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
frequency independent antennas....
Started by ●August 11, 2010
Reply by ●August 11, 20102010-08-11
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. �����������������������������������������������������������������������
Reply by ●August 11, 20102010-08-11
Jerry Avins <jya@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
Reply by ●August 11, 20102010-08-11
On 8/11/2010 5:04 PM, glen herrmannsfeldt wrote:> Jerry Avins<jya@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. �����������������������������������������������������������������������
Reply by ●August 11, 20102010-08-11
On Aug 11, 4:18=A0pm, "fisico32" <marcoscipioni1@n_o_s_p_a_m.gmail.com> wrote:> Hello Forum, > > a common wire antenna works the best when its size is equal to =A0half th=e> incident radiation wavelength. > That means that the physical size is a constraint in terms of the operati=ng> 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! > fisico32Hohlfeld & 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
Reply by ●August 11, 20102010-08-11
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
Reply by ●August 11, 20102010-08-11
On 08/11/2010 02:04 PM, glen herrmannsfeldt wrote:> Jerry Avins<jya@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
Reply by ●August 11, 20102010-08-11
On 08/11/2010 02:41 PM, Clay wrote:> On Aug 11, 4:18 pm, "fisico32"<marcoscipioni1@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
Reply by ●August 11, 20102010-08-11
Jerry Avins <jya@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
Reply by ●August 11, 20102010-08-11
Tim Wescott <tim@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
