My thoughts on transient antenna radiation - Part 1

Antennas have been used extensively since hertz first sent out signals wirelessly across the seas. There has been extensive work done in the field of antennas in the last 100 years. Still the radiation of a short pulse(transient response) from an antenna has a lot of outstanding questions that remain unanswered.

In the last 5 years, I have been working on one aspect of this problem which is the shape of the radiated electric field pulse and its relation to the geometry of the antenna. Reading books like Schantz, Stutzman and theile or Kraus, tells us that if you want to radiate a broadband pulse then a fatter, planar antenna works more efficiently than a thin wire antenna. Again this statement is arguable as the immediate question would be "what do you define as a more efficient antenna?" There is always two design trade-offs associated with broadband antennas: (1) How good do they match to a 50 ohm transmission line which feeds power to them?; and (2)How good do they radiate that power and in what direction?

The first question seems more difficult to answer to me at this time and so I will start with the second question. In my experiments, I have found that planar (fat) antennas with smooth edges (e.g. Ovals) tend to radiate a pulse with lesser time dispersion than antenna structures with sharp edges (e.g. diamond dipole). I think that antenna structures with sharp edges tend to cause changes in flow of charge and the rate of change of charge velocity is the current. Following Maxwells' curl equations we know that the radiated field is the delayed time derivative of the current density integrated over the surface of the antenna (Volume in general case). It is important to note that the partial time derivative involves not just the current density in an infinitesimal area on the antenna but also the delay associated with the distance from the infinitesimal surface to the point in space where the field is to be determined. Hence the shape and dimensions of the antenna play a vital role in defining the shape of the radiated electric and magnetic field pulse as they control the path taken by the charge flow.

I assert that the aforementioned reason is one cause of pulse distortion and not the only cause.

Schantz's book states that charges mainly flow along the edges of the antenna structure as there can be no field inside the conductor. Though FDTD simulations done by me show that the current follows the edges I do not agree with this statement for planar board antennas as the charges spread along the copper surface of the antenna and there is considerable difference in the radiated pulse shape as seen on the DSO in a anechoic chamber if only the edges of the antenna are considered and the copper in the middle is scratched off. It is obvious that this reduces the copper on the board and hence the surface integral would result in a different value for the radiated field strength. But it also important to note that the radiated pulse shape changes which is not stated in previous work. I do not know if what I am thinking is right or wrong in this regard.

Now coming back to the first question, the impedance, a measure of the obstruction to flow of current, of an antenna needs to be matched to 50 Ohms (normal transmission line impedance). Of importance is the geometry of the antenna structure when the transmission lines feeds to the planar antenna structure. If the structure is wide compared to the i/p transmission line the current entering the antenna spreads out and this means that the impedance of the antenna structure is lower at this point. If the structure is narrow compared to the i/p feed line, the current entering the structure encounters a large impedance. This mismatch in the i/p impedance would imply a loss of signal power as well as distortion of the i/p pulse shape.

Now looking at impedance from the frequency domain perspective would mean that the antenna structure needs to be such that it matches to 50 ohms over the transmission band which for certain applications such as ultra wideband is as large as 7 GHz. Impedance match over such a large impedance match is difficult to achieve.

I still do not have good insight on how geometry could be modified to acheive a good impedance match and provide omnidirectional or direction radiation with minimal pulse distortion depending on the application.

In the second part of this posting I will discuss my experiments and what I might be doing wrong and ask for insights from readers who might come across this column (If I am lucky)