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Tri-band Antenna Impedance Matching Network Help

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tsunami_all

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Hi,
I would like to impedance match a tri-band antenna using lumped elements.

Target band:
Band 1: 0.699 GHz - 0.960GHz
Band 2: 1.71GHz - 1.99GHz
Band 3: 2.11GHz - 2.17GHz

How feasible is this if the network of L, T, pi and/or combination of L T pi is used?
If this is feasible, what are the best techniques to use?

Thanks
 

It would help if you gave teh impedances you want to match otherwise it is just guess work
You can't just match 3 arbitrary impedances at 3 arbitrary frequencies with a fixed matching network.
A multiband antennna is usually designed to give a reasonable match at each operating frequency.
If the impedances are not close to 50ohms then you will need to actively tune the network each time you change band.
Look at the data sheet for your antenna, the manufacturer will almost certainly give some application notes.
If you're designing your own antenna then make the impedance close to 50ohms on each band by the antenna geometry. If you search, there are plenty of papers describing triband antenna design.
 
The antenna has been designed and fabricated.
Here are the impedance on each band.

Band 1: 0.699 GHz - 0.960GHz, Impedance: 11.756 - j135.919
Band 2: 1.71GHz - 1.99GHz, Impedance: 231.817 + j229.845
Band 3: 2.11GHz - 2.17GHz , Impedance: 63.544 - j186.479
 

As was stated, you can use a fixed frequency matching network only at one frequency. May be possible to use a wideband matching network, but your frequency range is too high for doing this (699MHz to 2.17 GHz)
Designing the tri-band antenna have two choices: first, to design the antenna in the way that resonate itself at those three frequencies (and don't need any matching network). Or, to use three separate antenna inputs (if is possible) each of them having (or not) its own matching network.
 
What happen if the resonance (the 3 bands mentioned above) of the antenna (before matching) are completely off from where they are suppose to be?
There is only one input to the antenna.
 

What happen if the resonance (the 3 bands mentioned above) of the antenna (before matching) are completely off from where they are suppose to be?
Relating the question to the impedances listed in post #3 (real impedance ratio 1:20, dominant reactive impedance), I don't expect that it's possible to achieve reasonable matching.
 

The antenna has been designed and fabricated.
Here are the impedance on each band.

Band 1: 0.699 GHz - 0.960GHz, Impedance: 11.756 - j135.919
Band 2: 1.71GHz - 1.99GHz, Impedance: 231.817 + j229.845
Band 3: 2.11GHz - 2.17GHz , Impedance: 63.544 - j186.479

If antenna have been fabricated, why? It is in that case very poor fabricated antenna.
Impedance curve shows poor matching for all frequencies and measured values from above does not even seem likely that they are correct measured or else is it an unusual antenna.
I did use your values from above but added some smoothing so that it did become a curve instead of three dots.
Smoothing is relative small and follows your impedance values rather close.

Your untuned antenna impedance is plotted below as yellow curve.
smith.png

As can be seen is it a strange looking impedance curve but I accept that this is a correct measured antenna impedance.
For decent impedance match should actual frequencies be within red circle, corresponding to VSWR less then 3:1.
This curve is way off.

It is correct that it is a very wide frequency range to tune. Result is often limited and good impedance match is not a guarantee for an effective antenna. This antenna curves do actually show signs that it is a non radiating resistor more then an antenna.

Just as an example for your antenna, as a proof of concept, yes it can be wide-band tuned to be acceptable impedance matched.
AnTune software did automatically select best topology and then selected optimal components. Result is as below.
graph.png

VSWR before matching is orange curve and yellow curve is after matching. All your required frequency ranges have now a VSWR better then 3:1.

Matching network is a relative simple T network:
schema.png


S-parameters used for calculations for the coils are from Murata LQW15. It is important to not change this type without recalculate the network, especially due the use of L2 68nH with self resonance frequency of 2.5 GHz, any else type or brand will behave rather differently.
C3 is sensitive for fringing losses, which may need adjustments to compensate for, depending om PCB layout.

Resulting values:
data.png

smith.png

Orange - original unmatched.
Yellow - matched curve.

Have also attached a zipped file containing S-parameters for your assumed antenna impedance after that I had smoothed it a bit and final S11 with tuning network applied, in case you want to double check result.
 

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  • S11_untuned_tuned.zip
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