MiniVNA antenna tuning question

[Spikee]

Hello,

I bought a miniVNA so that i'm able to test antenna matching on my rf designs.

Now I just got it out of the box, calibrated it and looked trough the manual.

I just tried this:


I just wanted to do a quick and dirty 915MHz antenna. According to the internet a 1/4th wavelength 915MHz antenna should be ~78mm long.
But my SWR / impedance measurement shows that ~64.9 mm gives a good impedance / swr value.

This is all relatively new for me.
So am I correct in to say that 1/4 wavelength equation gives general antenna length but the real length you have to use for the application is determined on many more factors -> thus ~64.9 mm is correct in my case ? or am I doing something totally wrong here  ?

[ConKbot]

What's the quarter wavelength of 915MHz in pvc insulation? I bet your 65mm is so where between that value and the air/vacuum number. Make up another antenna with no insulation and it'll probably be closer to your calculated value.

[davebb]

Hi also try a 1/4 wave counterpoise/ground
The yahoo group I for the minivna is very good
Dave 2E0DMB

[rfeecs]

Try sweeping over a broader frequency range, even the whole range of the analyzer.  You should be able to see the resonant frequencies more clearly.

[Spikee]

I made a 1/4 wavelength ground antenna and was able to get perfect impedance and swr.
Much harde with a 1/4 wavelength monopole.

I measured on of my RC 433MHz radio transmitter antenna and I was surprised that it was that much off:
(this unit this came with is capable of sending 2W, EU made , quite expensive).

And a measurement of one of my test circuits for RF radio impedance matching:

It supposed to be for 915MHz but as you can see it is a bit off.
I have re-checked my 50 ohm feed line on the pcb and my coplanar waveguide calculations were correct. This was a 4 layer board with 3.66 dielectric constant.
So it must have gone wrong in the impedance matching stage.

This is measured with antenna attached (bought for 915MHz). DUT port -> coax -> pin directly soldered on sma socket, coax gnd soldered to sma socket gnd.

This is the matching circuit from TI:

The chip's impedance is 45 + 15J
I assume that the first inductors , caps are the balun stage and from C21 -> and is the filter stage. C20 to block dc.
In the TI document it stated that it was a L match but it seems more like a two stage PI match ?

I did some searching for multiple stages of PI / L match but could not find much info / formulas.

I found some HAM's website which explained something about impedance matching and did a few calculations:


Could anyone shed some light on my findings ?
Am I just measuring it all wrong or .. ?

The layout was more or less a copy from TI, pcb thickness was different but the feed line was adjusted for that.
According to TI the thickness should not have that much of an influence on this.

[Spikee]

Regarding the TI measurement I posted about.
I think TI made an mistake in their calculations / schematic.

First I thought I was just measuring it wrong or pcb design or .. had that big of an influence.


At 915 MHz it showed ~ -5 dB RL. This graph looks just like what I measured.
So TI made a calculation mistake in their dev kits / schematics ? As this chip is characterized for 868 / 915 MHz.

---
To check I did a simulation on the matching circuit for the TI CC110 , 868/915MHz.
Result:

Not fantastic but at least it is in the frequency band it's supposed to be.

[Apollyon25_]

Do you have a photo of your test setup?

The problem I found with simulated results and the results you get from the VNA is typically one of ideal components in the simulation and parasitics in the actual parts and PCB.
This was quite a useful piece of software: http://www.fritz.dellsperger.net/smith.html
Though it looks like you have something similar already. Murata used to have a standalone app with all its components' parasitics... that was great for guesstimating the parasitics of a particular component at the freq of interest.

Just checking, but do you have a coax feed to your board? If so, did you re-calibrate to shift the measurement plane to the point on the PCB? This caught me out a couple of times when I moved setups or labs (work vs home).
I have calibrations saved for a couple of my usual test leads and adaptors. So much easier to re-load than re-cal.