# Quasi TEM mode and Axial Ratio???

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#### abhi_kolkata

##### Member level 5
hi,

i have a few very basic questions (may be very easy too - but i don't know the answers).
1) wat do we mean by - quasi TEM mode in the context of microstrip lines and mainly microstrip patch antennas?how it is different frm TEM mode in transmission lines like coax or two wire ones?
2) wat do we mean by Axial Ratio in microstrip patch antennas??And how it is related to the antenna polarization?wat is the axial ratio for a lineraly polarized(slant) microstrip antenna?

i don't know!!may be these are very funny questions.but i will be very pleased to know their answers in some details.

abhi

#### ysenthilece

##### Member level 3
1.
basically TEM is one in which both the electric and magnetic components are perpendicular to each other.

but in this case in addition to the perpendicular electric component , there are something called fringing fields at the edges of the centre strip .

but this decays expontentially similar to evanascent fields.......also they are negligible when thickness is small compared to thickness.

thus this closely resembles a TEM excluding fringing fields...

Quasi === means partly in english ..so this is Quasi TEM ..partially TEM

this same phenomenon comes in wiring in VLSI ..

2. axial ratio = Ey/Ex (z direction of propagation)
this is basically a factor used in the case of elliptic polarisation to denote how the fields are distributed wrt to the 2 axes...

for a linearly polarised wave ..axial ratio = ∞

#### sergio mariotti

ysenthilece said:
1.
basically TEM is one in which both the electric and magnetic components are perpendicular to each other.

but in this case in addition to the perpendicular electric component , there are something called fringing fields at the edges of the centre strip .

but this decays expontentially similar to evanascent fields.......also they are negligible when thickness is small compared to thickness.

thus this closely resembles a TEM excluding fringing fields...

Quasi === means partly in english ..so this is Quasi TEM ..partially TEM

this same phenomenon comes in wiring in VLSI ..
TEM is the propagation mode into the very common coax cables.
Also in some cases, may be considered TEM the propagation around the very common telephone pair.
"Quasi", an old latin word, = "very close to ... but not exactly the same"

All TEM lines at boundry eviromental conditions becames quasi TEM.
For microstrip lines, the "quasi" TEM depend on the fact that a small part of energy is radiated, a small part of energy is propagated surfacely (over dielectric) and it is sligtly dispersive.

ysenthilece said:
2. axial ratio = Ey/Ex (z direction of propagation)
this is basically a factor used in the case of elliptic polarisation to denote how the fields are distributed wrt to the 2 axes...

for a linearly polarised wave ..axial ratio = ∞

And for circular polarizzation, AR=0
Axial ratio is a Figure that give the goodness of antenna polarizzation.
Very good values for linear pol. are more than 35 dB
Very good values for circular pol. are less than 0.5 dB

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