Electrically short HF Monopole - high nearfield result

Question

Hi, I am simulating a monopole to see the electric field at 3m away with the following parameters;

8m long and 0.08m diameter, frequency range 2Mhz to 30Mhz, power 100W, 5dBi gain

Approaching the low frequency end ~2Mhz the electric field strength increases significantly.  ~ 600 V/m

Using hand-calc approximation for Far‑field field strength (V/m) = √(30 × P × Glin) / distance = ~ 32 V/m. Some testing data also showed roughly that same magnitude of results.

I've tried a few different solution settings (thin-wire port, edge port, waveguide (SEP/FEM) without success, I suspect is in relation to the length « wavelength, and would like some assistance if I am missing something / if the simulation will have to incorporate more complex modelling.

Thanks.

(Feko v2025.1.1)

HF_monopole.cfx

HF_monopole.pfs

Torben Voigt · Answer

Hi JJSS,

Everything looks good with the model. Keep in mind that, of course, the Monopol used in the measurement did not have an infinite ground plane, but certainly a much smaller one. With an infinite ground plane at 2.185 MHz, the field strength at a distance of 3 m and a height of 14 m is about 70 V/m.

Best regards,

Torben

JJSS · Answer

Hi Torben,

Unfortunately, I don't have access to the testing procedure, so it does make it difficult to validate between the model and test data. The frequency was 2.185 Mhz and 3 meters from the antenna. The height above ground I do not know.

I am surprised by the difference in magnitude between the expected values but at this time I no further information to progress with. If there was nothing incorrect with the model of the monopole, then I am happy to close the topic.

Torben Voigt · Answer

Hi @JJSS ,

At what frequency and at what height above the ground were the 70 V/m measured?

Keep in mind that there are always differences between the actual measurement setup (various potentially disruptive factors) and the simulation (ideal conditions, e.g., infinite PEC ground). But they should at least be quite similar, unless the measurement method was fundamentally different.