Antenna confusion

[sakitten123]

I have a custom PCB with nRF51822. I have copied the "QFAA QFN48 with internal LDO setup" schematic from Product Specification v3.1. However, I have not copied the PCB layout. The radio is working, but probably not very well. (I use a PCB antenna).

I'm new to antenna applications. How would I go about measuring the antenna performance? What equipment do I need? Seriously, I need a step-by-step guide.

I have read the Antenna tuning white paper (nWP-017) and I understand it in theory (engineering school was good for something). The examples at the end are good, but going from reading it and understanding to actually doing it is a big step. I have no idea where I would start. It talks about adjusting the length of the antenna, but how do I do this? Do I use a knife to scrape some of my PCB antenna off???

And about the shunt capacitor. Would I simply have some different capacitors available, and try and solder one of them, do the measurements, solder another one and do the measurements again, until I find the right value?

Do you think I should redo my antenna PCB layout to exactly match the reference layout (matching network to chip)? A quick price comparison of the matching network components vs. Johanson Technology Balun showed that the balun is cheaper? Have I made a miscalculation? If the balun in cheaper, why would I not choose the balun? Is there some advantage in having the matching network over the balun? The balun seems simpler... Is it?

I guess this is the pain of transferring from school to real life...

[Øyvind Karlsen]

Hi,

First off I recommend reading the blog post on General PCB design guidelines for nRF51. Following the reference layout is a good idea, if you want someone to review your layout, you can post your layout here(if it's something you prefer not to share you can make a mypage ticket to get in touch with our support team).

The measuring equipment you will need:

• Network analyzer(for measuring impedance, SWR etc), common abbreviations are VNA, ANA

• Spectrum Analyzer(for measuring power)

Network analyzer is probably the harder of the two to get running, as you will need to calibrate it manually before use. This is usually covered well by the manual. Once you have calibrated your ANA you will want to look at two different measurements, Smith chart and SWR. Ideally you aim to get your application towards the center of the smith chart, with an SWR of 2 or lower.

For the spectrum analyzer it's a little bit simpler, you set the frequency you know your chip is/should be transmitting at, hook up your device and look for the strongest signal, then you can read the output power from the instrument.

Be aware that these instruments are sensitive, so it's not a good idea to run high wattage(1W is a common maximum) into them. They are also very expensive, so it's not a great idea to break them.

Adjusting the length of the antenna is done to get the desired resonance frequency, generally you will want to cut the antenna to a length where you can pull it towards the middle of the smith chart with a simple shunt capacitor. For cutting the antenna use a pair of clippers(wire antenna) or a knife(PCB) to scrape or cut away the excess antenna.

To find what value of capacitor you should use you can either place random components in small steps, or you can do some smith chart calculations to figure out which values you should use, then fine tune with small steps up and down.

If you use a balun you will still have to do some matching, this is because antenna impedance depends heavily on its surroundings. It is also important to match your antenna for the use-case scenario, for example if it's a wristband, do the matching while it is at your wrist. Baluns give roughly equal performance to discrete components, and are normally used when there is a size constraint, so if you find that the balun is cheaper, go for it.

Hopefully this answers some of your questions, best regards,

Øyvind

[sakitten123]

Many thanks!

I read in the Quarterwave printed monopole antenna for 2.4GHz whitepaper that "For applications where range performance is not critical the antenna can be tuned by measuring radiated power from the antenna with a spectrum analyzer. For more accurate tuning a vector network analyzer must be used for impedance and SWR (Standing Wave Ratio) measurements."

What and how do you tune only using a spectrum analyzer? All tuning examples explains how to do it using a VNA. If I use the balun + shunt capacitor + PCB antenna, what will I see on the spectrum analyzer to indicate that I should scrape some of the antenna off or what value to choose for the shunt capacitor? Trial and error to achieve the highest output power possible? While a VNA would give more detailed info to more exactly tune the length and value, and hence being a "better" tuning method?

[Øyvind Karlsen]

Hi,

Yes, the VNA is the best instrument for tuning, but since it is a very expensive and specialized piece of equipement the white paper suggests that you can use a spectrum analyzer only, and optimize blindly for maximum power out.