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 feed-point impedance change radiation resistance?

Thanks

Scott Stephens wrote:
Since antennas have reactance and radiation resistance, ...

It might help to note that the antenna feedpoint
impedance is a virtual impedance - a ratio of total
voltage to total current. A dipole is a standing
wave antenna with a feedpoint impedance of:

Zfp = (Vfor+Vref)/(Ifor+Iref) [all phasors]

This type of impedance is covered by the (B) definition
of "impedance" in "The IEEE Dictionary". It is not the
same thing as an "impedor" which is covered by the
(C) definition of "impedance".
--
73, Cecil http://www.w5dxp.com

On 31 okt, 10:17, Scott Stephens wrote:
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 feed-point impedance change radiation resistance?

Thanks

Hello,

A small tuned loop behaves as a lumped RLC circuit around the center
frequency. Small means small w.r.t. lambda. It has small relative
BW (rel. BW = useful BW / Fcenter). It shows lots of ringing. Of
course, the model is only valid for the frequency range where size
lambda.

For small loops (especially loops on PCB), most losses are ohmic, just
a part of the losses is due to radiation. When you make a good air
loop (with air or vacuum capacitor), the Q factor can be over 1000 (so
your -3dB impedance bandwidth is very low at HF frequencies). Your
VSWR=2 useful bandwidth is about 70% of the 3 dB impedance bandwidth
of the antenna.

Thin half wave dipoles can also be modeled with a RLC circuit. The R
is frequency dependent, but in a limited frequency range, a simple
RLC circuit is useful. When more accuracy is required , or larger
frequency range, a transmission line model with lumped losses is
better. HW dipoles close to perfect conducting ground have narrow
useful BW, hence high Q factor and the RLC model matches better.

Thicker dipoles have wider bandwidth (so lower Q factor). In that
case even within the useful frequency range the radiation resistance
varies (it increases with increasing frequency). When the Thickness
of the dipole (think of a biconical dipole), is in the range of 0.15
lambda or more, Q factor will be that low, that you can hardly see
the exponential decaying sinusoidal wave (so it behaves more like a
heavily damped circuit).

If you have access to EM simulation SW you might simulate a
construction and compare the impedance versus frequency for your LRC
equivalent model.

Back to your question, most narrow band antennas are not critically
damped and have an impulse response with exponential decaying
sinusoidal wave shape. Are you in GPR or equivalent?

Best regards,

Wim
PA3DJS
www.tetech.nl
The mail is OK when you remove abc.

Cecil Moore wrote:
It might help to note that the antenna feedpoint
impedance is a virtual impedance - a ratio of total
voltage to total current.

I forgot to say that the surge impedance of a standing
wave antenna, like a transmission line, is quite different
from the steady-state value of feedpoint impedance.
--
73, Cecil http://www.w5dxp.com

Scott Stephens wrote:
Since antennas have reactance and radiation resistance, are they always
critically damped, or will they ring-down?
No, they are not critically damped, NOR are they a simple resonator.


Is this implied by a swr plot? Can I take 3db points as antenna
bandwidth and assume a radiation-resistance loaded-Q from that?

No (this is a common error when folks first hear about the Chu limit and
read about Antenna Q. They mistakenly equate Antenna Q with "tuned
circuit Q" and then leap to the idea that center frequency/bandwidth =
Q.. nope.. Q, in both cases, is the stored energy divided by the energy
lost per cycle. But the mechanism is different...)



Does feed-point impedance change radiation resistance?
No. A folded dipole has a feedpoint impedance of about 300 ohms and a
dipole has a feedpoint impedance of about 72 ohms, but they have the
exact same radiation resistance.

Google for "radiation resistance" and "surrey" to find some pages by Dr.
Jefferies at UofSurrey..

http://personal.ee.surrey.ac.uk/Pers...es/radimp.html
http://personal.ee.surrey.ac.uk/Pers.../antennas.html

http://www.ece.rutgers.edu/~orfanidi/ewa/ is an online textbook which
you may find useful


Thanks

Wimpie wrote:

On 31 okt, 10:17, Scott Stephens wrote:
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 feed-point impedance change radiation resistance?

Thanks

Hello,

Thin half wave dipoles can also be modeled with a RLC circuit. The R
is frequency dependent, but in a limited frequency range, a simple
RLC circuit is useful. When more accuracy is required , or larger
frequency range, a transmission line model with lumped losses is
better. HW dipoles close to perfect conducting ground have narrow
useful BW, hence high Q factor and the RLC model matches better.

As I suspected/feared. If I try to design a pulse generator for a TDR in
spice, I'll have to synthesize an appropriate frequency-dependent
radiation-resistor.

Thicker dipoles have wider bandwidth (so lower Q factor). In that
case even within the useful frequency range the radiation resistance
varies (it increases with increasing frequency). When the Thickness
of the dipole (think of a biconical dipole), is in the range of 0.15
lambda or more, Q factor will be that low, that you can hardly see
the exponential decaying sinusoidal wave (so it behaves more like a
heavily damped circuit).

Yes, I've noticed UWB antenna look like horns or loops of wide straps

If you have access to EM simulation SW you might simulate a
construction and compare the impedance versus frequency for your LRC
equivalent model.

Perhaps an inverse-Fourier transform of that Z vs. freq plot can give me
a time-domain impulse graph?

Back to your question, most narrow band antennas are not critically
damped and have an impulse response with exponential decaying
sinusoidal wave shape. Are you in GPR or equivalent?

Yes, I'm interested in TDR and GPR.

Thanks,

Scott, KB9ETU

Jim Lux wrote:

Scott Stephens wrote:

Is this implied by a swr plot? Can I take 3db points as antenna
bandwidth and assume a radiation-resistance loaded-Q from that?

No (this is a common error when folks first hear about the Chu limit and
read about Antenna Q. They mistakenly equate Antenna Q with "tuned
circuit Q" and then leap to the idea that center frequency/bandwidth =
Q.. nope.. Q, in both cases, is the stored energy divided by the energy
lost per cycle. But the mechanism is different...)

Looking up the "Chu Limit, I found:
(http://ceta.mit.edu/PIER/pier43/11.0....Bellett.L.pdf)

I just skimmed, it seems the mechanism of RLC network is the R, but for
antenna, "Q is formed from the independent contribution of all the modes".

Which, for a UWB or impulse-radar antenna, will be an interesting; a
frequency-dependent resistance to model.

From the paper it seems using a ferrite broadens a loop antenna by
raising radiation resistance across frequencies. Whereas dielectrics
increase energy storage (Q) by confining flux, unlike magnetics.

I've even heard of laser-ionized plasma being used for UWB antenna,
since it will quench to stop it from ringing.

Google for "radiation resistance" and "surrey" to find some pages by Dr.
Jefferies at UofSurrey..

Ok, I'll take a long-look.

http://personal.ee.surrey.ac.uk/Pers...es/radimp.html
http://personal.ee.surrey.ac.uk/Pers.../antennas.html

http://www.ece.rutgers.edu/~orfanidi/ewa/ is an online textbook which
you may find useful

Thanks

Cecil Moore wrote:

I forgot to say that the surge impedance of a standing
wave antenna, like a transmission line, is quite different
from the steady-state value of feedpoint impedance.

Thanks

On 31 okt, 20:12, Scott Stephens wrote:
Wimpie wrote:
On 31 okt, 10:17, Scott Stephens wrote:
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 feed-point impedance change radiation resistance?

Thanks

Hello,
Thin half wave dipoles can also be modeled with a RLC circuit. * The R
is *frequency dependent, but in a limited frequency range, a simple
RLC circuit is useful. When more accuracy is required , or larger
frequency range, a transmission line model with lumped losses is
better. HW dipoles close to perfect conducting ground have narrow
useful BW, hence high Q factor and the RLC model matches better.

As I suspected/feared. If I try to design a pulse generator for a TDR in
spice, I'll have to synthesize an appropriate frequency-dependent
radiation-resistor.

Thicker dipoles have wider bandwidth (so lower Q factor). *In that
case even within the useful frequency range the radiation resistance
varies (it increases with increasing frequency). *When the Thickness
of the dipole (think of a biconical dipole), *is in the range of 0.15
lambda or more, *Q factor will be that low, that you can hardly see
the exponential decaying sinusoidal wave (so it behaves more like a
heavily damped circuit).

Yes, I've noticed UWB antenna look like horns or loops of wide straps

If you have access to EM simulation SW you might simulate a
construction and compare the impedance versus frequency for your LRC
equivalent model.

Perhaps an inverse-Fourier transform of that Z vs. freq plot can give me
a time-domain impulse graph?

Back to your question, most narrow band antennas are not critically
damped and have an impulse response with exponential decaying
sinusoidal wave shape. * Are you in GPR or equivalent?

Yes, I'm interested in TDR and GPR.

Thanks,

Scott, KB9ETU- Tekst uit oorspronkelijk bericht niet weergeven -

- Tekst uit oorspronkelijk bericht weergeven -

Hello Scot,

Now it becomes a different story. You are talking about large relative
BW. The concept of radiation resistance is nice for small
structures, but in case of large structures (for example traveling
wave antennas), you get (for example) impedance transformation.

A flaring and widening parallel strip transmission line has almost
constant real input impedance for frequencies above the quarter wave
length, without any resistive damping. However when the design is not
OK, the radiation pattern can be frequency dependent and may show
notches in the desired direction for certain frequency ranges. Also
the radiation centre may vary with frequency.

Some wide band antennas create a nice impulse response by absorbing
most of the power in resistance (resistive loaded dipole). Others are
backed by wide band absorbing material, to avoid frequency selective
reflection.

So one can make an antenna with close to 50 Ohms real impedance over
wide frequency range (so no oscillatory behavior), but it does not
mean that such an antenna is good for your application as you also
have to consider radiation pattern (versus frequency).

Best regards,

Wim
PA3DJS
www.tetech.nl
The mail is OK when you remove abc.

I sense a little confusion about the differences between time and
frequency domain analysis. This is really common.

Impedance is a frequency domain concept, and works in the time domain
only in a very limited way. Impedance consists of two parts, a magnitude
and phase or a resistance and reactance. Reactance is a function of
frequency, so it has no simple equivalent meaning in the time domain
which encompasses a very wide range of frequencies all at once. Likewise
for magnitude and phase. So only resistance is really useful in time
domain analysis. Some impedances, like a low loss transmission line's
Z0, are essentially purely resistive, so they're useful. But complex
impedances, in general, are not.

With a TDR system you can readily see and interpret frequency-dependent
resistances like skin effect, "capacitive" and "inductive" regions
(where the Z0 of the transmission path is lower or higher than the
reference respectively), and a lot of other features. But it's difficult
to get an intuitive feel for the relationship between a TDR and
frequency domain analysis of a lot of circuits which change
characteristics rapidly with frequency (in other words, which have a
reasonably high Q).

A TDR generally produces a fast rising step or its derivative, a narrow
pulse. Viewed in the frequency domain, this step or pulse has energy
over a very wide range of frequencies, but very little in any narrow
range of frequencies. So if a circuit has high Q, there's usually not
enough energy at or near the resonant frequency to get the circuit to
ring at any appreciable amplitude, and you often won't even see the
circuit with a TDR. (By this I mean that, for example, a high Q series
resonant circuit looks like an open and a parallel resonant circuit like
a short, which are essentially their impedances except near resonance.)
A dipole is a pretty low Q circuit in the frequency domain, so you can
see a periodic time domain response that corresponds to its basic
resonance. It looks like a lossy, open circuited transmission line whose
characteristic impedance increases from the feed point outward. The
response, which I'm sure you can also find somewhere on the web, looks
something like a distorted (due to the changing Z0) square wave whose
amplitude diminishes with time (due to the radiative "loss").

Roy Lewallen, W7EL