hi everyone.
I've been reading a research letter about wearable antennas for biomedical application. and then in the abstract it said, the exact words are,"The structure achieves very good measured peak gain values of −33 dBi and − 16 dBi at 403 MHz and 2.45 GHz, respectively." it's not a good structure right? it's so lossy ...
can anyone explain or correct me if i'm wrong?
thanks.
the research letter i'm talking about is: A dual-band implantable antenna with wide-band characteristics at MICS and ISM bands. you can find it on internet.
Dipole antennas have 3 dB gian in air. The question how they measured the gain.It is very probable that the electromagnetic waves radiated by the antenna encounter a lossy medium such as the human body.
Best wishes
Negative gain means the antenna is lossy (mismatch, low efficiency, etc.). Whether that gain level is good or not depends on the application of the antenna. It is possible that for their application this is an acceptable gain level. You can see in the paper they compare with similar designs in literature and the gain levels are in the -20 and -30 dBi range.
You can also see that they test the antenna in saline solution to emulate human body environment conditions, this is likely the cause of the negative gain but I'm not sure.
Dipole antennas have 3 dB gian in air. The question how they measured the gain.It is very probable that the electromagnetic waves radiated by the antenna encounter a lossy medium such as the human body.
Best wishes
Wearable antenna has negative gain due to close proximity with lossy material such as clothing and human tissues and it is necessary to reduce the input power as much as possible for wearable antenna which automatically reduces the output radiation. You can find plenty of research articles on wearable antenna used for biomedical applications with negative gain.
@ Mohamed K. Emara thanks for the explanation. yes i know the gain depends on the application, but in most of the papers i read for wearable antennas in ISM band, the peak gain is about 2 to 4 dBi. so this paper with peak gain about -30 dBi was very odd to me.
Maryam Darabi the paper you referenced discusses an implantable antenna not a wearable antenna :)
As Mohamed K. Emara says, the antenna is implantable. It will not be large compared to a wavelength so will be starting from a baseline of 1 to 2 dB gain, even in air, but it is not good testing it in air because if it is matched when implanted it won't be matched in air, and its gain needs to take account of the body it has to get through. So it is tested in a body phantom, or something similar. At 2.45 GHz absorption is fairly strong. At 403 MHz the antenna is electrically small so will encounter the efficiency and matching problems of electrically small antennas, as well as loss in the body. It is not surprising that the gain of these antennas is -33 and -16 dB. If you work out the Friis equations I expect you will find this is perfectly fine for most short range applications.
@ Malcolm White @ Abdelhalim Zekry @Gourab Das thank you all. your explanations really helped me.
Hello,
A negative dB gain means that the antenna is not radiating very well at all in that direction in compared to a dipole antenna. But the gain, and directivity, are 3-D functions depending on azimuth and elevation. A directional antenna may have good gain in one direction and poor gain in other direction. The poor gain may come from the pattern shape, poor match, internal losses or external loading. This depends on the antenna and the application.
Thanks,
Usually negative gain in dB is not good. It is a poor radiator.
However, if your design goal is something different like very compactness then in that case negative gain will result.
Similarly in other applications, like others have stated, they may adjust with those gains only.
Agreed with Malcolm White , it makes sense, a very small antenna inside a phantom would be a lossy one.
Implantable antennas have a negative gain, Because these antennas have very miniaturized size and planar structure as well as multiband behavior. These restrictions as well as inside a phantom responsible for negative gain. But These antennas satisfy the application's requirement.
Lossy antenna can be use in back lobe canceller or side lobes blanker application to enhance es/ec feature in electronic warfare environment.
Vipin Kumar "It better to coin term of Electromagnetic energy absorbent structures." ? Better than what? Radar absorbent material (RAM) is an Electromagnetic energy absorbent structure but is not an antenna.
It is not good to have negative gain in Antenna, but if you are working on small antennas then it is difficult to get +ve gain with good impedance matching. These types of antennas are used in RFID, near field data transmission, etc... because here size is the most important thing. In broadband antennas also people are working with -ve gain antenna.
Taha Al Sheikhly when people refer to negative gain it almost certainly means negative decibels. This means the gain is less than that of an isotropic radiator, not that it is not radiating. If you do not understand decibels, look them up in an antenna book or wikipedia.
Hello Maryam Darabi ,
In general applications, the emphasis is given on increasing the antenna gain, which is directly dependent on the antenna efficiency and its directivity. In the simulation software, the gain and radiation patterns are provided for the far-field and in the case of practical replication and measurement, obstacles are avoided. For, medical applications, such as the implementation of antennas in the human body, the measurement of gain can be with respect to the lossy dielectric medium encountered in the human body and this may yield a lower gain with respect to an isotropic antenna than the gain it could have yielded for air medium.
Also, for broadband and related applications, you will find articles in which -10dB of S11 is more prioritized than the antenna gain itself.
thank you for your explanations Mr Md. Abdul Kaium Khan and Nasr Alkhafaji
Dear Maryam Darabi ,
It's the matter related to the application..
For implantable devices, negative gain antenna is good.
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