Bluetooth antenna
I had been working on some of the Bluetooth (2.4GHz) antenna.
Some design I implement is the IFA and the loop antenna.
I found that by overall the loop antenna VSWR control is better when in near with hand. Where as, the IFA VSWR can be easily shifted when near with hand.
I would like to get some feedback, comments or any experience encounter from the forummer on which antenna design type is more stable and preferable .
thanks,
spid3rx
Hello,
It is normal that when you approach the antenna with your hand, VSWR changes. Especially when you approach the antenna from the direction of maximum radiation. Your hand reflects some energy back into the antenna the will result in change in VSWR. When you approach your antenna within about 3 cm, the effect will be significant as you are in the near field where you modify the inductance and capacitance of the structure, hence the center frequency and matching.
When the VSWR doesn’t change much, it is mostly a sign of bad radiation efficiency. That means, lots of the electrical energy supplied to the antenna, is converted to heat instead of radiation.
When the VSWR becomes better when you have your hand close to the antenna, the matching/tuning isn’t correct. So you have to change the design somewhat. Best is to tune your antenna for the most occurring condition and see whether the VSWR is acceptable for other occurring conditions. There can be circumstances where you don't get VSWR<2 under all operation conditions (body-worn devices, hand held devices, etc).
Thank you for the comment.
Yes, I initially thought that I should design antenna which is not sensitive to the hand, because for the application, there his a tendency user's had will be near that portion of the housing. I have not go for efficiency measurement yet, it seems like I need to quickly go check in a chamber.
Is there any antenna type that I should be worth trying ? I only tried IFA and loop.
thanks
Hello,
What type of antenna one will use depends on available space, availability of relative large ground plane (that can be used as image), available equipment and personal experience/preference.
Hello,
I assume your antenna is on a PCB in the same plane as the ground plane. As long as your VSWR is OK for your band of interest, I think the IFA will not be bad. When orientation of the device is random, the radiation pattern isn't that important. If you have space (I think not) you can use a quarter wave that is shorted artificially with a capacitive head (so you get a T). You can feed it in the same way as the IFA. The radiation pattern is different from that of the IFA (it has maximum radiation perpendicular to the leg of the T.
In case of having time, knowledge, but no money, you can do your assessment of radiation efficiency/pattern yourself without an anechoic chamber, because of the small wavelength and small antenna, you can measure close to the antenna under test, hence the effect of reflections is mostly negligible. Of course you need some detector (spectrum analyzer, diode detector, down converter+oscilloscope) and a reference antenna to compare with your antenna under test.
For measuring reflection (VSWR) you need a directional coupler or a slotted line.