I need help. The resonant frequency of the monopole antenna varies greatly from the theoretical result.

Your "monopole" configuration seems to involve misunderstandings. You can't make a monopole without a ground plane, ideally of infinite extension.

FvM said:

您的“单极子”配置似乎存在误解。您无法在没有接地平面的情况下制造单极子，理想情况下接地平面应无限延伸。

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Thank you for your answer. I don't know much about antennas. Do I need to buy a single antenna with a ground plane, similar to the antenna in the following literature?

Antenna Lover said:

Theoretically, the resonant frequency in water should be the antenna operating frequency 2400MHz divided by the relative dielectric constant of water in the square root(80), which is 268MHz.

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If you can change the resonant frequency of an antenna from 2400MHz to 268MHz (one decade), just placing it in the water, you will get a prize.

Antenna Lover said:

Thank you for your answer. I don't know much about antennas. Do I need to buy a single antenna with a ground plane, similar to the antenna in the following literature?

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The setup is at least nearer to a water imersed monopole. I'm not sure, if the water volume is large enough to absorb most of radiated waves, if not, you'll still see reflections at the air interface in measurement results.

Not only resonance frequency, also impedance will be massively affected by medium permittivity. But what's the purpose of your experiments?

Don't think that you can buy this kind of antenna. But you can make it yourself without much effort.

Do you mind to post a link to quoted literature?

FvM said:

The setup is at least nearer to a water imersed monopole. I'm not sure, if the water volume is large enough to absorb most of radiated waves, if not, you'll still see reflections at the air interface in measurement results.

Not only resonance frequency, also impedance will be massively affected by medium permittivity. But what's the purpose of your experiments?

Don't think that you can buy this kind of antenna. But you can make it yourself without much effort.

Do you mind to post a link to quoted literature?

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In fact, the purpose of my experiment is to verify that the resonance frequency of a monopole antenna in water can satisfy the theoretical law, and subsequently, I want to put the antenna into soil with different water content in order to observe the resonance frequency change. But so far there has been no good results in the water.Do you have any other suggestions, thank you very much!
Here's the PDF of the reference：(1)whip antenna (2)disc antenna

vfone said:

If you can change the resonant frequency of an antenna from 2400MHz to 268MHz (one decade), just placing it in the water, you will get a prize.

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Why is that? Is it not possible to realise the results of the experiment that I would like to see?

Antenna Lover said:

Is it not possible to realise the results of the experiment that I would like to see?

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@vfone is thinking of regular antenna operation (i.e. efficient radiation), but you want to use it as a sensor. That's perfectly fine.

The sqrt(eps_r) scaling is valid if you have an infinite hemisphere sphere filled with that material, not just a glas of water in the near field. At the moment, your antenna will "see" a layer of eps_r followed by a large air volume.

You will need to calibrate your setup with known water content. Conductivity of the wet soil will also have an effect, it changes attenuation and the width of your resonance.

Thirty-three years ago, I built a prototype soil moisture sensor for Greg Arnold for what later turned out to be "Design of a dielectric based soil moisture sensor", G. J. Arnold, M. L. Stone, R. L. Elliott. I found I had to increase the distance of the sensing circuit elements from the soil, or the dielectric constant between dry soil and wet soil was too great for any sensor to function..

If I were to try to do it today with your tools, I would start with a dipole on a piece of circuit board or plastic fed by 50-ohm stripline covered in Kaplan tape. The sensor must be waterproof at all times. Then, cover both sides with thicker and thicker layers of low-loss, low-dielectric plastic until the best results are found. If the method shows any promise, a full wave loop will have an 80 to 150-ohm impedance, which might help if SWR becomes problematic.

Good Luck, Gordon