug 2010 17:25 On 8/11/2010 5:04 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. > > 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.) Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: Clay on 11 Aug 2010 17:41
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. Using just these two constraints and Maxwell's equations you can prove frequency independence. Details in this paper: SELF-SIMILARITY AND THE GEOMETRIC REQUIREMENTS FOR FREQUENCY INDEPENDENCE IN ANTENNAE ROBERT G. HOHLFELD & NATHAN COHEN |