Is a passive antenna sold on its own covered by the RED?

No. The Guide to the RED (section 1.7.4.6) says passive antennas are not covered by the Directive when placed on the market as a single commercial unit. Radio equipment with its antenna attached is subject to all RED requirements, and the interface to that antenna need not be assessed unless the radio part is also sold without it. Active antennas (with active components that interact with the RF signal) are always covered, and so are amplifiers, filters and antenna tuners intended to be connected to an antenna (section 1.7.4.7). If the radio is delivered without an antenna, the EU declaration of conformity (Annex VI, point 8) and the instructions (Article 10(8)) describe the antennas it may be used with.

Can I change the antenna of an FCC-certified Part 15 radio?

Only within the rules of 15.203 and 15.204. The radio must be designed so that only the antenna furnished by the responsible party can be used: a permanently attached antenna or a unique coupling, never a standard antenna jack (15.203). It may be operated only with antennas it was authorized with; an antenna of the same type and of equal or lower directional gain may be used without retesting, while a different type or a higher gain needs the change procedures of 2.1043 (15.204(c)(4)). Testing is done with the highest-gain antenna of each type, at maximum output power (15.204(c)(2)).

How does antenna gain enter the 2.4 GHz power limits?

Differently on each side of the Atlantic. Under FCC 15.247(b) the limit is conducted output power (1 W for digital modulation), and it assumes antennas of 6 dBi or less; above 6 dBi the conducted power is reduced by the excess gain, with relief for fixed point-to-point links (15.247(b)(4) and (c)(1)). Under ETSI EN 300 328 V2.2.2, the RF output power is defined as e.i.r.p. (20 dBm maximum), so every dB of antenna assembly gain comes off the conducted power the radio may deliver.

What return loss should a matching network achieve?

No regulation sets one. A return loss of 10 dB (S11 of minus 10 dB) is a common design convention: it means 10 percent of the incident power is reflected, a VSWR of about 1.92 and a mismatch loss of about 0.46 dB. A VSWR of 2 corresponds to 9.54 dB return loss and 0.51 dB mismatch loss. Return loss says nothing about efficiency: a lossy antenna can be perfectly matched and still radiate poorly, which is why radiated measurements (TRP, TIS) decide.

Why are small sub-GHz antennas so narrowband?

Because of the Chu limit on the radiation Q of an electrically small antenna. In the form given by McLean (IEEE Transactions on Antennas and Propagation, 1996), the minimum Q is 1/(ka)^3 + 1/(ka), where a is the radius of the sphere enclosing the radiating currents. For a 10 mm sphere this gives about 172 at 868 MHz against about 9.4 at 2.44 GHz. Bandwidth scales inversely with Q, so the same volume is far harder to match over a sub-GHz band. On a PCB the ground plane carries radiating current too, which is why its size matters.

Which OTA tests does PTCRB require for an IoT device?

RF performance evaluation is mandatory for every device type going through PTCRB or IoT Network Certified (NAPRD03 V6.26, section 2.9), using the CTIA Certification Test Plan for Wireless Device Over-the-Air Performance in effect when the request is submitted. For an IoT device, a three-question table (data per transaction, number of transactions per day, fixed orientation to the horizon) decides between TRP plus TIS and near-horizon metrics (NHPRP, NHPIS). After an antenna change, the Primary lab decides which OTA tests are repeated (PPMD V4.2, section 11.6.4).

What does the GNSS antenna have to work with?

GPS L1 is at 1575.42 MHz and L5 at 1176.45 MHz, and the satellites transmit right-hand circular polarisation. The GPS SPS Performance Standard (2020) gives a minimum received power of minus 158.5 dBW (minus 128.5 dBm) for the L1 C/A code, under the conditions of IS-GPS-200. With signals that weak, any loss ahead of the first amplifier (filter, trace, mismatch) adds directly to the system noise figure, which is what the LNA and SAW ordering trade-off is about.

Antenna design and impedance matching for IoT

Hugues Orgitello updated 7 Oct 2026 15 min read

The antenna is where radio design meets the rules. Its gain enters the e.i.r.p. limits of EN 300 328 and the 6 dBi assumption of FCC 15.247; its connector decides whether a Part 15 radio meets 15.203; changing it after certification triggers 15.204 in the United States and a lab review under the PTCRB rules. This page, updated on 7 October 2026, first sets out what the texts actually say about antennas (FCC Part 15, the RED and its Guide, EN 300 328, PTCRB NAPRD03 and PPMD, the CTIA OTA test plan), then covers the engineering: antenna families, free-space wavelengths per band, the small-antenna limit that makes sub-GHz matching hard, return loss arithmetic, the matching workflow, ground plane, detuning, OTA metrics, the GNSS front end and MIMO correlation. Physics is given with its textbook or journal reference; numbers that only a datasheet can give are left to the datasheet.

What the texts say about the antenna

Text	Provision	What it requires
FCC 15.203	Antenna requirement	The intentional radiator must be designed so that no antenna other than the one furnished by the responsible party can be used. A permanently attached antenna or a unique coupling is sufficient; a standard antenna jack or electrical connector is prohibited. Some devices are excepted, including professionally installed ones, the installer then being responsible for the antenna
FCC 15.204(c)(1) to (3)	Antenna types	The radio may be operated only with antennas it is authorized with; "type" means similar in-band and out-of-band patterns. Tests use the highest-gain antenna of each type at maximum output power; the application lists the acceptable antenna types
FCC 15.204(c)(4)	Changing antenna	Same type and equal or lower directional gain: may be marketed and used without retesting. Different type or higher gain: only through the 2.1043 change procedures
FCC 15.212(a)(1)(iv)	Modules	A modular transmitter meets 15.203, 15.204(b) and 15.204(c); the antenna is permanently attached or uses a unique coupler at every connection, cable included
FCC 15.247(b)(4)	Antenna gain	The conducted power limits assume antennas of 6 dBi or less; above that, conducted power is reduced by the dB of excess gain
FCC 15.247(c)(1)	Fixed point-to-point	2400 to 2483.5 MHz: 1 dB of power reduction per 3 dB above 6 dBi. 5725 to 5850 MHz: no reduction
RED, Article 2(1)(1)	Definition	Radio equipment includes a product "which must be completed with an accessory, such as antenna" to emit or receive
RED Guide, section 1.7.4.6	Antennas	Passive antennas placed on the market as a single commercial unit are not covered. Radio equipment with the antenna attached is subject to all RED requirements; the radio to antenna interface need not be assessed unless the radio is also sold without the antenna. Active antennas are always covered
RED Guide, section 1.7.4.7	Equipment connected to antennas	Amplifiers, filters, splitters, converters, antenna tuners and switches intended to be connected to an antenna are covered
RED, Article 10(8) and Annex VI point 8	Accessories	Instructions and the EU declaration of conformity describe accessories, such as antennas, that let the equipment operate as intended. The Guide adds that a radio delivered without antenna must state the antenna technical features, generic or by reference
EN 300 328 V2.2.2, clause 4.2.4	Antenna types	Integral or dedicated antennas only; dedicated antennas are assessed in combination with the equipment. An antenna on an internal connector to the PCB counts as integral (clause 3.1)
EN 300 328 V2.2.2, clause 4.3	Power and spurious	RF output power is mean e.i.r.p., 20 dBm maximum. Spurious limits apply at the antenna port when there is a connector; for integral antennas they are e.r.p. below 1 GHz and e.i.r.p. above
PTCRB NAPRD03 V6.26, section 2.9	OTA	RF performance evaluation is mandatory for all device types going through PTCRB or IoT Network Certified, to the CTIA OTA test plan in effect at submission
PTCRB PPMD V4.2, section 11.6.4	Antenna change (ECO)	The Primary lab decides whether TRP, C-TIS, TIS, RSE, NHPRP or NHPIS testing is needed, and whether full OTA testing or spot-checks are enough
PTCRB PPMD V4.2, section 11.11.7	Parent and child devices	The antenna subsystem of a child device must be similar in design and performance to the parent and based on the same design specification

Two consequences for design. First, in the EU the antenna gain is inside the limit: with a 20 dBm e.i.r.p. ceiling, a 3 dBi antenna assembly leaves at most 17 dBm of conducted power. Under 15.247, gain up to 6 dBi costs nothing against the 1 W conducted limit. A design sized at the FCC limit has to be backed off for the EU. Second, the antenna choice is frozen early: a later change is a 2.1043 change on the FCC side, a reassessment under the RED and a lab decision under PTCRB.

Antenna families

Family	Typical use	Strengths	Limits
Chip antenna (ceramic)	2.4 GHz, 5 GHz, GNSS, UWB, some sub-GHz parts	Small, comes with a vendor reference layout, clearance and matching starting point	Performance depends on the board and ground plane it sits on, not only on the part
IFA, meander, monopole (PCB trace)	2.4 GHz, 5 GHz, sub-GHz on larger boards	No extra part, pattern close to omnidirectional	Occupies board edge, sensitive to substrate and surroundings
PIFA	Cellular multi-band, larger devices	Several resonances, sits above the ground plane	Needs height above the PCB
Patch (ceramic or microstrip)	GNSS, UWB	Directional, circular polarisation possible	Needs a ground plane, narrow bandwidth
External (whip, helix, dipole)	Sub-GHz, cellular, gateways	Away from the board, larger possible volume	Connector and 15.203 constraints, mechanical exposure

Chip antenna datasheets give gain, efficiency and bandwidth measured on the vendor evaluation board. Those figures belong to that board: on a different ground plane, in a different enclosure, they change. Treat them as a starting point and the reference layout as part of the specification.

Free-space wavelength per band

The wavelength is c/f. Antennas on a PCB are shorter than these values because of the dielectric and the ground plane, by an amount only a simulation or a measurement gives.

Technology	Band	Wavelength	Quarter wavelength
Sub-GHz SRD, EU	863 to 870 MHz	347.4 to 344.6 mm	86.8 to 86.1 mm
15.247 band, US	902 to 928 MHz	332.4 to 323.1 mm	83.1 to 80.8 mm
Cellular low band (example 700 MHz)	700 MHz	428.3 mm	107.1 mm
GPS L5	1176.45 MHz	254.8 mm	63.7 mm
GPS L1	1575.42 MHz	190.3 mm	47.6 mm
BLE, Wi-Fi 2.4 GHz	2400 to 2483.5 MHz	124.9 to 120.7 mm	31.2 to 30.2 mm
Wi-Fi 5 GHz (U-NII-1 to U-NII-3)	5150 to 5850 MHz	58.2 to 51.2 mm	14.6 to 12.8 mm
Wi-Fi 6 GHz, EU (Decision (EU) 2021/1067)	5945 to 6425 MHz	50.4 to 46.7 mm	12.6 to 11.7 mm
Wi-Fi 6 GHz, US (U-NII-5 to U-NII-8)	5925 to 7125 MHz	50.6 to 42.1 mm	12.6 to 10.5 mm
UWB channel 5 / channel 9	6489.6 / 7987.2 MHz	46.2 / 37.5 mm	11.5 / 9.4 mm

The regulatory side of each band is covered elsewhere: EN 300 328 for 2.4 GHz in the EU and 15.247 in the US, EN 303 687 for 6 GHz RLAN in the EU and 15.407 in the US, EN 303 413 for GNSS receivers. See RED tests and FCC standards.

The small-antenna limit

An antenna that fits inside a sphere of radius a much smaller than the wavelength has a high minimum radiation Q, and bandwidth falls as Q rises. Chu derived the bound in 1948; McLean (1996) gave it in closed form for a linearly polarised antenna:

Q_min = 1/(ka)³ + 1/(ka), with k = 2π/λ.

Worked example for a sphere of radius 10 mm (the sphere must enclose all radiating currents, including those on the ground plane):

Frequency	ka	Q_min
868 MHz	0.182	about 172
1575.42 MHz	0.330	about 31
2441.75 MHz (centre of 2.4 GHz band)	0.512	about 9.4

The same volume is roughly eighteen times more constrained at 868 MHz than at 2.4 GHz. That is the physics behind two field observations: small sub-GHz antennas are narrowband and easily detuned, and the ground plane is part of the antenna (making it larger increases a). The bound is a lower limit on Q for an ideal lossless antenna; real antennas sit above it, and losses widen bandwidth only by wasting power.

Return loss, VSWR and mismatch loss

With reflection coefficient |Γ|: return loss RL = -20 log|Γ|, VSWR = (1 + |Γ|)/(1 - |Γ|), mismatch loss = -10 log(1 - |Γ|²).

S11	abs(Γ)	VSWR	Reflected power	Mismatch loss
-6 dB	0.501	3.01	25.1 %	1.26 dB
-9.54 dB	0.333	2.00	11.1 %	0.51 dB
-10 dB	0.316	1.92	10.0 %	0.46 dB
-14 dB	0.200	1.50	4.0 %	0.18 dB
-20 dB	0.100	1.22	1.0 %	0.04 dB

Minus 10 dB is a widespread design convention, not a requirement of any regulation or test plan. The table shows why chasing a deeper match has little return: going from -10 to -20 dB recovers 0.42 dB. A poorly radiating antenna (lossy, detuned by metal, too small a ground plane) can still show an excellent S11, because the power is dissipated rather than reflected. Efficiency and radiated power are measured in a chamber, not inferred from S11.

Matching network workflow

The matching network transforms the antenna impedance at the feed to the 50 ohm reference of the radio.

Topologies

Topology	Elements	Use
L-network	One series, one shunt	Two elements reach any impedance from 50 ohm, but only from one side of the chart for a given arrangement; narrowband
Pi-network	Shunt, series, shunt	Footprint for both L arrangements; the unused position is left unpopulated or fitted with a zero ohm link. Common on reference layouts
T-network	Series, shunt, series	Same flexibility, less common on IoT boards

Placing a pi footprint costs little and keeps both L solutions available after the enclosure is known. The components must be specified at the operating frequency: an inductor behaves as an inductor only well below its self-resonant frequency, and the Q of both inductors and capacitors at frequency sets the loss in the network. Use the vendor RF models (S-parameters), not the nominal value.

Measurement steps

Provide a measurement point at the antenna feed (a coaxial probe point or a pigtail soldered at the matching network).
Calibrate the vector network analyser at the end of the cable (open, short, load), then move the reference plane to the feed with port extension.
Measure in the final mechanical configuration: board in the enclosure, battery, display, cables, mounting.
Read S11 on the Smith chart across the band and identify the impedance at band centre.
Compute the network (any Smith chart tool or circuit simulator), fit parts, measure again. Iterate.
Check the result in radiated terms (TRP and TIS, or efficiency in a chamber): a good S11 is necessary, not sufficient.
Check spread: a few units and, where relevant, temperature, because component tolerances move a high-Q match.

The cable itself carries current on its outer shield and can radiate, changing the measurement. Ferrite on the cable, or a measurement from the radio port using the radio itself, are common ways to check that the cable is not part of the antenna.

Ground plane and clearance

For monopole, IFA and chip antennas, the PCB ground plane is the other half of the antenna. Three consequences follow from the physics above:

The keep-out area in the antenna datasheet is part of the antenna. No copper pour, trace or via inside it on any layer unless the datasheet allows it. Its dimensions come from the datasheet of the specific part, never from a generic rule.
The ground plane size limits how low in frequency the antenna works well, as the Chu bound shows. Check the board size used for the published datasheet figures; a smaller board will not reproduce them.
Placement at a board edge or corner with the keep-out extending off the ground plane is the arrangement nearly all reference layouts use; an antenna surrounded by copper does not radiate as designed.

For a GNSS patch, the ground plane under the patch shapes the pattern and the axial ratio; the patch datasheet gives the ground plane size used for its figures.

Detuning in the assembled product

Source	Mechanism	What to do
Plastic enclosure close to the antenna	Dielectric loading lowers the resonance	Tune in the closed enclosure
Battery, display, shields, metal parts	Coupling, pattern distortion, loss	Keep them away from the keep-out; validate assembled
User hand or body	Loading and absorption	Test in the use positions; the CTIA plan has a document on near-field phantoms (CTIA 01.72)
Cables in use (USB, sensor leads)	Common-mode current, cable radiates	Test with and without cables; ferrite or filtering where needed
Switching regulators, clocks, high-speed buses	Noise coupled into the receiver	Shows up as degraded TIS rather than TRP; layout separation, filtering
ESD on an exposed antenna	Damage to the front end	Protection rated for RF capacitance on the antenna line

The size of each effect depends on the product. The point is the order of work: S11 on the bare board, then on the assembled product, then radiated measurements.

OTA metrics and test plans

OTA measurements are made on the complete device in a chamber. TRP (Total Radiated Power) integrates radiated power over the sphere; TIS (Total Isotropic Sensitivity) is the sensitivity integrated over the sphere. Both include the antenna, the matching, the front end and, for TIS, the self-interference of the product. PPMD V4.2 also defines C-TIS (combined over all receive branches when there are several), and the near-horizon metrics NHPRP and NHPIS for devices installed in a fixed orientation.

CTIA test plan

The CTIA Certification Test Plan for Wireless Device Over-the-Air Performance was restructured in Version 4.0 (published 14 February 2022) into 17 topic documents, among them CTIA 01.01 (scope and applicability), 01.20 (SISO anechoic chamber), 01.21 (SISO reverberation chamber), 01.22 (SISO millimetre wave), 01.40 and 01.41 (MIMO), 01.50 to 01.52 (radio technologies, including location), 01.70 (measurement uncertainty), 01.71 (device setup and positioning) and 01.72 (near-field phantoms). PTCRB references the "Active Version", so check the CTIA test plan page for the current release of each document at submission.

PTCRB: what NAPRD03 V6.26 requires of IoT devices

Item	NAPRD03 section 2.9.2
IoT characteristics table	(1) more than 100 Kbytes per transaction? (2) more than 24 transactions per day on average? (3) installed in a fixed orientation relative to the horizon?
Always	OTA only in the highest-order RAT, in each supported band of it, plus bands supported only in a lower-order RAT
All three answers "N"	TRP on all supported bands and channels; TIS on the mid channel for bands above 1 GHz, on low, mid and high channels below 1 GHz; anechoic or reverberation chamber
(1) and (2) "N", (3) "Y"	NHPRP ±30° and NHPIS ±30° in an anechoic chamber in place of TRP and TIS (reverberation chamber with TRP and TIS if the device is too large)
Proximity power reduction	TRP measured per CTIA 01.01, section 2.1.3
Cellular plus Wi-Fi simultaneous operation	Cellular desensitisation testing per CTIA 01.20, section 4.9

Target operators may add their own requirements; see AT and T NAF cellular IoT, Verizon Open Development and T-Mobile IoT lab.

3GPP

In 3GPP specifications, TS 38.521-1 is the NR conformance specification for radio transmission and reception in FR1 standalone; Part 2 covers FR2. The radiated performance of the antenna is specified separately: TS 37.544 for UTRA and E-UTRA OTA conformance testing, and TS 38.161 (requirements) with TS 38.561 (test methodologies) for NR FR1, where 3GPP calls the receive metric TRS, Total Radiated Sensitivity.

GNSS front end

Parameter	Value	Source
L1 centre frequency	1575.42 MHz	GPS SPS Performance Standard 2020
L5 centre frequency	1176.45 MHz	GPS SPS Performance Standard 2020
Polarisation	Right-hand circular	GPS SPS Performance Standard 2020
Minimum received L1 C/A power	-158.5 dBW (-128.5 dBm), under IS-GPS-200 conditions	GPS SPS Performance Standard 2020, section 2.2.1.1

A passive patch feeds an LNA on the board; an active antenna has the LNA in the antenna and is powered through the coaxial line. In the EU an active antenna sold on its own is radio equipment (RED Guide, section 1.7.4.6).

The order of filter and LNA follows from the cascade noise formula (Friis, 1946): F = F₁ + (F₂ - 1)/G₁ + ... . A passive filter ahead of the LNA adds its insertion loss directly to the noise figure; behind the LNA, its contribution is divided by the LNA gain. Filter first protects the LNA from strong nearby transmitters (cellular, Wi-Fi on the same product) at a noise cost; LNA first gives the lowest noise figure but leaves the LNA exposed. Which one wins depends on the transmitters on the board and on the LNA linearity, both found in the datasheets and the link budget.

MIMO and diversity

The figure of merit for two antennas is the envelope correlation coefficient (ECC), 0 for uncorrelated and 1 for identical signals. Blanch, Romeu and Corbella (2003) showed it can be computed from S-parameters:

ECC = abs(X)² / [(1 - abs(S₁₁)² - abs(S₂₁)²)(1 - abs(S₂₂)² - abs(S₁₂)²)], where X is the complex conjugate of S₁₁ times S₁₂, plus the complex conjugate of S₂₁ times S₂₂.

The formula assumes lossless antennas in a uniform multipath environment. Small IoT antennas are lossy, so the S-parameter value is optimistic; the pattern-based calculation from chamber measurements is the reliable one. Separation, orthogonal polarisation, different pattern coverage and ground-plane shaping (slots, neutralisation lines) are the levers. The CTIA plan has its own MIMO documents (01.40, 01.41), and PTCRB section 2.9.4 covers receive diversity and MIMO.

Filter	Function
Diplexer	Combines two non-overlapping bands on one antenna, for example 2.4 GHz and 5 GHz
Duplexer	Separates transmit and receive on one antenna for FDD cellular
SAW and BAW filters	Band selection and rejection of nearby transmitters; the choice between them depends on frequency, power handling and temperature drift, per datasheet

Filters also act on what EN 300 328 counts: for an integral antenna, transmitter spurious emissions are measured radiated (e.r.p. below 1 GHz, e.i.r.p. above), so harmonics leave through the antenna and the filtering before it.

Claims the texts contradict

Claim	What the text says
"A passive antenna needs CE marking under the RED"	Passive antennas placed on the market alone are not covered (RED Guide, section 1.7.4.6). Active antennas and equipment intended to be connected to antennas are
"Any antenna with the same gain can replace the certified one under FCC rules"	Same type and equal or lower gain only; "type" means similar in-band and out-of-band patterns (15.204(c)(1) and (c)(4))
"An SMA connector is fine on a Part 15 product"	A standard antenna jack or electrical connector is prohibited; a permanent antenna or unique coupling is required, except for professionally installed devices (15.203)
"15.247 sets an e.i.r.p. limit"	It sets conducted power with a 6 dBi antenna assumption (15.247(b)(4)). EN 300 328 is the one that sets e.i.r.p.
"S11 below -10 dB is the regulatory requirement"	No regulation or test plan sets an S11 value; regulators and operators assess radiated power and sensitivity
"The CTIA OTA test plan current version is 3.x"	Version 4.0 (14 February 2022) split the plan into topic documents; PTCRB uses the active version at submission (NAPRD03 section 2.9)
"3GPP TS 38.521 covers TRP and TIS for FR1 devices"	TS 38.521-1 covers conducted transmission and reception; FR1 TRP and TRS are in TS 38.161 and TS 38.561, LTE OTA in TS 37.544
"OTA is optional for IoT modules and devices"	RF performance evaluation is mandatory for all device types under PTCRB and IoT Network Certified (NAPRD03 section 2.9)
"Any antenna change is a new certification" (PTCRB)	It is an ECO; the Primary lab decides which OTA tests are needed and whether spot-checks suffice (PPMD section 11.6.4)

Going further

Bluetooth SIG qualification: RF and protocol qualification of the BLE radio behind the antenna
UWB FiRa consortium certification: UWB channels and antenna spacing for ranging
Wi-Fi Alliance certification: interoperability across 2.4, 5 and 6 GHz
PTCRB module certification: cellular conformance and OTA for North American operators
RED scope: modules, antennas and receivers under the RED
FCC procedure: modular approval and the 15.212 conditions
Glossary: definitions of TRP, OTA and EIRP

Frequently asked questions

Is a passive antenna sold on its own covered by the RED?: No. The Guide to the RED (section 1.7.4.6) says passive antennas are not covered by the Directive when placed on the market as a single commercial unit. Radio equipment with its antenna attached is subject to all RED requirements, and the interface to that antenna need not be assessed unless the radio part is also sold without it. Active antennas (with active components that interact with the RF signal) are always covered, and so are amplifiers, filters and antenna tuners intended to be connected to an antenna (section 1.7.4.7). If the radio is delivered without an antenna, the EU declaration of conformity (Annex VI, point 8) and the instructions (Article 10(8)) describe the antennas it may be used with.
Can I change the antenna of an FCC-certified Part 15 radio?: Only within the rules of 15.203 and 15.204. The radio must be designed so that only the antenna furnished by the responsible party can be used: a permanently attached antenna or a unique coupling, never a standard antenna jack (15.203). It may be operated only with antennas it was authorized with; an antenna of the same type and of equal or lower directional gain may be used without retesting, while a different type or a higher gain needs the change procedures of 2.1043 (15.204(c)(4)). Testing is done with the highest-gain antenna of each type, at maximum output power (15.204(c)(2)).
How does antenna gain enter the 2.4 GHz power limits?: Differently on each side of the Atlantic. Under FCC 15.247(b) the limit is conducted output power (1 W for digital modulation), and it assumes antennas of 6 dBi or less; above 6 dBi the conducted power is reduced by the excess gain, with relief for fixed point-to-point links (15.247(b)(4) and (c)(1)). Under ETSI EN 300 328 V2.2.2, the RF output power is defined as e.i.r.p. (20 dBm maximum), so every dB of antenna assembly gain comes off the conducted power the radio may deliver.
What return loss should a matching network achieve?: No regulation sets one. A return loss of 10 dB (S11 of minus 10 dB) is a common design convention: it means 10 percent of the incident power is reflected, a VSWR of about 1.92 and a mismatch loss of about 0.46 dB. A VSWR of 2 corresponds to 9.54 dB return loss and 0.51 dB mismatch loss. Return loss says nothing about efficiency: a lossy antenna can be perfectly matched and still radiate poorly, which is why radiated measurements (TRP, TIS) decide.
Why are small sub-GHz antennas so narrowband?: Because of the Chu limit on the radiation Q of an electrically small antenna. In the form given by McLean (IEEE Transactions on Antennas and Propagation, 1996), the minimum Q is 1/(ka)^3 + 1/(ka), where a is the radius of the sphere enclosing the radiating currents. For a 10 mm sphere this gives about 172 at 868 MHz against about 9.4 at 2.44 GHz. Bandwidth scales inversely with Q, so the same volume is far harder to match over a sub-GHz band. On a PCB the ground plane carries radiating current too, which is why its size matters.
Which OTA tests does PTCRB require for an IoT device?: RF performance evaluation is mandatory for every device type going through PTCRB or IoT Network Certified (NAPRD03 V6.26, section 2.9), using the CTIA Certification Test Plan for Wireless Device Over-the-Air Performance in effect when the request is submitted. For an IoT device, a three-question table (data per transaction, number of transactions per day, fixed orientation to the horizon) decides between TRP plus TIS and near-horizon metrics (NHPRP, NHPIS). After an antenna change, the Primary lab decides which OTA tests are repeated (PPMD V4.2, section 11.6.4).
What does the GNSS antenna have to work with?: GPS L1 is at 1575.42 MHz and L5 at 1176.45 MHz, and the satellites transmit right-hand circular polarisation. The GPS SPS Performance Standard (2020) gives a minimum received power of minus 158.5 dBW (minus 128.5 dBm) for the L1 C/A code, under the conditions of IS-GPS-200. With signals that weak, any loss ahead of the first amplifier (filter, trace, mismatch) adds directly to the system noise figure, which is what the LNA and SAW ordering trade-off is about.