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Choose an antenna to suit the radio’s feed, available space, circuit board and enclosure—not just its nominal frequency. A half-wave dipole or quarter-wave monopole is a sensible starting point when there is room; smaller loaded, helical and loop antennas can fit tighter designs, but usually involve trade-offs in efficiency, bandwidth or sensitivity to nearby materials. The antenna must be measured and assessed in the assembled device because a good impedance match alone does not prove good radiation or range.
What an antenna does—and why resonance is not enough
An antenna converts energy at a radio’s feed into electromagnetic waves, and converts received waves back into electrical signals. It is a resonant structure, but resonance by itself does not tell you how well it performs. Input impedance affects how much power transfers from the radio into the antenna; radiation resistance and losses affect how much of that accepted power is radiated rather than dissipated.
A feed can therefore look well matched while the antenna remains inefficient, or while its radiation pattern does not serve the intended link. A vector network analyzer (VNA) can measure feed impedance and help tune a design, but a VNA reading alone does not establish radiation efficiency, coverage, range or regulatory compliance.
Start with electrical size, not a catalog dimension
Antenna dimensions relate to wavelength, so frequency provides a useful first estimate. In free space, a quarter wavelength at 915 MHz is about 82 mm. Actual resonance depends on the antenna geometry and its surroundings, however; on a PCB, dielectric material and layout alter the effective wavelength.
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- Frequency Range & Gain: 900 MHz - 930 MHz. (915 MHz) / 3 dBi
- Antenna Connector & Impedance & SWR: RP-TNC-Male(Hole Inside) / 50 Ω / ≤1.8(SWR Increases with a 90-degree Antenna Orientation.)
- Total Length of the Antenna: 26 cm(10.2 in)
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- Packing List: 2 x Indoor Antennas
Texas Instruments’ 2005 application report SWRA046A gives a 915 MHz PCB example in which a quarter-wave monopole is 47.5 mm on FR4, assuming dielectric constant 4.2, board thickness 1.5 mm and trace width 1 mm. Those assumptions yield an effective dielectric constant of 2.97. This is an example under specified conditions, not a universal PCB antenna length.
Real boards add effects that a simple wavelength estimate cannot capture. The TI report identifies parasitic capacitance to ground, inductance from bends, package effects and non-ideal ground-plane dimensions as factors that change impedance. Use calculated dimensions to choose a starting geometry, then measure the actual board.
Choose a geometry that fits the feed and device
The basic choice is often between a dipole fed differentially and a monopole fed single-ended. A monopole relies on a ground plane or counterpoise as part of its antenna system; it is not just a shortened dipole with one half removed. Available volume, bandwidth needs, mounting position and proximity to an enclosure or wearer also matter.
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| Geometry | Feed and space | Practical trade-offs and fit |
|---|---|---|
| Half-wave dipole | Differential feed; needs room for both arms. | TI recommends it where space allows. An ideal half-wave dipole has 2.15 dB gain relative to an isotropic radiator in the direction perpendicular to its axis (SWRA046A, 2005). Orientation matters because radiation is not uniform in every direction. |
| Quarter-wave monopole | Single-ended feed; needs a ground plane or counterpoise. | TI recommends it where space allows. Ground-plane size, shape and orientation are part of the design. The report advises placing the counterpoise near the feed and, where possible, extending it perpendicular to the radiator. |
| Loaded stub | Compact radiator; implementation and feed depend on the design. | Can reduce physical size, but shortening an antenna makes matching, bandwidth and efficiency more constrained. Measure it in the intended assembly. |
| Transversal-mode helical antenna | Compact helical option; feed and layout depend on the implementation. | Useful when a straight radiator will not fit, with the usual compact-antenna trade-offs. Nearby materials and the final mounting arrangement can affect performance. |
| Small loop | Compact loop; feed arrangement depends on the implementation. | TI identifies small loops as a possible body-worn option. This is an engineering tendency, not a guarantee that a particular loop will be insensitive to a wearer or enclosure. |
The report’s general warning is that “An extremely small antenna can not be efficient and tolerance-insensitive at the same time.” The original spelling is preserved. A design that fits a small enclosure may still work well for its application, but it should not be assumed to match the efficiency, bandwidth or tolerance of a larger antenna.
For a premanufactured antenna, follow the manufacturer’s layout and evaluation-board guidance. TI cautions that “The manufacturer’s specifications are only achieved if the ground plane has the same size and shape as the manufacturer’s evaluation board” (SWRA046A, revised August 2005). A nominally compatible frequency does not guarantee equivalent performance on a different PCB.
Why the enclosure, board and user change antenna behavior
The antenna is part of the complete radio assembly. PCB dielectric, ground plane, enclosure, nearby metal or dielectric objects, and the user’s body can change tuning or radiation. Electrical antennas can be more sensitive to dielectric objects in their reactive near field; that is a general tendency, not a guarantee for every design.
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TI’s SWRA046A illustrates how large an application-specific change can be: in one 915 MHz test-module setup, maximum effective radiated power (ERP) was +10.85 dBm for a free stub module and −4.4 dBm with the module near a test person’s arm. That is a 15.25 dB reduction in the maximum-radiation direction for that particular fixture and geometry—not a general estimate of body loss for other devices.
That is why “tuning a 915 MHz antenna” cannot be reduced to cutting a trace to one standard length. The board stack-up, antenna placement, ground return, enclosure and nearby user all contribute to the final result. Recheck the design after changes to any of them.
A practical workflow for choosing and tuning
- Define the radio and use conditions. Identify the intended frequency or band, whether the radio feed is differential or single-ended, the usable PCB volume, the enclosure materials and the normal operating orientations. Include expected proximity to a person, metalwork and other nearby objects.
- Select a starting geometry. Use a dipole or monopole when space permits and the feed supports it. If the design must be smaller, consider a loaded stub, transversal-mode helix or loop with the understanding that compactness can constrain efficiency, bandwidth and tolerance.
- Design the feed and counterpoise together. For a monopole, treat the ground plane or counterpoise as part of the antenna. Keep its size, shape and placement consistent with the intended layout; do not assume an evaluation-board specification will transfer unchanged to a different board.
- Measure the antenna in the assembled device. Use a VNA to measure feed impedance and determine practical radiator length, as TI recommends in SWRA046A. Measure with the intended PCB and enclosure assembled, and account for the measurement setup so the instrument connection does not become an unintended part of the antenna.
- Adjust and remeasure. Change radiator geometry and, where the design uses them, matching components based on measured results. An archived TI practical example reports that matching elements were optimized with network-analyzer measurements for the particular antennas (Matthew Loy and Iboun Sylla, “Fundamentals of ISM-Band and short range device antennas, Part 4,” EE Times, March 2, 2007). Recheck after changing the enclosure, ground plane or nearby components.
- Validate radiation and the radio link. Test radiation behavior and the complete link in relevant orientations and environments. A feed match does not show that the antenna radiates efficiently in the needed directions or that the device has the desired coverage.
Choose a radio band for the application, not range promises
Lower frequencies can offer range advantages and may be less dependent on line of sight, but frequency alone cannot predict a device’s range. Antenna size and pattern, obstructions, environmental noise, available power and the application’s design constraints all affect the result. Analog Devices’ “Getting Started with a Radio Design” describes band selection as a balance between an application’s established band and the trade-offs among design parameters. Output power is also bounded by applicable regulation.
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“ISM band” does not mean that every short-range communications transmitter has blanket permission to operate at a listed frequency. Internationally designated ISM frequencies and the rules for radio communications or short-range devices are distinct regulatory questions.
- For Canadian ISM equipment context, ISED’s ICES-001 Issue 5 (July 2020), Annex A, lists 902–928 MHz around 915 MHz and 2,400–2,500 MHz around 2,450 MHz. That table concerns the standard’s ISM-equipment scope; it does not by itself authorize a communications transmitter.
- ITU-R SM.2179-2 (2023) lists examples of standards used in CEPT countries, including EN 300-220 for equipment from 25 MHz to 1,000 MHz and EN 300-328 for wideband data-transmission equipment in the 2.4 GHz ISM band. These references are not a complete compliance determination.
Check the current rules for the device category and jurisdiction where the product will operate, including current standard editions and national implementation. Frequency, power, equipment class and other conditions can affect what is permitted.
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