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Why does DN038 pcb helical antenna need shortening to achieve 915MHz?

Ron Segal1
Prodigy 150 points
Other Parts Discussed in Thread: CC-ANTENNA-DK2

We tried to follow exactly the dimensions specified in DN038 to fabricate a helical pcb antenna for 915MHz frequency.

When SWR is measured using a VNA the resonant frequency is around 678MHz rather than the 915MHz target.

(For analysis there is a 0 ohm resistor in the path. VNA calibrated for sweep from 600MHz to 1000MHz. The soldered pigtail shown was port extension compensated 481ps) 

Subsequently we had to remove (grind away) 3 whole helical turns to increase the frequency to reach the target value of around 915-920MHz:

This doesn't seem right with standard 1.6mm FR4 pcb and exact helical dimensions implemented in Altium Designer.   Similar measurement on a second board. VNA setup proven by checking against a reference antenna with know frequency.  

Be keen to hear from more RF savvy engineers of potential reasons for such a big frequency "discrepancy". Thanks.

  • 0
    TI__Mastermind 33660 points

    Hi,

    Interesting that you are measuring such a low frequency for DN038 since we have increased the size of this antenna from DN038 in the CC-Antenna-DK2 with an extra link.

    When measuring the antenna, it should be placed in free space and not directly on the lab bench since this could affect the results. I usually measure the antennas hanging of the bench in air.

    Removing three links to get 915 MHz does not sounds right so I'm dubious that you are measuring correctly. Can recommend the CC-Antenna-DK2 kit for antenna measurements and debugging purposes. 

  • 0 in reply to RGW
    TI__Mastermind 33660 points

    DN038 was originally designed for an EM board whilst plugged onto the motherboard (TrxEB). This antenna is dependent on the GND plane size and the antenna matching network for resonance at 915 MHz.

  • 0 in reply to RGW
    Prodigy 150 points

    Thanks, you're absolutely correct, the problem was measurement too close to the bench. In the air the frequency shoots up. Such a stupid error.  We'll order a CC-Antenna-DK2 kit. Very useful to have some calibrated reference antennas.

  • 0 in reply to RGW
    Prodigy 150 points

    Thanks for your reply.  From my silly error, the ground plane size clearly does have a very significant impact on resonance.