How to design impedance matching circuit for loop antenna??

[Jeetkumar]

Hi All,

I have a 1 inch copper tube bended into circular structure of 30 cm inner to inner diameter as shown in figure, i am using this tube for wireless power transmission system, i need to match this to 50 ohm coaxile cable, first of all i dont have any idea how to calculate impedance of tube and than how to make matching circuit, any help is appreciated.



Thanks in advance

 

[E Kafeman]

With loop inductance and frequency known, can you calculate expected capacitance for resonance.
This value can be needed to be adjusted depending on how you are connecting to the antenna and if it should include coupling effect from RX loop and its load.
Connecting 50 Ohm source can be done in many ways, which to select depends on several factors, but for TX can these methods work:
Most simple from matching viewe to connect your 50 Ohm source is like **broken link removed**.
Drawback with this method is that it easy becomes detuned and then drops efficiency dramatic.
A alternative is this one.
Tuning looks simple but it can be a bit tricky to find best matching. I recommend that fine tuning of reactive components are done with a variable cap as in the first example.

 

[Jeetkumar]

Thanks Kafeman,

I am using it to resonate all the four coil, 2 at tx side and 2 at rx side at a resonant frequency of 17.9 Mhz, the thing is that i have calculated inductance and capacitance of coupling coil of tx side, which is 30 cm in diameter as mentioned in my 1 st comment, but i dont know the way how to calculate/ design matching circuit to 50 ohm efficiently. I have simulated in hfss software, but there we directly match the circuit, but in practical we need to design. The inductance and capacitance value which i got are as follow L=606nh and C= 130pf.

I found your 1st method useful, but still not sure of the exact way how to proceed.

 

[jiripolivka]

In practice it is preferred to use variable capacitors to optimize cable to antenna coupling with an inductive loop. Consult ARRL Antenna Book, and use a VSWR meter or antenna bridge to adjust an optimum matching.

 

[E Kafeman]

Take any coil, feed it from your signal generator at some distance from the coil under test. Connect a spectrum analyzer in serial with 3.3 pF (or any relative small value) to DUT. Diode detector/LED can be used as indicator if no analyzer or RF voltage instrument is available. Tune C1 until a peak is found. Should be around 130pF or 2*65 pF.
Then adjust C2 position at loop until R=50 Ohm. If possible measure with a vector network analyzer. Adjust value of C2 until reactive impedance is 0.
If no VNA is available can a two ch. oscilloscope in XY mode be used to measure reactive effect.

 

[biff44]

Low freq. antennas are easy to tune up. But you need some sort of signal source and some sort of test broadband antenna/detector. You tune the frequency back and forth, and maximize the detected signal by tuning the capacitances.

You can't do much without at least that basic equipment.

 

[FvM]

The question is about impedance matching, not just tuning the resonant circuit. You should be aware, that the loop antenna's radiation resistance and also the copper resistance of the series resonant circuit are very low (far below 1 ohm) and thus not easy to match. You can refer to antenna text books for expectable radition resistance values. According to a formula that I found, it will be around 2 mOhm.

Rr = 20 * pi^6 * d^4/λ^4

For possible matching techniques, you can e.g. refer to the "Gamma Matching Arm" in the TI HF Antenna Cookbook and Design Notes, dedicated to long range 13.56 MHz RFID antennas. **broken link removed**
**broken link removed**

P.S.: The loop series resistance, which is mainly based on skin effect, will be already 13.4 mOhm. So it makes the majority of loop real impedance, that has to be matched to 50 ohm. This is under the assumption, that there are no absorbing objects or receiver loops in the vicinity, that can increase the real impedance.

 

[Jeetkumar]

Thanks Fvm, the link which you send me was really helpful, lets hope everything works out well.

 

[Jeetkumar]

Hi Guys,

i have got network analyzer and firstly i tried to see o/p without matching circuit, i got s22 as -6.58 db around 18.26 Mhz (which is nearby to my resonance frequency as far as simulation is considered)but s11 was coming around -11db, and that to around 41 MHz, so what would be the problem with it?, is it because of not connecting matching circui,?s11 was coming around 41 Mhz, or do you think there is some other problem.

 

[E Kafeman]

S11, can you provide a Touchstone s1p table, 10-50 MHz?
S22, if that is the antenna at Rx side, impedance match should not be against 50 Ohm, antenna should be matched against your actual load.
Do same 10-50 MHz measurement both for antenna and actual load. It is between these both impedances you need to calculate a matching network.
If you need help to calculate matching network, provide these files too.

 

[Jeetkumar]

Thanks Kafeman,
I have mailed you the files.

 

[FvM]

Although a single reflection factor magnitude value doesn't tell much, the numbers don't sound credible to me, after knowing the loop impedance X and R values.

 

[RCinFLA]

Think of it as an unloaded resonate tank circuit. With light coupling via small caps (compared to resonating cap) coupling input and output and make a freq sweep measurement for resonate freq and -3db down freq points. The resonate freq divided by the 3 db bw with give you the Q. Q = Rp/X. Figure a match circuit to take 50 ohms to Rp.

Present of your body and other thing near loop will change Rp (usually lower it).

 

[E Kafeman]

Sorry, but the problem was moved outside of this forum.
Here is a part of the matching results: Link
We can probably not solve whole problem for the moment as some completing job must be done in my opinion.
FvM, you is right, there was room for some corrections but I think Jeetkumar now know what to do.