I want to ask you about Bluetooth antennas. I saw that some modules like HC-07 have a zig zag antenna on them:

HC-07 image

And others like BTM-331 do not have such an antenna:

BTM-331 image

Is this antenna mandatory? How does this antenna influence the good working of a Bluetooth module?

Here is the back photo of the BTM-331, the antena is top right corner between the 2 grounds(that half circle). Please exclude the fact that the reset pin is not connected to the GND , I will solder that ti GND because I saw in datasheet that reset is active HIGH.

BTM-331_back image

  • \$\begingroup\$ Do you have a BTM-331 on-hand? Perhaps the antenna is on the back side of the board. \$\endgroup\$
    – JYelton
    May 10, 2014 at 20:20
  • \$\begingroup\$ Yes I have both modules, but I want to use BTM because of its smaller size. I can provide a picture with the back of the module if this help. \$\endgroup\$ May 10, 2014 at 21:16
  • \$\begingroup\$ @JYelton - Any new information or should I buy one with zig-zag antena and dispose the BTM-331? Maybe the antena is that chip on the top-left (front picture) that has an u on it. Any Idea? \$\endgroup\$ May 11, 2014 at 12:52
  • \$\begingroup\$ Unfortunately no new information. I'd say if the BTM-331 does have an antenna, its performance and range would be worse. On the other hand, if it lacks an on-board antenna, then you have the opportunity to add one, which could be better than the zig-zag version. Personally I'd use the one with the clearer datasheet. :) \$\endgroup\$
    – JYelton
    May 11, 2014 at 15:22

3 Answers 3


Zig-zags in PCB traces can be for impedance matching, but also for control of signal integrity and timing.

In the case of antennas, impedance matching and minimizing reflection are the main reasons. Depending on the frequency and physical size of the PCB, a designer may include the antenna as a trace that is a single bar, a "L" or "T" shape, a loop, or a zig-zag. The goal is to match the length of the trace to the frequency in such a way that avoids reflecting power back to the circuit but also corresponds to a common denominator of the wavelength.

Bluetooth operates at 2.4GHz, which has a wavelength of ~125mm. Antennas are often 1/2 or 1/4 the wavelength, so you might find that the zig-zag trace is somewhere close to 31.25mm if "stretched" out.

The BTM-331 doesn't show an obvious antenna in the image, but might have one on the reverse side of the PCB or even included inside an ultra compact ceramic chip antenna like this WRL-00144.


Per the addition of the photo of the back side of the BTM-331, it clearly lacks a PCB trace-based antenna. Either it has a pin to connect an external antenna, as Dzarda suggests, or it uses an antenna that is very small.

If it uses an ultra compact antenna, its performance/range won't be very good. If it allows for an external antenna, then performance could be much better. The question then is whether you are comfortable buying or creating an appropriate antenna, and what sort of performance is acceptable.


Looking at the BTM-331 Datasheet, it only has a RF pin. You need to provide the antenna structure on your motherboard.

From the datasheet:

Pin 26 | RF_IO ANT | Antenna Interface

  • \$\begingroup\$ The datasheet I found here says "The on board antenna eliminate the need of matching circuit on the main board and therefore save the costs of production test." But certainly the information may be incorrect. \$\endgroup\$
    – JYelton
    May 10, 2014 at 20:36
  • \$\begingroup\$ Ok, I will post a picture with the back. I asked this question because I did not succeded to connect to BTM module but I must do better tests. And I also saw that on the data sheet when I decided to buy the BTM module. \$\endgroup\$ May 10, 2014 at 21:20
  • \$\begingroup\$ @JYelton: I suspect they lie. \$\endgroup\$
    – Dzarda
    May 10, 2014 at 22:18

The antenna is necessary for operation, but may be external on some modules instead of etched on the pcb. External antennas, if properly matched, can provide much better results, especially an increase in range.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.