How does loop and monopole antennas behave near objects?
I am doing a few measurements in the 433 MHz range on an electrically small loop antenna and a 1/4 wavelength helic monopole. I've been measuring their resonance frequencies by measuring S11 using a VNA. I've done this both with the antennas in free air, and surrounded by (but not in contact with, about 3 cm from antenna to material) densely packed, wet plant matter. Now I expected the resonance frequency of both antennas to drop as they are now very close to a material with a lot of water, which means a higher permittivity (ε) than air. The helic antenna dropped a lot as I had expected (~80 MHz) but the loop antenna only dropped very little (~5 MHz). Why is this? From what I've read my initial guess is that it's because the helic antenna is electric and thus influenced a lot by the change in the permittivity of the surroundings, while the loop is mainly magnetic in the near field which means it's susceptible to changes in the permability (μ) of its surroundings rather than the permittivity but as the permability of most materials, organic materials included, is close to μ0 the change from air to plant matter isn't that great, hence the small change in resonance frequency.
Is this correct? Could one of you explain it better and/or give a few reference links I could use to read up on it to understand what is happening? I'm not strong in antenna theory as I'm just an electronics engineer dabbling in this area and trying to make sense of what I see.
Thank you for any help in advance.
I agree with your assumption, that the magnetical antenna will be less affected by capacitive load. Unfortunately, it can be expected to have a poorer performance according to application literature, e.g ti.com AN003. But it may be better under special enviromental conditions.
For the general small antenna behaviour, I would refer to profound text books like Ballanis.
Any antenna is defined in a "free space", or, close to a conductive "ground plane" like ground.
If you bring any conductive or dielectric object close to it, the "free space" condition is broken. The result can be calculated for simple boundary and specified materials, but the best is to make experiments. Also the feed cable should be located in a "neutral" plane to avoid its effect upon antenna performance and radiation pattern.\
Modeling is one thing that can roughly predict antenna behavior in real conditions. But good antennas come with measured parameters.
I absolutely agree, however in this case I am more interested in a rough outline of the mechanics behind the phenomena rather than getting a good antenna.