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Helical PCB Antenna for 868MHz

Knud Erik Glud
Prodigy 150 points
Other Parts Discussed in Thread: WMBUS

Hi

I have made a 868MHZ design with a Helical PCB Antenna like the one in the Design Note DN038 by Richard Wallace.

The PCB is a 1.0mm FR4 type.

When I measure the resonance for my antenna it shows app. 1100MHz, but in the design note it looks as the resonance is close to 868MHz.

I have tried to change the number og windings on the PCB but I cannot bring the antenna to a resonance at 868MHz.

If I match the antenna to 50 ohm @ 868MHz, it looks like I get an antenna with low effeciency, i.e. the transmission range is too short compared to other designs.

So my questions are: Do the DN038 antenna have resonance at 868MHz before matching and how good is the performance compared to other antennas (1/4 wave whip).

Thanks in advance

Knud Erik glud

  • 0
    TI__Mastermind 39860 points

    The resonance without matching components is approx 1.3 GHz. 

    Not sure how you change the number of windings. If you want to move the resonance down in frequency you need more windings (i.e. make the antenna longer), and it is not clear to me how you add windings to a PCB antenna.

    The antenna in DN038 is actually very good for its size. If you refer to the design note (sections 3.4.1 and 7) you will see the CTIA report and the efficiency is 65% (=-1.8 dB).

    As stated in the design note the impedance is far from 50 ohms (10-j88 ohm)  and matching components are required. Use a network analyzer to measure the resonance / impedance / S11. Make sure to compensate for the phase delay to measure the impedance correctly. 

  • 0 in reply to Sverre
    Prodigy 150 points

    Hi Sverre

    The resonance is exactly the same as I measured, I just thought that my antenna was out of resonance.

    About adding windings on a PCB antenna, You take two boards and cut one half of an antenna, solder it to the end on the other board. But I have found that the resonance was very hard to move down to 868. In fact I managed only to get it down to 1.1GHz.


    I have matched my antenna using a network analyzer, and after the matching i can measure an impedance of 47-j7 ohm, not perfect but good enough taken into account the measurement errors.

    Another thing is the receiver sensitivity.

    We have tried to measure the this by having two units sending telegrams to each other. We also get the RSSI value from both boards.

    Each board is placed in a shielded box. Between the boards there is a variable attenuator so we can find the attenuation where the receiver stops receiving.

    By this setup we can measure the reciever sensitivity to app. -90dBm (Freq: 868.95MHz, BR = 38kHz WMbus mode T). But the datasheet says -105dBm. Must we expect to have such a low sencitivity?

  • 0 in reply to Knud Erik Glud
    TI__Mastermind 39860 points

    Matching to 47-j7 is actually very good (In my book this is perfectly matched).

    Agree that the sensitivity is on the low side. Before doing radiated measurements though you should verify the conducted performance to rule out other parts of the design as the reason for the degraded sensitivity. Disconnect the antenna and solder a semi-rigid coax cable after the LC filter.