Monopole transceiver Antenna (Actual vs Simulations)

Hi,

I am doing some studies on monopole antennas on an actual transceiver sensor.

The sensor has a monopole antenna that is about 5 cm in height. And it is used to transmit at 300 MHz.

But 5 cm is only 0.05 lamda at 300 MHz. I did some simulations in HFSS and it shows that S11 at 300 MHz is only about -1 dB, and not a lot of the input power is radiated. Morever, from simulation, this monopole resonates only at about 1.5 GHz (lamda /4).

Is there something that I have missed out on that the system is able to use such a short monopole to radiate effectively, when HFSS simulations show that it is not an effective antenna at all?

Is there something that I should include for the Simulation for a more accurate model?

Thanks and regards.

The simulation is basically in accordance with antenna theory (and actual behaviour). An electrical short antenna has a
capacitive impedance with a very small real part (= radiation resistance). It can be tuned with a serious inductance and
impedance matched with a matching network, resulting in rather small bandwidth and low efficiency due to losses.

As another remark, a monopole (also a short monopole) requires an infinite ground plane in theory. The ground plane of a
real monopole has typically dimensions similar to the antenna itself, so it should be rather analyzed as a kind of asymmetric
dipole.

HFSS and similar tools can of course perform a realistic analysis of the complete structure, including the real ground plane,
and the required matching network.

Hi FvM,

Thanks! So if I am able to find the actual impedance of the antenna, and add in the conjugate match of the antenna impedance, would I be able to improve the matching and thus the efficiency of the antenna even though its length is much shorter than lamda/4?

I still quite new to HFSS. Any idea as to how I am able to compute antenna impedance in HFSS? Do I use a Lumped Port to get the antenna impedance?

S11 is simply another representation of antenna impedance. I understand, that you know how to get it in your simulation.
I'm not particular familar to HFSS, so I can't answer specific questions in this regard.

In addition, I suggest to consult a textbook that is discussing small antennas, e.g. Balanis Antenna Theory.