Title: Co Ax Cable as Impedance Transformer Post by: Gito on June 16, 2013, 04:14:44 AM 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 Title: Re: Co Ax Cable as Impedance Transformer Post by: AB2EZ on June 16, 2013, 09:31:33 AM 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 Title: Re: Co Ax Cable as Impedance Transformer Post by: Steve - K4HX on June 16, 2013, 11:02:43 AM 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. Title: Re: Co Ax Cable as Impedance Transformer Post by: W1RKW on June 16, 2013, 12:09:40 PM 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. Title: Re: Co Ax Cable as Impedance Transformer Post by: AB2EZ on June 16, 2013, 02:05:25 PM 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 Title: Re: Co Ax Cable as Impedance Transformer Post by: Gito on June 16, 2013, 08:11:22 PM 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. Title: Re: Co Ax Cable as Impedance Transformer Post by: Steve - K4HX on June 16, 2013, 08:54:34 PM 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). Title: Re: Co Ax Cable as Impedance Transformer Post by: KA2DZT on June 16, 2013, 10:02:45 PM Gito,
What type of antenna are you trying to feed that is 25 ohms?? Fred Title: Re: Co Ax Cable as Impedance Transformer Post by: Gito on June 16, 2013, 11:00:19 PM Fred AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
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 |