antenna-theory.com

[Abu Maria.]

hello
i am new member in this forum
sorry i dont speak english good...sorry
my qestion
in the book of balanis
He said that , the antenna radiates in its edges , why the radiation will pass through the edges?
and why it pass only in 2 edge only ?
thank you
sorry for my english

[bigSteve]

It is not the edges of the patch antenna that radiate. It's the fields coming off the edge of the patch antenna. These fields are the "fringing fields" and are described here:
http://www.antenna-theory.com/antennas/patches/antenna.php#fringing

The fields at the edges of the patch add up in phase, and this gives rise to the radiation.

[Abu Maria.]

thank you bigsteve
i know that is not the edges that radiate, but i meant that fields comming in the edges
but
i like understand
why we have this mechanism (physically) ? and why it happen in 2 edges only ,?I found this in Balanis book (model cavity , model line transmission) in this models he speak only of 2 edgs ( ex: he say in balanis: a rectangular microstrip antenna can be represented as an array of two radiating narrow apertures (slots),...)
thank you.

[bigSteve]

The cavity model is nonsense. Antennas are best understood intuitively - so when you use the cavity model and start discussing magnetic currents at the edges you have no physical intuition. The cavity model is a poor model in my opinion.

You should be able to figure out why the edges produce the fringing fields responsible for radiation from the link I posted previously, or from this video:
http://www.youtube.com/watch?v=4qyb_hslP3A

Basically, on the two radiating edges, the fringing fields are adding up in phase. On the non-radiating edges, the fringing fields are "balanced" and don't produce radiation because the total of the fields cancel each other out.

[Abu Maria.]

thank you bigsteve...sorry again , I use the google translator...
the model of the cavity we'll talk again later.
thank you very much
when feeding is in center of the patch , we have not a radiation , because in edge we have voltage=0 , ?

when feeding is placed on the edge of w , we have radiation???
or no ???
thank you.

[bigSteve]

When the feed is in the center of the patch antenna, it will not radiate. The patch antenna does not radiate because the impedance (Zin=0) is zero. This is because the voltage is zero and the current is a maximum (Z=V/I).

When the patch antenna is fed at the edge, it will radiate. The impedance is typically very high (Zin = 200-400 Ohms). So the patch will radiate, but the impedance matching can be improved by moving the feed point closer to the center of the patch antenna.

[Abu Maria.]

thank you very much bigsteve
i forgot
why the antenna can be viewed as an open circuit ? is because we have choose L=lambda/2 , or there is another cause physical ?
because when we choose L higher than lambda/2 , we have not current equal zero , and we have another radiation
+
why the 2 bord does not radiate ? why please ?
thank you.

[bigSteve]

The patch antenna is an open circuit because you can view it as a transmission line that is open. That is, the patch terminates in nothing; it is open.

I have no idea what 2 bord is.

[Abu Maria.]

thank you big
but
when for exemple L=lambda/3 , current different of zero , although the patch terminates in nothing!
sorry for the inconvenience

[Abu Maria.]

i think that L=lambda/2 , give a current equal zero in the edges.

[admin]

It doesn't matter what L is. The current has to be zero at the edge of the patch. Current has to go somewhere. If the patch is terminated in an open circuit, then the current is zero at the end.

[Abu Maria.]

thank you big , thank you admin
so
when the length of the patch L equal to lambda/4 , we have a radiation , because , the patch is terminated in an open circuit , then current equal to zero in the end of the patch , and voltage will be max or min , in the end of the patch , in the start of the patch cuurent will be max and voltage will be zero. then we have radiations in one edge only. true or false ?
thank you
admin
can you tell me , why a patch antenna radiate in 2 edge only , and the other two edge not radiate

[Abu Maria.]

then...true or false!!
and
who know , or who can explain physically why 2 edges doesn't radiate...
thank you

[admin]

False

The voltage and current are always out of phase on a patch antenna because it is an open circuit

If you feed a quarter-wavelength patch at the edge, then the current is zero at the edge of the patch, which means the current is a maximum a quarter-wavelength away. If the current is a maximum, and the voltage is 90 degrees out of phase, then the voltage is zero. Then the impedance Z=V/I will be zero, so no power is delivered to the antenna, and it won't radiate.
The patch should be about a half-wavelength long and fed between the center and an edge.

Two of the edges radiate because the voltage is constant across the width of the patch, as seen here:
http://www.antenna-theory.com/antennas/patches/antenna.php

Along the length of the patch, the voltage is varying like a sinusoid. Since the voltage is zero at the center, it will be positive on one side and negative on the other. Hence, the fields cancel out on average and no radiation occurs along these edges.

[Abu Maria.]

thank you
last question
why we have Fringing Effects ? physically why ?
is there because we have a patch smaller then ground plane so we have field line courbed ???