Near Field Notes

Note 3RFRev 1Watch the video · 0:22

Antenna ground

How much ground does a 2.4 GHz PCB antenna need?

About 40 mm behind the antenna. With 10 mm, 22% of the power gets out, and a perfect matching network only lifts that to 43%.

The ground, cut

From 40 mm down to 10 mm

85%40 mm of ground77%35 mm of ground67%30 mm of ground56%25 mm of ground44%20 mm of ground33%15 mm of ground22%10 mm of groundof the power radiatesTI AN043 at 2.44 GHzRange at 40 mm1.00× the range0.95× the range0.89× the range0.81× the range0.72× the range0.62× the range0.51× the range85%40 mm of ground77%35 mm of ground67%30 mm of ground56%25 mm of ground44%20 mm of ground33%15 mm of ground22%10 mm of groundof the power radiatesTI AN043 at 2.44 GHzRange at 40 mm1.00× the range0.95× the range0.89× the range0.81× the range0.72× the range0.62× the range0.51× the range85%40 mm of ground77%35 mm of ground67%30 mm of ground56%25 mm of ground44%20 mm of ground33%15 mm of ground22%10 mm of groundof the power radiatesTI AN043 at 2.44 GHzRange at 40 mm1.00× the range0.95× the range0.89× the range0.81× the range0.72× the range0.62× the range0.51× the range
  • Range
  • Range at 40 mm

Share of the radio’s power radiated at 2.44 GHz. Range in free space.

The antenna and its ground

TI’s CC2511 USB dongle

TI’s AN043 antenna is a copper meander across the end of a 15.2 mm wide board. Two legs reach down over the edge of the ground on layer 2. One is shorted to it and the other is fed from it. The ground is the other half of the antenna, and current runs along it too. Shrink the board and that half gets cut short.

The antenna
Fig. 3Feed, 50 ΩShorted legGround, layer 2The meander, 2.44 GHz2 mmFig. 3Feed, 50 ΩShorted legGround, layer 2The meander, 2.44 GHz2 mmFig. 3Feed, 50 ΩShorted legGround, layer 2The meander, 2.44 GHz2 mm

Read from TI’s own Gerber of the CC2511 USB dongle, on 1.0 mm FR4.

The board
Fig. 3bAntennaCut to 10 mm40 mm of ground15.2 mm wideFig. 3bAntennaCut to 10 mm40 mm of ground, on layer 215.2 mm wideFig. 3bAntennaCut to 10 mm40 mm of ground, on layer 215.2 mm wide

The ground runs back from the antenna on layer 2. Each run cut it to one length, from 5 to 120 mm.

Four boards
Fig. 3c110 mm22%220 mm44%340 mm85%460 mm73%Fig. 3c110 mm22%220 mm44%340 mm85%460 mm73%Fig. 3c110 mm22%220 mm44%340 mm85%460 mm73%

The same antenna on 10, 20, 40 and 60 mm of ground, to scale, with the share of power each radiates at 2.44 GHz.

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Where the power goes

Specimens 1 and 3
Radiated, lost or sent back
  • Radiates
  • Heats the board
  • Reflects
Fig. 3d1 · 10 mm22%29%49%3 · 40 mm85%7%8%Fig. 3d1 · 10 mm22%29%49%3 · 40 mm85%7%8%Fig. 3d1 · 10 mm22%29%49%3 · 40 mm85%7%8%

Share of the power a 50 Ω radio offers at 2.44 GHz.

74

Key to antenna ground

How much ground does a 2.4 GHz PCB antenna need?

  1. 1aRoom for 40 mm of ground behind the antenna85% radiates
  2. 1bRoom for only 10 mmGo to 2
  3. 2aA matching network tuned to it43% radiates
  4. 2bThe antenna as built22% radiates

A perfect match stops the reflection. The 10 mm board still turns 57% of what it takes in into heat.

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Cut the ground yourself

5 to 60 mm, every 2.5 mm
25 mm
Antenna ground

With 25 mm of ground, 56% radiates

rr40=PP40=56%85%=0.81
  • As built
  • Perfectly matched
  • Range at 40 mm
5 mm0.37× the range7.5 mm0.45× the range10 mm0.51× the range12.5 mm0.57× the range15 mm0.62× the range17.5 mm0.67× the range20 mm0.72× the range22.5 mm0.77× the range25 mm0.81× the range27.5 mm0.85× the range30 mm0.89× the range32.5 mm0.92× the range35 mm0.95× the range37.5 mm0.98× the range40 mm1.00× the range42.5 mm1.01× the range45 mm1.02× the range47.5 mm1.02× the range50 mm1.01× the range52.5 mm1.00× the range55 mm0.98× the range57.5 mm0.95× the range60 mm0.93× the range5 mm0.37× the range7.5 mm0.45× the range10 mm0.51× the range12.5 mm0.57× the range15 mm0.62× the range17.5 mm0.67× the range20 mm0.72× the range22.5 mm0.77× the range25 mm0.81× the range27.5 mm0.85× the range30 mm0.89× the range32.5 mm0.92× the range35 mm0.95× the range37.5 mm0.98× the range40 mm1.00× the range42.5 mm1.01× the range45 mm1.02× the range47.5 mm1.02× the range50 mm1.01× the range52.5 mm1.00× the range55 mm0.98× the range57.5 mm0.95× the range60 mm0.93× the range5 mm0.37× the range7.5 mm0.45× the range10 mm0.51× the range12.5 mm0.57× the range15 mm0.62× the range17.5 mm0.67× the range20 mm0.72× the range22.5 mm0.77× the range25 mm0.81× the range27.5 mm0.85× the range30 mm0.89× the range32.5 mm0.92× the range35 mm0.95× the range37.5 mm0.98× the range40 mm1.00× the range42.5 mm1.01× the range45 mm1.02× the range47.5 mm1.02× the range50 mm1.01× the range52.5 mm1.00× the range55 mm0.98× the range57.5 mm0.95× the range60 mm0.93× the range
25%50%75%100%0204060 mm12%17%22%28%33%38%44%50%56%61%67%72%77%81%85%87%88%88%87%84%81%77%73%25%50%75%100%0204060 mmPerfectly matchedAs built12%17%22%28%33%38%44%50%56%61%67%72%77%81%85%87%88%88%87%84%81%77%73%25%50%75%100%0204060 mmPerfectly matchedAs built12%17%22%28%33%38%44%50%56%61%67%72%77%81%85%87%88%88%87%84%81%77%73%

Share of the radio’s power radiated at 2.44 GHz. Range in free space.

Longer boards
  • As built
  • Perfectly matched
Fig. 3eRadiates25%50%75%100%04080120 mm88%60%90%Ground behind the antennaFig. 3eRadiates25%50%75%100%020406080100120 mm88%60%90%Ground behind the antennaFig. 3eRadiates25%50%75%100%020406080100120 mm88%60%90%Ground behind the antenna

Share of the radio’s power radiated at 2.44 GHz, every 2.5 mm to 60 mm and every 5 mm after.

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Conditions

Antenna
TI AN043 meandered inverted-F, from TI’s Gerber of the CC2511 USB dongle reference design, rev 1.3
Board
4-layer FR4, 1.0 mm, εr 4.5, 15.2 mm wide
Ground
Layer 2, 0.25 mm below the antenna, cut to each length from 5 to 120 mm
Source
50 Ω at 2.44 GHz
Measure
Radiated power over the power the source can deliver
Solver
openEMS, rechecked on a 0.125 mm mesh
Check
TI’s dongle resonates at 2.525 GHz in the model and 2.49 GHz in TI’s measurement, 1.4% apart

Simulated only. TI’s 2.49 GHz is the one measured number on this page.

Limits

  • Range here is free-space range. Walls, hands and cases change it.
  • The model has no case, hand or laptop. TI’s own measurement shows a laptop can move the result.

Files

  • antsim/geom.pyThe antenna’s copper, read from TI’s Gerber
  • antsim/run.pyOne board, reduced to its power balance
  • post/matched.pyThe best matching network for each board
  • runs/Every ground length, 5 to 120 mm

Sources

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