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I have designed a PCB trace antenna for a BLE module. I used an existing reference design, which is offered to be used in your own design. I am able to connect to the radio, and on a spectrum analyzer the frequency is where it's supposed to be (right in the middle 2.442GHz), but the (effective) range is terrible. With the dev board module I get close to 90 meters without losing the connection but with my board I get about 8-10 meters before it drops the connection. The transmit power seems good but I have a very poor understanding of the amplitude of the frequency. When comparing the dBm of my design and the reference design there is very little difference in the dBm. So, my questions are how can I have the correct frequency but such poor range, what attributes to the range, and what can be done to a PCB trace antenna to increase its range?

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  • \$\begingroup\$ Have you properly done the impedance matching? \$\endgroup\$ – Angs Feb 27 '15 at 23:28
  • \$\begingroup\$ Yes, 50Ohms. I practically copied the reference design. The antenna trace and surrounding components is exactly the same. The only difference is my ground plane is slightly larger. \$\endgroup\$ – DigitalNinja Feb 27 '15 at 23:47
  • \$\begingroup\$ Same stackup as reference design? \$\endgroup\$ – dext0rb Feb 28 '15 at 0:03
  • \$\begingroup\$ The reference design gives the PCB specs, 2 layers and 1mm thickness. I kept this the same. They also run the same exact code. I'm really confused, especially after observing the two on a spectrum analyzer (results look the same). I just don't know enough about RF. \$\endgroup\$ – DigitalNinja Feb 28 '15 at 0:08
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Copying an antenna design is hard, it won't work first time even if you keep all dimensions the same. Variations in the PCB dielectric coefficient will change the tuning.

Tuning the transmitter is different from tuning the antenna - the transmit frequency you're measuring is fixed by the crystal oscillator. The antenna is a filter that's independently tuned, and with your test equipment you can't easily sweep it, so you need to tune for best performance at your chosen frequency.

Fortunately its easy to tune the antenna, by cutting and/or extending it. This is the normal process when porting an antenna design to a new PCB, or even just changing PCB manufacturers or board suppliers.

Cutting process Try cutting the tip of the antenna shorter, 0.5 mm at a time. If the results get better, keep going, otherwise extend the track with some wire or copper tape, and start trimming again.

Measuring your results Set up a measurement so you can tell how you are doing, either use the two modules talking to each other, measure the RSSI or measure the range, which is much less accurate. Or measure the transmitted power with a spectrum analyser and a standard antenna. Keep the board under test close to the receive antenna. Rotate the board to get the maximum signal. Then you can start trimming, always measuring in the same position.

If you're going into production with the board, you'll need to control the dielectric. Specify the PCB material carefully, insist that it's from the same batch, etc. You can also keep some spare pads for tuning, zero ohm jumpers, a second capacitor pad in the tuning network, or some other way of adjusting the impedance on an existing board.

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  • \$\begingroup\$ Thanks for your answer. I think I underestimated just how critical different parameters of the PCB are in the RF performance. \$\endgroup\$ – DigitalNinja Mar 2 '15 at 19:14

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