PCB antenna for 868-915MHz

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1. 868/915 MHz antenna overview

1.1. Characteristics

1.1.1. Impedance matching @ resonance

• The antenna is resonant at a given frequency when its impedance is purely resistive at this frequency. This resistive impedance is very likely to be different from 50 Ω). A criterion to measure this resonance is when the S11 is minimum.

• The antenna is usable over a certain bandwidth around this resonance and is generally defined when S11 < 6db (25% of reflected power max)

• This resonance depends on several factors
  • Antenna physical dimensions
  • Local environment

1.1.2. Efficiency

• This is the capability of the antenna to convert RF power into effective radiated power
• Perfect impedance matching doesn't mean perfect radiation
  • A 50 Ω load will present a perfect S11 but no efficiency
• Efficiency formula: Rr / (Rr+Rl) where Rr = radiation resistance and Rl = Loss resistance
  • Rr = 37 Ω and Rl = 25 Ω ==> efficiency = 0.59 = 59%
  • Rr = Antenna design dependent
  • Rl = Antenna environment dependent (ground, etc)
• To optimize efficiency, we need to
  • Increase Rr (widen Rf conductor, increase antenna physical length, ...)
  • Decrease Rl (increase GND plane for monopole antenna, ...)

1.1.3. Gain and radiating diagram

• Antenna have a "gain" referred to ideal isotropic antenna (gain) expressed in dBi
  • Gain can be either positive, or negative
• Gain is not likely constant in all directions, radiating diagram depend on
  • Antenna type/design
  • Local environment

1.2. Antenna types

1.2.1. SMA antenna

PROS: High performances

CONS: Cost + hardly implementable (3-D mechanics)

1.2.2. Wire antenna

PROS: Mechanically simple with good performances

CONS: hardly implementable (3-D mechanics)

1.2.3. Chip/ceramic antenna

PROS: Excellent for very small form-factors

CONS: Low gain, very sensitive to local environment and narrow bandwidth

1.2.4. PCB antenna

PROS: Cheap and easily adaptable to customer's PCB

CONS: Tuning required and special care to be brought on GND plane and distance with enclosure

1.2.5. Real life: antenna rules of thumbs

• Final performances always depend on local environment
  • Local GND + enclosure + nearby metallic objects
• A too much "reduced-size" antenna can NOT do miracles
  • It brings either reduced efficiency, and/or gain and/or bandwidth
• Antenna vendors always highlight great figures in a non-realistic environment
  • GND plane not realistic from an form-factor PCB
  • No nearby obstacles

2. 868/915 PCB antenna

2.1. Main PCB antenna types

2.1.1. Which printed antenna for my PCB?

• If the RF range is a major criteria, a longer antenna (vs 1/4 wavelength) is recommended
• Where can I install my 868 MHz PCB?
  • As far as possible from other antennas, shielded blocks, and metallic parts to increase isolation
  • As much as in clear as possible, with close proximity with a wide GND plane
• Ideally, the choice of antenna type and place should be decided at the very beginning of the PCB design
  • Absolutely avoid to allocate PCB antenna space once the main layout is frozen

2.2. 868/915 MHz PCB antenna, practical examples

2.2.1. PCB helical antenna

2.2.1.1. Physical dimensions

2.2.1.2. Simulations

2.2.1.3. Measurements

2.2.1.4. Comparison

	Gain	Resonance	Bandwidth
Measurement	-9 dBi	870 MHz	15 MHz
Simulation	-10,8 dBi	864 MHz	14 MHz

Very good correlation between measurement and simulation

2.2.1.5. Impact of mechanical enclosure

Huge detuning of the antenna when placed in a final form-factor: it is mandatory to re-tune the antenna to overcome this dispersion.

2.2.1.6. Impact of antenna position versus GND plane

Changing the antenna position vs GND highly impacts antenna performances. this is why antenna must be as much in the clear as possible.

2.2.2. MEANDER antenna

2.2.2.1. Physical dimensions

2.2.2.2. Simulation results

2.2.2.3. Conclusion of MEANDER antenna

• Antenna is physically bigger than the helical one, so the positive gain

3. Conclusion

The choice of an antenna

• Depends on many criteria but its choice must be done at the very beginning of the project.
• Is agnostic from the RF solution driving it, but requires a dedicated matching circuitry to optimize power transfer in both RX and TX mode.