I am aware of the fact that electrically small antennas are poor radiators with
very small gain and low efficiency. One main cause of this is poor matching.
An extremly electrically small antenna has been designed at 865 MHz which is less than lamda/12. It has a good matching (-15 dB) over the entire Bandwidth,
without using external matching network.
But, It shows - 8 dBi realised gain with low efficiency.
However, in my case the matching is good and the dielectric loss is negligeable as material has low loss tangent.
Which is in this case the cause of the very bad gain and low efficiency? I am wondering where the power is lost, given that the matching is good.
Could you direct me to some references explaining alternative causes of low efficiency and negative loss in electrically small antennas,
besides poor matching?
In case of electrically small antennas, even when the antenna input matching is perfect, EM waves sometimes do not leave the antenna surface in an efficient way. Consider a comparison between a half-wavelength dipole and an electrically small (ideal) dipole. The realized gain around an antenna is actually the amount of power leaked from the antenna surface to the free space. In the case of the half-wavelength dipole, efficient leakage is possible, which is not the case for electrically small dipole, consequently lowering the realizable gain value. Hope it helps.
https://www.researchgate.net/post/Can_anyone_tell_me_why_I_am_getting_negative_value_for_gain_in_designing_of_meandered_dipole_antenna_at_40_MHz
Hi Debraj,
A good match is not an indicator of efficiency, just a measure on reflected power against transmitted power. An ESA radiation resistance Rrad is quite low. So the resistance Rres in the antenna and the losses in the matching network compare well with the radiation resistance and lower the antenna efficiency. [The reactive match will have a lossy part.]
You may want to verify if the design factors all such parameters sufficiently. And reverifying the measurement process may sometime help.
Regards
spt
In case of electrically small antennas, even when the antenna input matching is perfect, EM waves sometimes do not leave the antenna surface in an efficient way. Consider a comparison between a half-wavelength dipole and an electrically small (ideal) dipole. The realized gain around an antenna is actually the amount of power leaked from the antenna surface to the free space. In the case of the half-wavelength dipole, efficient leakage is possible, which is not the case for electrically small dipole, consequently lowering the realizable gain value. Hope it helps.
The type of the investigated antenna has not been mentioned here. But, dielectric loss is used. Therefore, the antenna has dielectric or substrate.
Therefore, the excited mode is not the radiated mode. This is reso ating mode. Even resonator gives good matching at resonnat Freq. Antenna gives radiation, resonator does not.
Therefore, investigate the internal fields and identify the modes. If the mode is identified accurately, follow conventional excitation technique for the same antenna.
If the mode is complex and unidentified, then then the may be resonating mode, not radiating modes.
Sudipta Maity Thanks for your reply. Could you please tell me how can I investigate the the mode in HFSS? Usually, I know how to check the E filed and Surface current in a structure but doesnt know how to distinguish in between different mode e.g TE 01 or TM 01 or TE 01 or TE 10. It would be nice if you can give an example with picture.
A small antenna has to have high currents in order to radiate because radiation amplitude is the integral of current over length. That to get that current you either need a very low impedance feed, a transformer, or a resonant circuit to store energy to build up the current. All of these are lossy.
If a resonant circuit is used it is hard to keep the loss of the circuit lower than the radiated power. Radiation Q is defined as stored energy divided by power radiated per radian. Radiation Q is at least inversely proportional to the volume that will hold the antenna. As the antenna gets smaller it becomes impossible to have a matching or resonant circuit with a Q that high, so most power is lost in the matching circuit. The antenna and matching circuit can be well matched, but most of the power goes into the matching circuit. See Small Antennas by Wiley, or papers by Wheeler. If you want a wide bandwidth it is even worse.
Dear Debraj Kar, generally speaking, the antenna that you designed is storing the energy within the entire structure. Good matching indicates that the energy travels to the structure, yet it does not indicate that the same energy converts into radiation and bounces of your antenna into the free space. Since you haven't mentioned the type of your antenna (MSA, horn, leaky-wave, and etc.), it is very difficult to point to a certain reference for your problem. You should first designate the radiation regime of your antenna, meaning how your designed antenna generally radiates the input energy.
The type of the structure is not mentioned. For example, if it has defects in ground plane then it modifies the aperture field distribution. If the aperture distribution becomes tapered then it reduces the gain.
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