Co Ax Cable as Impedance Transformer

[Gito]

 I have a question  To Ask


As we knew people used to make 1/4 wave co ax as an impedance trans former

For instance A 25 Ohm load /Antenna using 1/4 wave 50 Ohm Co ax feeder has an output of 100 Ohm   Z1 X Z2 =  Z0 X Z0

so the impedance raises from 25 Ohm to 100 Ohm as the length of the co ax get longer from 0 cm to ....Cm( quarter wave length)

What make me think ,That there is a 50 Ohm impedance point  in this length if made  shorter than 1/4  wave length

A resonance Antenna with an impedance of  25 Ohm can be fed with a length of 2 wave length/or more 50 ohm Co Ax,using this tech nick ,by pruning this length ,we can can get a 50 hm load at the end of the  Co ax ,   25 to 50 Ohm (transformer action of the Co Ax

Am I right

Giio.N

[AB2EZ]

Gito

At any point along the coax, the impedance is a complex number: Z = R + jX. In the specific example that you mentioned, there is a point along the coax (1/4 wavelength from the 25 ohm load) where R= 100 ohms and X = 0 ohms.

Between the load and the 1/4 wavelength point, these is a point where R= 50 ohms, but X does not equal zero ohms. At that point, X is approximately 35 ohms.

You can easily find out more details on line, or in the print literature, by using search terms like: "Smith chart".

http://www.amanogawa.com/archive/LossLessSmithChart/LossLessSmithChartWide-2.html

Stu

[Steve - K4HX]

One way to transform your 25 Ohms to 50 Ohms is with a quarter wavelength of 37.5 Ohm cable. Where do you get such a cable? Parallel two 75 Ohms cables. To be precise, the cable impedance should be 35.35 since it has to be equal to sqrt (Z1 x Z2) or sqrt (25 x 50).  But usually 37.5. is close enough.

Another approach is to us a non-synchronous transformer. Here you would connect the 25 Ohm load to a 50 Ohm cable that is 29.33 electrical degrees in length (or 0.0815 wavelength). In turn, you would connect this cable a second cable, 25 Ohms impedance (made of two 50 Ohm cables in parallel) also 29.33 electrical degrees in length.

[W1RKW]

Stu,
is that website safe? I'm getting a security alert.

Yes, the Smith Chart is the tool of choice for situations such as this.

[AB2EZ]

Gito:

If the question is really how to transform a 25 ohm input impedance antenna to a 50 ohm input impedance... then a good choice for some applications would be to use an appropriate ferrite core to build a broadband matching transformer.

With 7 turns on one winding and 5 turns on the other winding... the turns ratio will be 1.4:1, and the impedance transformation ratio will be 1.96:1.


Bob:

I don't know if that web site is safe. Since the topic is Smith charts, and since the application runs on Java... I decided to use it even though I don't have any reason to trust the source.

Stu

[Gito]

Thanks


That is the  Question Why must we  always used a 1/4 wave long Co Ax,
As we know  25 ohm  load goes from 25 ohm to 100 Ohm at the 1/4 wave long point and goes back to 25 Ohm  at the half wave long.
 
Actually a  Co Ax impedance transformer is  Cutting/ pruning a length of Co Ax and it happen that a 1/4 wave length  gives a standard calculation As Z1 X Z2 = Z0 X Zo        Zo - Co Ax impedance

Steve used two    1/4  length 75 Ohm  connected parallel  to have a 35  impedance :Co Ax

And in the other hand pruning  a half wave length Antenna ,we can get a 35 ohm  impedance point .

Yes of Course we can used a Ballun trancformer

But I think why do we need this as we can find  more easy way by pruning this length of Co Ax

Am I right

Gito

Gito.

[Steve - K4HX]

Yes, you can prune a quarter wavelength coax in your example and obtain a 50 Ohm resistive point. But you have a reactive component too. If you prune the length to about 0.15 wavelength the reactance will be inductive. If you add length until you have about 0.35 wavelength, the reactance will be capacitive.

The three methods previously suggested will result in a 50 Ohm resistance with little or no reactive component (assuming the 25 Ohm load was purely resistive).

[KA2DZT]

Gito,

What type of antenna are you trying to feed that is 25 ohms??

Fred

[Gito]

Fred

I am thinking  combining four 50 ohm  resonant Antenna  with a power divider.

Where two of this antenna is combined  with  two 1/4  wave length Co Ax

,giving a 25 ohm output load

the other two  is also combined and also gives an 25 Ohm load

This two 25 0hm load is also combined with two 1/4  wave length Co Ax

Each output has a 100 ohm load ,combining this two gives a 50 0hm load

Why this 100 Ohm must be  combine with 1/4 wave Co AX,of course to get a 50 ohm load

Suppose we don not use a  1/4 wave co ax (shorter) than maybe the
  load will be 40 Ohm with a little L (inductive)

using a long  feeder  at the transmitter side  the impedance can be matched by trimming the co ax by the transmitter side site

Since  shorter than a quarter wave is inductive,we can trim the Co Ax/ Feeder length at the Capacitive side,longer than 1/4 wave but shorter than a halve wave length

So this two L an C cancel each other


Gito


Gito