quarter wave transformer impedence matching

[shopi Ram]

Hi,

i wanted to match a complex impedance antenna using a quarter wave transformer to 50 ohm feed. It has been given in Balanis that

Another technique that can be used to match the antenna to the transmission line is
to use a λ/4 transformer. If the impedance of the antenna is real, the transformer
is attached directly to the load. However if the antenna impedance is complex, the
transformer is placed a distance s0 away from the antenna, as shown in Figure 9.19(b).
The distance s0 is chosen so that the input impedance toward the load at s0 is real and
designated as Rin . To provide a match, the transformer characteristic impedance Z1
should be Z1 =√RinZ0, where Z0 is the characteristic impedance (real) of the input
View attachment untitled.bmp

Then what is the impedance of the line btw the quarter wave transformer and antenna i.e, along the length So???

thanks
shobi

 

[tyassin]

Hello,

If you have your antenna impedance in the Smith chart normalized to Zo=50 ohms, then adding a line with 50 ohms make your impedance turn clockwise in the chart. This has a constant radius. So at some point this will cross the horizontal line(Real axis) ie. real input impedance. This impedance can then be transformed by a quarterwave transformer.
You can also do a transformation in a single step using a single line with an impedance different from Zo, then your lenght will also be different from lambda/4. In this way you tune the real and imaginary parts at the same time.

Regards

 

[shopi Ram]

Hi,
Thanks Tyassin, that was useful.
I have another doubt.pls help me with that also.......

corporate feed is actually a power divider.It has three ports. Let P1 be input P2 and P3 be output. If the input impedance (impedance of P1) is 50 ohms ,then the impedance of P2 and P3 is 100 ohms (I referred few text books,most of it have it this way and they use 70 ohm quarter wave transformer to match it with 50 ohm line) .
Why is it???
Can't we use a simple 50 ohm line there too????we can put one quarter wave transformer near the antenna(assume my antenna has only resistance)to match .bye this way we can eliminate the need for all other quarter wave transformer can't we?????

Also ,If u could help me with some material for this corporate feed design it would really be useful....

Thanks in advance
Shopi

 

[tyassin]

Hi and happy new year.

Well what is important is what is the impedance that P2 and P3 is connected too? If it is 50 ohms then you need to transform these to 100 ohms which is the reason of using quarter wave transformers of 70.7 ohm impedance. Then you transform the 50 ohm connected to P2 and P3 to 100 ohm and having two in parallel gives you the wanted 50 ohm at P1.
You will always need some transformers. The problem is that you what 50 ohms and this mean you have to up-transform impedances.
The last thing you ask I am not really sure what you mean. Do you have a single antenna with two feed or and array or what?

Regards

 

[shopi Ram]

Hi,

Happy new Year Sir.
I am designing a 4X1 microstrip patch antennae and the input impedance offered by each is 100 ohms. I am using a corporate feed and the source impedance is 50 ohms. I am referring Balanis for the design.But it is not very detailed

Can u suggest some good papers or books for corporate feed design ????

 

[tyassin]

Hi

I can not recommend any particular books or papers. But the matching should be pretty straight forward if you equal power split.
What you could do is to combine two antennas at one point. This would give an impedance of 50 ohms. Now you have two branches which you should transform up to a 100 ohms using a quarter wave transformer with a impedance of 70.7 ohms. Now you can merge the two lines together which will give 50 ohms.

you could also just connect all four lines together at one point which would give an input impedance of 25 ohms. Then using a quarterwave transformer of 35 ohms to give you the wanted 50 ohms input impedance.

Hope this helps.
Regards