miniVNA Tiny for bluetooth antenna design?

[mhexrobot]

Hello

I was wondering if anyone has used the miniVNA Tiny for designing matching networks and antennas, particularly SMD matching networks and PCB trace antennas for Bluetooth Smart?

Unfortunately I work for a company with no foresight or willingness to invest in R&D so am forced to look for the cheapest option. This device looks dubious to me and I vaguely recall reading something about the lack of phase information but cannot seem to relocate this. It appears to be designed for HAM radio users, which I am not, so I don't really know what to expect...

I would be very keen to hear the opinions of any that have used this device, especially for what I am describing but also just any general remarks about it's features/annoyances..

Regards

m

[leafi]

Hi

I purchased a VNA TINY and we have used it for this purpose. I am not an RF engineer so my knowledge on the subject may not be perfect however it does show gain and phase. You can also plot the results on a smith chart.

Specifically what we were doing was to hook up a BT antenna and we can see how our housing and the human body affected the tuning and Q of the antenna. Obviously we couldn't see antenna gain changes. It is not the fastest sweeping device however for 500 bucks it does the job. It also came with a SMA calibration kit. (a cheap looking one but it seems to work) I do not have a real VNA to compare it to but it seemed pretty decent especially for the price.

There are some annoyances I have but it may be that I am just not familiar with the VNA application.

If you ask me specifically to try to do something I can try to do it and post the results.

Keep in mind for trace antennas your feed length with the cable can play a role. Again I am not an RF guru but we had problems with my spectrum analyzer with a sweep generator (scalar network analyzer) making good measurements. (Granted we also had problems with a real VNA we rented) basically the measurements always changed. The designer who made the board did not put a connector down on the board for antenna calibration so we had to try to dead bug one on or solder coax. (I really would not recommend doing that).

Mostly what our RF consultant wants with our current design is to have the antenna alone with a feed point. Basically get the 50 ohm feed line out and only tune the antenna.

So basically I'm pretty happy with mine of course ignorance is bliss. 

[mhexrobot]

leafi, thanks for the quick and informative reply.

I'm not an RF engineer either but what I am trying to do is tune an antenna matching circuit from a manufacturer supplied sample. The component layout is similar but it has been adopted from a 4-layer PCB USB stick device to a 2-layer standalone thing and so I expect some careful tuning is required. I'd also like to try different antenna designs and plastic housings to try and get the best out of the little space available.

I was a little worried about feed length as you mention. I could possibly place a connector on board but I was a little worried about what the effect of then removing this after tuning would be. I was hoping I could use a very short length of coax soldered directly to the PCB with the shield as close as possible to the feed point. I thought this way I can calibrate the VNA at its connectors and don't have the weight and tension of a big cable rip up components or traces. By the way, what did you do in the way of SMA terminated cable for a test lead?

Were you also able to use the miniVNA as a spectrum analyzer to look at RF output for harmonics?

[leafi]

Hey sorry for the delay.

From my experience with it the person who laid out the antenna I was trying to mess with used a TI reference design however he did not have the ground plane next to the antenna as it should. Due to this the antenna was horrible. I would definitely recommend a connector. Also keep in mind you have to be able to isolate the antenna from the transmitter and any low pass filters you have in the antenna net.

Our RF consultant ended up having me cut the antenna off (it was on a rigid flex) and solder a coax cable to it but it is extremely fragile. I would think that if you copy the OEM guide it should be pretty close. What we were planning on doing on this current design (a different one but same thing) was to make the antenna a little long so the tuned frequency is low. Then we could cut back the antenna trace to increase the resonant frequency.

From our first attempt with the HP VNA we rented the measurements were all over the place. Each time I took a measurement it was different and a lot of it was due to cables and connections. Keep in mind you have to calibrate the VNA and from that point it should be 50 ohm. So if you have a feed line that is 2 inches to your board that is not part of the calibration it will throw it off. I had a very low loss high quality RF cable which was like 2 feet and it was making it impossible to get a real value. Some of it was the cable but it was also the missing ground plane as everything affected the tuning and it was all over the place.

If you have an inline u.FL connector you can get SMA to u.FLs on ebay very cheap. This will help minimize the feed length. Also having a real connector will give you a better ground for the connector.

They also do see RF switches where they make connection when you plug them in. Personally I would do the u.FL and make it so there is no stub as best as you can.

I have not tried to see if the mini VNA will operate as a spectrum analyzer. I do not think it will but I am not certain. Keep in mind harmonics of 2.4Ghz will be beyond the range of the mini VNA anyway.

What you really can do and I did a few weeks back was to hook up the antenna and see how the plastics and human body change the Q and tuning. We were using a few Taoglass antennas and it was quite noticeable on their return loss.

Keep in mind if you go the coax route (which you can do and we did do on the first one) the coax is a huge torque point compared to what the circuit normally has there. Just bending micro coax cable can rip pads / traces off the board. It can be done and this was how our RF consultant asked for it but you have to be really careful and it is not something I will ever do on purpose since I had a lot of swearing on the project.

I hope I didnt miss anything it is getting late. Sorry about the long delayed response. I didnt get any emails saying someone responded. Personally I think the Mini VNA is well worth the money. Even since the company didnt pay for it. I have also used it at home to look at cellular antennas and wifi antennas to see how they are behaving. I just need a good USB spectrum analyzer now. I have a RF explorer but it is sooooo slow. I think I want to buy a signal hound BB60C or A but they are above the pain threshold for purchases so Im hesitant. I have a 3GHz and an 8GHz spectrum analyzer at work but they are too big to carry around. A signal hound would let me take it with me which would be nice if I have to go out into the field or a customers facility... or my house.

[hcglitte]

Hi,

I also have the miniVNA tiny. I have not compared it to an expensive unit.

But, I did measure an antenna at 868 MHz. I used a semi rigid coax and ferrite beads around the coax. Calibrated on-board with short, open and 50 Ohm using 0603 components. Them performed the measurement, then calculated the required matching elements. Soldered them onto the board. Did new measurement. Now I was at smith center. So, the miniVNA seems to work.

I guess that it should be easy to find out whether the unit actually shows the correct impedance. One could make a known impedance load and measure it to check if this load is shown correctly by the device, then design a matching network and see if you end up where you are supposed to.