Question about the input impedance of an antenna

[Mica188]

Input impedance

Hi,

I'm a little bit confuse. Concerning the input impedance of an antenna, I have excited a dipole with a pluse (gaussian). The output data at each frequency is associated with one input impedance. Is this mean that to have that energy radiated I have to match this impedance to the antenna ? For example, the input impedance is 34 Ohm and radiation power is 4.4 e6 watts. So, the antenna has to have an impedance of 34 Ohm to radiated this power (let say no loss).

Regards,

mica

 

[House_Cat]

Re: Input impedance

An antenna is a matching device between free space and a transmitter. The impedance of free space is approximately 377ohms. The input impedance of the antenna is matched to free space by the construction of the antenna (its characteristic inductance and capacitance).

To get the maximum transfer of energy into free space, the input transmission line has to match the input impedance of the antenna. The output impedance of the final amplifier also has to match the impedance of the transmission line. In other words, all impedances must be matched to get the maximum transfer of energy. When all the impedances are matched, the antenna system is resonant, and the inductive reactance cancels the capacitive reactance - all that is left is the real part of impedance.

I'm not sure if this is what you need explained, but it is the essence of antenna theory.

 

[jiripolivka]

I think your problem is linked mainly to the way you excite the antenna- by a Gaussian pulse. Such signal is rich in harmonics and the "standard" approach based on sinusoidal signals is not working.
I think that you can find a good advice in books on UWB antennas (ultrawideband). Those antennas are analyzed by moving charges in conductors which is so far not well understood. In general a suitable antenna for short-pulse excitation should be matched over a very wide band which is rather difficult.
I can recommend you to see "www.picosecond.com" where a white paper on such antennas is available.

 

[biff44]

An RF pulse has a very big bandwidth. If the antenna has a broadband input impedance of 34 ohms, then you need it drive it with a broadband 34 ohm source.

In all likelihood, your antenna will not have a pure 34 ohm resistive input impedance, but its real and imaginary parts will vary over frequency. You need to Match the driving source to that antenna "load impedance". That means cancelling out the imaginary part, and getting the source impedance to match the load impedance.

Impedance mismatches will reflect power that could have radiated (i.e. it is lost to heat). Also, non-uniform impedance matches will change the waveshape of the pulse.

 

[FvM]

Yes. Unfortunately the original poster hasn't visited edaboard since many years. But he has maintained more advanced discussions on UWB antennas after this thread.