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Hi, I am fairly new to RF and have been working in electronics as a hobby for decades, but have recently gone back to school and am working toward an engineering degree, either electrical or mechanical, not sure yet but am getting the math and physics out of the way to go either way. So please forgive any lack of understanding on my part. I am working on a project that needs me to determine the values of a coil antenna that will be used in an RFID device at 13.56MHz, I need to design an impedance matching network for the chip that will run the antenna, so I need to know the resonant frequency, inductance, and parallel resistance, does the nano measure parallel resistance? I am using the nanosaver software which has a field for Rp but the values I'm getting at resonance are very high and inconsistent, this is what I would expect if you were to measure the reactive impedance of the antenna at resonance, but the parallel resistance at resonance is effectively the radiated power of the antenna, yes? In any case, I'm a bit lost as to whether the nano isn't going to measure what I need or whether I'm asking it the wrong questions... Thanks!

On Sun, Oct 6, 2019 at 05:13 AM, <sean@...> wrote: > ... but the parallel resistance at resonance is effectively the radiated power of the antenna, yes?

Since ohms are obviously not watts, please give us the formula that you are using for that conclusion.

I am aware of that, like I said, I'm new to RF, but I understand that without a non reactive part of impedance in the antenna, there's no power going into the antenna right? If the LC is at resonance, the impedance is infinite, therefore, there can be no watts. I am assuming the parallel R at resonance partly consists of the power radiated into space, and partly other effects of the current flowing through the LC tank. In the case of RFID, because the passive receiver is powered by the RF field, part of the power sent to the antenna is dissipated in the receiver circuitry through coupling. What I'm trying to figure out is: am I chasing the right thing when it comes to designing a matching network? and is the nanoVNA capable of measuring the real component of impedance at the resonant frequency with any accuracy.

On Sun, 6 Oct 2019 at 12:34, <sean@...> wrote:

What I'm trying to figure out is: am I chasing the right thing when it comes to designing a matching network?

With respect to matching networks, have you seen W2AEW's Youtube
channel?
Also, do follow the link to W0QE's Youtube channel: SimSmith is very
nice software.

--buck

Sean,Are you aware that the simulation software call MicroCap is now free?

Try simulation first to give you a rough idea of your component values, then prototype and test.?
Regards
Larry

On Sun, 6 Oct 2019 at 12:34 PM, sean@...<sean@...> wrote: I am aware of that, like I said, I'm new to RF, but I understand that without a non reactive part of impedance in the antenna, there's no power going into the antenna right?? If the LC is at resonance, the impedance is infinite, therefore, there can be no watts.? I am assuming the parallel R at resonance partly consists of the power radiated into space, and partly other effects of the current flowing through the LC tank.? In the case of RFID, because the passive receiver is powered by the RF field, part of the power sent to the antenna is dissipated in the receiver circuitry through coupling.? What I'm trying to figure out is: am I chasing the right thing when it comes to designing a matching network?? and is the nanoVNA capable of measuring the real component of impedance at the resonant frequency with any accuracy.

Also, LTspice from Linear Technology (now part of Analog Devices) has been freely avaiable for a long time. There is a Yahoo group, LTspice, which provides excellent support and information.

DaveD

Sent from a small flat thingy

thanks, the smith chart tutorial was nice, the matching network I'm dealing with is a bit more involved as it consists of an emc low pass filter on the rfid chip amp, then the matching L network going to the coil antenna, there are some useful tools provided by the chip manufacturer to design the matching circuitry but the parameter I'm missing is the parallel R at resonance, I don't think it's equivalent to the DC resistance of the coil... so that's where I'm a bit stuck trying to figure out a way to measure it.

IMO, you are probably better off measuring the series impedance and converting it to a parallel impedance because of the inaccuracy of the NanoVNA when measuring very high impedances.

Hi Sean,

Your VNA will be used to measure your antenna or other elements in shunt form or it could be used to measure in series through form, that would be a S21 measurement. However, for sake of simplicity let's confine ourselves to SERIES form. That case provides you with S11. And S11 when properly handled will result in the SERIES impedance. You can than convert that series Z to its equivalent parallel form. The method for doing this is quite well documented. Just look for "converting series impedance to its parallel form". Currently, the nano save PC program does that conversion. However, its worthwhile understanding the arithmetic behind the scenes. Its not hard.

Once you have the series form of the Z for the antenna which may be complex, you will have to conjugate match that to your other device. There are a number of utilities that preform that task. Again, the math behind this process is not hard. A portion of the Z, the real part contains both the radiation R as well as the portion that is LOSS. That ratio between radiation R and loss R is sometimes referenced as the antenna efficiency factor.

If you have issues with any of this I highly recommend you get a copy of the TEXT Solid State Radio Engineering by Krauss, Bostain, Raab and READ CHAPTER THREE. The discussion there is EXCELLENT. Hell... Read the whole BOOK!.

Regards, Alan

Rp = (Rs^2 + Xs^2)/Rs
Xp = (Rs^2 + Xs^2)/Xs

This is in the ARRL handbooks. Just have to keep an eye on the sign of the reactance.

As W5DXP mentions, the impedance range is pretty limited. Above a few hundred ohms the errors get pretty excessive. For the high Z's I use old Gen Rad bridges.

thanks, this is all helpful. I found through a few different evaluations of the info I was getting, a range of R, that got me in the ballpark and made tuning the circuit fairly painless (if anyone considers desoldering and resoldering 0603 smt's painless). I'm hoping to measure the total Q of the network and antenna now, using the nano, is this basically done setting logmag display on channel 1, hooking up the matched network and antenna to channel 1, and putting a small coil on channel 2, putting them in proximity and measuring the falloff rate of channel 1 from the maximum amplitude?