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.