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Latest Findings with my Nano VNA device

June 26

de k3eui Barry



Each week I learn more about what the Nano VNA can teach me.

We all quickly picked up that this device can measure SWR on an antenna.

But knowing an SWR (say 2:1) value does not tell you WHY the SWR is that value, nor how to improve it.

The Nano VNA tells me the IMPEDANCE (R and X) and shows PHASE (timing errors) which is far better information than merely SWR.

I've tested all of my antennas in my back yard.

But I want to "adjust" the values and see what happens - how can I improve these antennas?



So I set up a "test circuit" on my bench: a 50 ohm R, a 200 pf Capacitor, a 10 uH coil (from an old MFJ antenna tuner).

I put these three components in SERIES.

So at resonance, which I set for 3.65 MHz, the impedance is minimum in a series circuit, so the current is maximum.

Let's see what the Nano VNA shows, as I adjust the L and the C for resonance in the middle of the 80m band.

I did a sweep from 3.0 to 5.0 MHz.


So note first the Smith Chart: I 'hit the Bull's Eye' at 3.65 MHz. YEA. That means SWR of 1:1 at that frequency.

Look at the SWR graph. Nothing surprising. Minimum SWR at the resonant frequency.


Look at the S11 (reflected voltage) Return Loss (I plotted it as a negative value, which makes engineers cringe) and it is deepest at 3.65 MHz. YEAH

The R and X values are just weird. I have to think about those graphs.

But note the REACTANCE (red) changes more quickly than the RESISTANCE (blue): why is that? More discussion.

The PHASE graphs are just too good to be true! At resonance, the PHASE angle is ZERO degrees. YEA. I get it.

The voltages (forward/reflected) are back "in-phase" only at the resonant frequency: X(L) = X(c)


So this circuit is set up to MIMIC a real antenna. I can adjust all three values (R,C,L) easily and play with the circuit.

I'll include a photo of the bench RLC circuit as well.

The Nano VNA is much more than an antenna "analyzer".

de k3eui Barry

On 6/26/21 6:47 AM, Barry K3EUI wrote:

Latest Findings with my Nano VNA device

June 26

de k3eui Barry



Each week I learn more about what the Nano VNA can teach me.

We all quickly picked up that this device can measure SWR on an antenna.

But knowing an SWR (say 2:1) value does not tell you WHY the SWR is that value, nor how to improve it.

The Nano VNA tells me the IMPEDANCE (R and X) and shows PHASE (timing errors) which is far better information than merely SWR.

I've tested all of my antennas in my back yard.

But I want to "adjust" the values and see what happens - how can I improve these antennas?



So I set up a "test circuit" on my bench: a 50 ohm R, a 200 pf Capacitor, a 10 uH coil (from an old MFJ antenna tuner).

I put these three components in SERIES.

You have a S21 plot, but isn't your circuit a single port? Or are you putting R, L, C in series between center conductor of Ch0 and Ch1? (In which case, Ch0 sees RLC + 50 ohms in series, and Ch1 sees the same)

I may be wrong but those appear to be wirewound resistors. They will act
like inductors at RF. To prove this, create a dummy load with just those
resistors and measure with the vna and compare with a "load" terminator. I
would replace the resistors with non inductive ones.

I could be wrong and have been previously ?

Marco

Good Catch.

I did the S21 (through the RLC series circuit center conductor of coax) but I forgot - the S21 is already loaded at 50 ohm internal.
so the first time I did the trial, I had R=100 ohm and I missed the Bull's Eye on the Smith Chart.
So I just bypassed the 50ohm R (with some internal inductance) and got these graphs.

But good catch in both cases.
Wire-wound resistors will also have inductance.
Now I will have to go back and measure the L of my "resistor" 10W Radio Shack at 3-5 MHz.


It did show me that all of the graphs indicated the same concept:'
At resonance of series RLC circuit (R=50 ohm)
The SWR is lowest at 3.65 MHz, the resonant frequency
The Smith Chart passed through the Bull's Eye only at 3.65 MHz
The R and X graphs still confuse me a bit, but X is changing much faster than R around resonant frequency 3.65 MHz
The PHASE graph continues to make me smile: yes, at resonance, the phase changes sign
The RETURN LOSS is just superb and very deep (except that I am still plotting this as negative dB, and yes, I know RL is positive)

This is fun (especially during a rain storm)

de k3eui Barru