Suppose if i want to design microstrip antenna for ISM Band and for lossy tissue how can we compute the length, width and effective permittivity of medium.
The effective permittivity depends on the antenna nature. In the case of a microstip nature antenna, you can see its formula here:
https://www.microwaves101.com/encyclopedias/microstrip
After estimating the effective permittivity, the guided wavelength can be calculated and, in turn, used for calculating the antenna dimensions.
As we increase permittivity of the substrate used for the patch antenna design, the size of the antenna reduces. One more advantage is beamwidth of the antenna increases.
But, bandwidth and gain of the antenna reduces with respect to the increase in the permittivity value.
The use of substrate material with higher dielectric constant in microstrip patch antenna design, results degradation of antenna performance but size of the antenna reduces. With increasing substrate thickness (h), the resonance frequency decreases but the bandwidth increases. Check this paper for basic concepts.
Conference Paper Effect of dielectric constant on the design of rectangular m...
http://www.antenna-theory.com/definitions/permittivity.php
If permittivity (er) is increases (1) lamda_g will be decreased, hence resonant frequency will reduce (2) maximum amount of E-fields will be confined in the dielectric media. Hence, less radiation, Hence, gain will reduce. (3) Amount of Electric stored energy will increased. hence, Q will increase (4) as Q is increasing, BW will decrease (5) lamda_g will be decreased, hence compact antenna can be designed (6) More no of bands will be accommodated. hence, multiband antenna is possible
With lower permittivity dimensions required are more...........
In Balanis 14th chapter it is clear..........
Tanq for your answer mr Sudipta Maity,
i have one doubt in your answer,what is the relation between lamda_g and resonance frequency??
Lamda_g is the guided wave length in the media. If we consider air-field metallic waveguide, we will get simply lamda or lamda_0. If the metallic waveguide is filled with dielectric having er>1, the lamda will be termed as lamda_g (guided wave length).
Dear Vikash,
In the question you have mentioned lossy biological superstrate (generally of high permittivity) on the top of the microstrip patch operating at ISM band. You haven't mentioned the substrate on which you want to fabricate the antenna and the same can be any standard substrate available!
For designing patch antenna, we need to consider the fringing effect with utmost care. Therefore, we do calculate effective permittivity and effective length during designing the patch antenna. In general, we consider air as the superstrate and calculate effective permittivity; but, you need to use the permittivity of specific biological tissue instead of air during effective permittivity calculation. Thereafter, use that effective permittivity for effective length calculation of patch antenna. Then only, you can use your ISM band patch antenna for specific application on biological tissue.
Hope, it would help. Thank you.
Thank you sudipta mity sir, in your answer >> If permittivity (er) is increases (1) lamda_g will be decreased, hence resonant frequency will reduce .but in general frequency and wavelength are inversely proportional.
Dear Tathababu,
I think you have missed the point given by Dr. Sudipta Maity. As the permittivity of substrate increases, lambda_g corresponding to lambda_zero (free space wavelength) at all frequencies reduces. For fixed dimensions of antenna, it can now accommodate resonant lambda_g corresponding to higher lambda_zero because of increase in substrate permittivity. Therefore, reduction in resonant frequency is observed.
Hope it would clarify your doubt. Thank you.
hi there, permittivity increase/decrease has some interesting impact on the antenna. For example, in case of microstrip patch antenna, substrates with higher value of permittivity has reduced size. But this results in somewhat narrow bandwidth. If u increase the size of the substrate, u can achieve wider bandwidth when compared to the previous consideration.
But, the overall size of the antenna increases, which is not favourable if u r working for an application in which compactness also plays a major role.
In such cases, u can choose permittivity with higher value..so that size of the antenna is reduced. And if u need to increase bandwidth, u can go for concept of slots, ground variations, etc.
Conforming with the colleagues above, the dimensions of the patch decrease as the dielectric constant increase. They scale as W, L is proportional to 1/sq. root of epsilonr. This is straight forward as the epcilonr increases the capacitance of the patch increases proportionally. Then for same frequency one needs the same C which means that one has to scale the every dimension by sq.root of epsilonr.
For W and L calculator please refer to the link:https://www.pasternack.com/t-calculator-microstrip-ant.aspx
Best wishes
Hi,
There is another way of explaining this...
With an increase in permittivity, the capacitance increases, and this increases the amount of charge stored (within a constant Electric Field) between the plates for a fixed sized patch. Since, the resonant frequency is given by 1/(2*pi*sqrt(LC)); with an increase in C, the resonant frequency decreases.
Now to maintain the initial resonant frequency, the only way is to reduce the size of the patch. By this way, the charge stored between the capacitors is reduced and hence the resonant frequency of the patch increases and reaches its initial value.
As the effective area of the patch is reduced, this reduces its gain.
With Regards,
Suhas D.
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