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Scott Stephens
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 Posted: Fri Oct 31, 2008 2:27 pm Post subject: What>s the step-response of an antenna like? |
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Since antennas have reactance and radiation resistance, are they always
critically damped, or will they ring-down?
Is this implied by a swr plot? Can I take 3db points as antenna
bandwidth and assume a radiation-resistance loaded-Q from that?
Does the feed-point used affect radiation resistance and step-response?
Thanks |
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Benj
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| Posted: Fri Oct 31, 2008 5:34 pm Post subject: Re: What>s the step-response of an antenna like? |
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On Oct 31, 4:27 am, Scott Stephens <obnox...@sbcglobal.net> wrote:
[quote]Since antennas have reactance and radiation resistance, are they always
critically damped, or will they ring-down?
Is this implied by a swr plot? Can I take 3db points as antenna
bandwidth and assume a radiation-resistance loaded-Q from that?
Does the feed-point used affect radiation resistance and step-response?
[/quote]
So many questions!
There is nothing that says antennas must be critically damped. That
depends on the Q of the circuit which is defined by the ratio of the
stored energy to the dissipated energy which in the case of an antenna
includes the energy lost in transmission which is symbolized as
radiation resistance. Basically it all depends on the geometry of the
antenna.
And SWR plot on the other hand relates to matching of the antenna to
it>s transmission line. That of course relates to relation between the
load on the end of the line (antenna) and the characteristics of the
line itself. So yes, SWR does reflect the antenna parameters to a
degree, but it>s not really related directly to antenna electrical
properties. It>s actually related to antenna properties seen at the
feed point.
Although obviously feed point properties change with the feed point,
the antenna properties are dependent upon it>s geometry. If whatever
feed point you choose is properly matched with an appropriate line,
then no, antenna properties are not affected by the feed point. Of
course, those properties as SEEN at the feed point are dependent upon
the feed point. Does that make sense?
As for "step response", this treats an antenna as a filter. If one
considers the transform of a step function into the frequency domain,
you can see that the bandwidth of the antenna filters that frequency
domain step to give the response to it. If only the far field is
considered, then the transmitting characteristics of the step are
found. OK? |
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Scott Stephens
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| Posted: Fri Oct 31, 2008 11:52 pm Post subject: Re: What>s the step-response of an antenna like? |
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Benj wrote:
[quote]Of course, those properties as SEEN at the feed point are dependent upon
the feed point. Does that make sense?
[/quote]
Yes, certainly. I>ve seen how different antennas use different methods
to match the transmission line to the antenna to effect an impedance match.
[quote]As for "step response", this treats an antenna as a filter. If one
considers the transform of a step function into the frequency domain,
you can see that the bandwidth of the antenna filters that frequency
domain step to give the response to it. If only the far field is
considered, then the transmitting characteristics of the step are
found. OK?
[/quote]
I>m interested in TDR/Radar-like applications, like Ground-penetrating
radar. I could put a transistor on the antenna feedpoint itself.
Ideally, I transmit a simple pulse. But if the antenna behaves as a
filter, as you say, (I suspect a yagi or log-periodic would!) the
antenna will transmit an exponentially decaying sinusoid, not a
critically damped pulse.
I see UWB transmitters using horn-like structures, and loops of wide
metal strap.
An Xnec2c EM MOM simulation can give me a plot of real and complex Z vs.
Frequency. I want a plot of the time-domain, not the frequency domain.
Perhaps an inverse-Fourier transform?
How about a spice-model for an antenna? I>ll start googling around for
that... |
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funny.haha
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| Posted: Tue Nov 04, 2008 8:21 am Post subject: Re: What>s the step-response of an antenna like? |
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Scott Stephens wrote:
[quote]Benj wrote:
Of course, those properties as SEEN at the feed point are dependent upon
the feed point. Does that make sense?
Yes, certainly. I>ve seen how different antennas use different methods
to match the transmission line to the antenna to effect an impedance match.
As for "step response", this treats an antenna as a filter. If one
considers the transform of a step function into the frequency domain,
you can see that the bandwidth of the antenna filters that frequency
domain step to give the response to it. If only the far field is
considered, then the transmitting characteristics of the step are
found. OK?
I>m interested in TDR/Radar-like applications, like Ground-penetrating
radar. I could put a transistor on the antenna feedpoint itself.
Ideally, I transmit a simple pulse. But if the antenna behaves as a
filter, as you say, (I suspect a yagi or log-periodic would!) the
antenna will transmit an exponentially decaying sinusoid, not a
critically damped pulse.
[/quote]
If you are talking about the impulse response of a narrowband antenna,
it will be under damped, but not critically damped.
[quote]
I see UWB transmitters using horn-like structures, and loops of wide
metal strap.
An Xnec2c EM MOM simulation can give me a plot of real and complex Z vs.
Frequency. I want a plot of the time-domain, not the frequency domain.
Perhaps an inverse-Fourier transform?
[/quote]
The field simulator can give you the impedance of the antenna. If you
IFT, you will get the impulse response of the antenna.
[quote]
How about a spice-model for an antenna? I>ll start googling around for
that...
[/quote]
THese are my two cents. |
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Benj
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| Posted: Tue Nov 04, 2008 5:05 pm Post subject: Re: What>s the step-response of an antenna like? |
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On Nov 4, 12:48 am, "funny.haha" <funny.h...@live.ca> wrote:
[quote]If you are talking about the impulse response of a narrowband antenna,
it will be under damped, but not critically damped.
[/quote]
Of course the bandwidth of any given antenna is determined by it>s
geometric structure. Hence damping is determined by that as well (as I
said previously).
[quote]The field simulator can give you the impedance of the antenna. If you
IFT, you will get the impulse response of the antenna.
[/quote]
And since there is a simple relationship between the impulse response
and the step response you can use one to find the other. |
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extremesoundandlight@yaho
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| Posted: Fri Nov 07, 2008 5:11 pm Post subject: Re: What>s the step-response of an antenna like? |
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On Nov 4, 10:05 am, Benj <bjac...@iwaynet.net> wrote:
[quote]On Nov 4, 12:48 am, "funny.haha" <funny.h...@live.ca> wrote:
If you are talking about the impulse response of a narrowband antenna,
it will be under damped, but not critically damped.
Of course the bandwidth of any given antenna is determined by it>s
geometric structure. Hence damping is determined by that as well (as I
said previously).
The field simulator can give you the impedance of the antenna. If you
IFT, you will get the impulse response of the antenna.
And since there is a simple relationship between the impulse response
and the step response you can use one to find the other.
[/quote]
Time domain reflectometry (TDR) looks at the step response of a
system.
Some people say that the pulse response of a system can be obtained
from the step response by delaying a second copy of the step response
one unit interval of time, then subtracting from the step response,
This is almost true.
Why, you ask, is it 'almost' true? Well, it would be true if the
driver had the same strength, edge rate, and so on, in both the
positive and negative directions. In a real application, it makes
sense to account for these differences. But for the passive part of
the interconnect itself, the assertion will be true. |
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Benj
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| Posted: Sat Nov 08, 2008 6:02 am Post subject: Re: What>s the step-response of an antenna like? |
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On Nov 7, 12:11 pm, "extremesoundandli...@yahoo.com"
<extremesoundandli...@yahoo.com> wrote:
[quote]Time domain reflectometry (TDR) looks at the step response of a
system.
Some people say that the pulse response of a system can be obtained
from the step response by delaying a second copy of the step response
one unit interval of time, then subtracting from the step response,
This is almost true.
[/quote]
Correct. I should have said "easily mathematically obtained". A true
impulse response can never be obtained because a true impulse does not
exist. All one can get is approximations, which usually tell you all
you want to know.
Once again reality is more real than mathematics. |
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