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In my application, the device is going to be placed on top of a metal disc, less than a centimeter away from the pcb!

Now I have prototyped up the device using a nRF24L01+ breakout board with only a chip antenna, and I can communicate with the device from a few meters away (obviously from a receiver with line of sight to the PCB, not from underneath the disc). However, if I want to enhance my design and enhance the reliability of the wireless link what kind of antenna should I use?

Should I switch to a lower RF frequency band?

From what I have read around the internet, a patch antenna with a ground plane underneath might be a good antenna to use, because the effects of the metal disc underneath the whole pcb will be reduced since our design involves a ground plane underneath the antenna anyway. This seems to take up a lot of space as opposed to a IFA antenna etched directly on a pcb.

However, being quite new to all this RF theory, I basically don't really have a clue. Any ideas?

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cksa361, I am confused by your bounty because I am not sure what else you need answered except for the comment you left me, so I will answer that.

When you pick between different chip antennas you will normally not find one that is significantly better than another in every way unless a new process was developed. In most cases you will have to pick the one that better fits your application, and to you this will seem significantly better.

Radiation Strength

The larger one, Mica, has better radiation characteristics(more power out for the same power in). This means that for two transmitters radiating the same power, this antenna will get greater range. This also means the Mica antenna will receive from a source at a great distance much better.

Size

The Taoglas is smaller. This is self explanatory, if space is an issue, or if the odd size of the mica causes problems, the Taoglas wins.

Tuning

You will need to tune the matching circuit for the Mica one. For the Taoglas it seems they have a set matching circuit. This problem is approached in the question about measuring output impedance. This can be challenging if you do not have the right equipment. A mistuned antenna will severely hurt your range, if you cannot tune you may find the Taoglas will be easier to use.

Edit: I misread the datasheets, they both have reference designs. If you are going to vary your layout from the reference design (ie. You do not have room to do the exact say layout) on either antenna you will need to re-tune the circuit, as was linked in the paragraph with the strike-though.

I hope this helps.

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  • \$\begingroup\$ Kortuk, I have a similar question and you appear to be the resident RF expert. :-) I've got an IFA at the edge of a small board and the board is mounted in a metal "bowl" with a glass top. (imagine a watch case made of metal) - the metal case is NOT grounded. I know the metal case will seriously impede the antenna's abilities, but would it be better to select something like the Mica antenna or a more conventional IFA that requires the lack of ground plane under it? The board is a tight fit (mm of gap) in the case and the antenna is in the one corner. \$\endgroup\$ – akohlsmith Nov 20 '10 at 20:28
  • \$\begingroup\$ This is a messy situation. If the antenna is going to be right next to your bowl, you might have something that you cannot tell about. I would guess the chip antenna would do better, but you really might just have to test. Also note that this will screw up your tuning, you will need to re-tune when you place the board in location. Do you own a VNA? \$\endgroup\$ – Kortuk Nov 21 '10 at 4:09
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How to cope with IFA

So, if you place a large ground plan below an antenna you create an image of the antenna and equal distance from the ground plane but on the other side, as if the ground plane is not there. The only important part being any solution exists above the ground plane, and there is 0 below it.

Not getting caught up in the solution, try placing the ground plane plate one half wavelength below your IFA radiator. This should cause the radiation up to be doubled. You can test this, move it around, measure your RSSI with different distances, pick the one that you like the most.

I hope that is clear, let me know If i can do more.

Your solution

From a general perspective, you will be happy at about 2.4GHz, it has a lot developed at it, this reduces cost. An IFA antenna seems to get you a nice form factor, it is actually a type of patch antenna, and if the ground piece causes a problem just look into other patch. These are commonly used because of low cost and low form factor.

Other options

If you need to radiate up from the disk(ie. The direction that passes through the disk) then the patch antenna is a very easy method. There are also chip antennas designed to have a ground plane. Both these will radiate very well.

If you need to radiate in the place of the disk, to its sides, this is where you cannot go wrong with a half wavelength dipole. Specifically the quarter wavelength monopole, using the ground disk as a conductor. This can be a very effective configuration.

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  • \$\begingroup\$ I took it that the distance was controllable, if it is not, please test what I said, and if it does not work, look into other solutions. \$\endgroup\$ – Kortuk Nov 4 '10 at 4:49
  • \$\begingroup\$ unfortunately the distance is not uncontrollable to the disc. The device is merely placed on top of it. these discs also sometimes come with a rubber coating of unknown thickness (< 1cm). \$\endgroup\$ – cksa361 Nov 4 '10 at 5:03
  • \$\begingroup\$ i would suggest a basic patch or monopole then. \$\endgroup\$ – Kortuk Nov 4 '10 at 16:21
  • \$\begingroup\$ Antenova Mica -- antenova.com/?id=408 or Taoglas --> taoglas.com/images/product_images/original_images/…. Which one should I get? Both allow ground planes underneath the chip antenna which is good. \$\endgroup\$ – cksa361 Nov 19 '10 at 6:43
  • \$\begingroup\$ The larger one, antenova.com/?id=408, has better radiation characteristics(more power out for the same power in), taoglas.com/images/product_images/original_images/… is smaller. You will need to tune the matching circuit for the Mica one. For the Taoglas it seems they have a set matching circuit. You need a VNA for designing the matching circuit, so if you do not have one you may have bad results with the Mica antenna. \$\endgroup\$ – Kortuk Nov 19 '10 at 14:59

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