matching of an antenna

[yasine79]

hi all!

i designed two similar antennas(transmitter and reciever). the maching of the antennas is (S11= -5dB). it is not good!
But the (S21=-14dB in Air),(S21=-30dB in water). the Antennas are used in the near field . the distance between both antenna is 70mm, the frequensy is the ISM BAND.

is the the matching so important also in this case?


the antenne is a dielectric feeded with a coaxial cable.

thank you

 

[albbg]

The VSVR is approx 3.6:1 that is not good but, I think, not so bad as well. The mismatch will cause losses of the order of 1.6 dB (mismatch losses) and about 32% of power reflected back to the transmitter; this means only 68% power will reach the antenna and also the final stage of the transmitter must be able to accept the higher voltage swing due to the reflected power.
I think that if the S21 you reach is satisfactory from your point of view and the final stage has enough room to receive the additional power reflected by the antenna you can neglect the poor matching

However, since you wrote the antennas work in both air and water, I expect the S11 will change in this two different media. Furthermore it could be the S11 will change also by coupling of the two antennas.

 

[biff44]

ism bands:
6.780 MHz
13.560 MHz
27.120 MHz
40.680 MHz
433.920 MHz
915.000 MHz
2.450 GHz
5.800 GHz
24.125 GHz

 

[yasine79]

hi ,
you are right. i mean 2.45 GHz.

 

[jiripolivka]

Matching an antenna in water is really quite different from that in air! If you immerse your antenna in a dielectric, its dimensions must be scaled by square root of k. For water, nine times.
In salty water, the loss is high. Deionized distilled water is best but the matching circuit will differ if located in air or under water.

Expect a lot of tuning.

 

[yasine79]

hi!
* its dimensions must be scaled by square root of k* . you mean radius and height? in which book did you find it?

thanx

 

[jiripolivka]

hi!
* its dimensions must be scaled by square root of k* . you mean radius and height? in which book did you find it?

thanx

The "k" is the dielectric permittivity, the real part of it. Any textbook of electromagnetics includes this fact. Any structure immersed in a dielectric must have dimensions scaled from those in air.

 

[albbg]

Matching an antenna in water is really quite different from that in air! If you immerse your antenna in a dielectric, its dimensions must be scaled by square root of k. For water, nine times.
In salty water, the loss is high. Deionized distilled water is best but the matching circuit will differ if located in air or under water.

Yes this is due to the wave propagation speed in a medium, that is given by 1/sqrt(mu*epsilon). In our case mu is=1 (it differs from 1 in media with magnetic properties).
This means the electrical length, change by a factor sqrt(k). Changing the electrical length means to change the resonance frequency by the same factor.
increasing k means lowering the resonance frequency.
However the dielectric constant you have to consider is the actual one; I mean I suppose your radiating element is not directly in contact with the water, but there will be some radom and probably some air between radiating element and radom so the dielectric constant you have to consider will be that given by composing Kair, Kradom and Kwater so will be less that that of the water (roughly 80) and will depend from the various thickness
The calculation of the effective dielectric constant could be not trivial. The large the air volume around the radiator the less the impact on detuning.
S21 will be instead always affected by the water absorption.