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.