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Txline between non-50ohm antenna and receiver.

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littlemage

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I've one non-50ohm antenna, and the matching circuit and receiver, but there's some distance between the antenna and matching circuit, Txline is needed between them for connection, but in this case, which kind of Txline should be used, low characteristic impedance Txline, 50ohm Txline, or high characteristic impedance Txline?
Thanks in advance!
 

hi
the impedance of the line should be the same as antenna before matching circuit
This is simple or I'm making some mistake?
 

What is the impedance of the antenna ?

If your antenna has 50ohm impedance, the below holds good.
you will have to use a quarter-wavelength transformer to convert your non-50ohm transmission line to 50ohms.

For example: If you want a 50ohm line drawn from a 100ohm line, the 1/4 wavelength transformer will have an impedance of 70.7ohms (sq rt (100x50)).
 

Ideally the antenna matching circuit should be right beside the antenna.
In your situation the best way to do it is to use a simulator and integrate that Txline into the antenna matching circuit.
 

vedagarbhamsandeep said:
What is the impedance of the antenna ?

If your antenna has 50ohm impedance, the below holds good.
you will have to use a quarter-wavelength transformer to convert your non-50ohm transmission line to 50ohms.

For example: If you want a 50ohm line drawn from a 100ohm line, the 1/4 wavelength transformer will have an impedance of 70.7ohms (sq rt (100x50)).
Click to expand...

of course this is some kind of matching itself.but appropriate for real impedance to real one transformation and if the impedance is not real it does not work.I agree with vfone
 

The problem is that if the antenna is not matched by design, it probably has a complex impedance. For example 20+j15 ohms. So you can not just hook up a 20 ohm transmission line to it and expect to see 20 ohms resistive at the far end....the reactive part makes it spin around on the smith chart.

So, ideally you would match right at the antenna. If not, you need to figure out the transformed impedance at the far end of the line, and match to it. The longer the transmission line, the harder it will be to obtain a good broadband match, though.
 
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    WimRFP

    WimRFP

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Thanks biff44, that's the real case I encountered. The antenna impedance will spin around on the smith chart widely comparing with no Txline, that really makes wide band matching difficult.
And my another concern is whether this Txline will impact the antenna efficiency's transmission to receiver, if so in which case (low impedance or high impedance or other) the loss will be small.

biff44 said:
The problem is that if the antenna is not matched by design, it probably has a complex impedance. For example 20+j15 ohms. So you can not just hook up a 20 ohm transmission line to it and expect to see 20 ohms resistive at the far end....the reactive part makes it spin around on the smith chart.

So, ideally you would match right at the antenna. If not, you need to figure out the transformed impedance at the far end of the line, and match to it. The longer the transmission line, the harder it will be to obtain a good broadband match, though.
Click to expand...
 

One trick you can do is figure out the impedance at the antenna terminals, add a series L or C chip to make the impedance real at midband, and then run a transmission line who's characteristic impedance is the same as the antenna/series chip real part. That way, at least it does not rotate all over the place (for narrowband). Then at the chip where you might have more layout room you can do a simple impedance transformation.
 

Thanks, this is helpful.
But one of my concern is though I can have a matching between receiver and after the Txline, I can't get match between antenna & Txline input, will the reflection is big between antenna & Txline input under this case? Then it cause the antenna's efficiency drop though the match between Txline & receiver is good. Is this right?
 

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