Question about antenna frequency - Page 1
Forum section: electronics / rf-microwave

author: pcm81
author_rank: Regular Contributor
timestamp: July 16, 2022, 01:43:14 am
content: I have a narrow band receiving antenna for 2.2GHz signal. When talking to tech support to resolve an issue the matter of oil contamination came up and the tech support noted that oil film on the antenna will effectively change the frequency of this narrow band conformal antenna, probably causing the issues we're seeing.I am curios as to how oil contamination can change frequency of the antenna.I can see how it may have a dampening effect, but by what mechanism can it change the frequency response? Wouldn't if effectively need to change the electrical length of the antenna to change its frequency? The only exposed contacts are for grounding connections. Is the oil film creating some kind of capacitive effect with antenna patches or what? Just curios about mechanism by which frequency change effect due to oil contamination can be achieved, not expecting a solution to the problem, just better understanding of physics at play.Thanks ahead

author: radiolistener
author_rank: Super Contributor
timestamp: July 16, 2022, 05:15:21 am
content: Any kind of material around antenna within half wavelength distance affects it's resonant frequency, directivity and efficiency.This is because there is a skin effect. AC current flows not within wire but on their surface and the speed of that current depends on electromagnetic wave speed in the environment near the wire. Since different environment has different EM wave speed, different environment around antenna leads to change electrical length of antenna wires.In other words, your antenna electrical length is different when it's placed in air and in oil environment. Usually antenna length is specified for vacuum environment (free space) which is very close to air environment. But if you place it in the oil enthronement, it's length will be different and as result resonant frequency will be different.Even air around antenna affects it's resonant frequency, but EM wave speed in the air is almost the same as for vacuum, so the difference is too small and can be ignored. But for other environment (such as water, oil, soil and other) EM wave speed can be very different so it leads to a high change of antenna resonant frequency.There is also reflection effect on the border between materials with different EM wave speed (for example air and oil), it leads to adding reactance (L or C), so the resonant frequency of antenna will be changed.It's hard to take into account all effects in the mind, because this is very complicated process, so it's better to use some antenna simulator software to see how different material and it's geometry configuration near antenna can affect antenna properties.

author: pcm81
author_rank: Regular Contributor
timestamp: July 16, 2022, 08:29:02 pm
content: Quote from: radiolistener on July 16, 2022, 05:15:21 amThanks. You gave me a very important piece of information that i was missing about the environmental effects within half wavelength being critical to electrical length. I was initially thinking exclusively in terms of shorts to ground or shielding effects of ground plane equivalents, however capacitive coupling within 1/2 wavelength makes sense.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: July 18, 2022, 01:10:35 am
content: conductive material couples inductively or capacitivelynon-conductive material can act as a dielectricboth routes can change the effective electrical length of the antenna.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 18, 2022, 01:25:57 am
content: Quote from: pcm81 on July 16, 2022, 01:43:14 amSorry, but you have told us very little about this antenna.What does it look like?, (what type, in other words).Where is it situated?, out exposed to the elements?, inside a building?, or inside a device?What do you call "narrowband" at 2.2GHz?2,2GHz is not "bleeding edge", & there are plenty of antennas out there on that frequency & higher, which survive year after year subject to all sorts of environmental pollution, with no discernible degradation in performance.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 18, 2022, 09:38:07 am
content: It comes down to the fact that people think of 'the antenna' as an isolated part on their PCB/in their system. But in the end, the antenna is the interplay between everything that is near.I think radiolistener's 'half wavelength' rule is actually a pretty bad one. Rules like that are always an approximation, but the rules I've always heard is 'within the near field' of the antenna, as for a 256 element array, something within 2 wavelengths of the center elements will still have a huge influence. I've seen cases of antennas at 700 GHz (wavelength < half a mm) being detuned by metal being moved 5 cm away.

author: radiolistener
author_rank: Super Contributor
timestamp: July 18, 2022, 01:35:10 pm
content: Quote from: TheUnnamedNewbie on July 18, 2022, 09:38:07 amActually it depends on a largest antenna dimension D. Near field distance is a good fundamental parameter of antenna, and it's actual distance isR = 2 * D^2 / lambda, where D is max dimension of antenna and lambda is wavelength. In most cases you can approximate it with half wavelength distance for 1/4 wave antenna.But if you're using antenna array, it's near field region can be more largest.Also note that near field effect drops down as1/R^2or even1/R^3at short distance. So if some material falls into the near field zone on a long distance, it's effect will be much smaller than on short distance.Also note, that there is some limit which prevents to minimize near field zone by using a very small antenna dimension, the mimimum limit for reactive near field zone is aboutRmin = lambda / (2*pi)AtR = 2 * D^2 / lambdadistance near field waves still have some effect, but it is low enough to ignore it on the background of far field plain wave effects. This distance is taken by criterion where E and H field phase error is smaller than pi/8.