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Since 2016, Apple has been able to make these laptops with no visible “window” of plastic or glass for RF signals to travel through. As an electrical engineer, I’m intrigued.

If you look at older MacBook pros, they have a black plastic hinge. For the Pixel chrome book, it’s got a whole glass half of the lid for that.

My guess was the hinge or the glass screen, but neither would work when the lid is closed, and you can very much use your MBP while the lid is fully closed with WiFi and Bluetooth (think when you have external displays).

How does Bluetooth and WiFi work on the 2018 MacBook Pro models?

  • If I remember correctly, the antenna is housed in the plastic trim piece that cover the hinge. – Allan Aug 15 '18 at 10:33
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    I'm voting to close this question as off-topic because it entails asking how and why Apple does something which is off topic under the scope of the site. – Nimesh Neema Aug 15 '18 at 12:20
  • @NimeshNeema this is a technical question - how does something work - not an Apple “philosophical” question of how or why they chose to do something in a certain way. – Allan Aug 15 '18 at 14:10
  • @Allan You would be right about MBPs up to 2015. In the latest "Touch bar" generations, the hinge is also all aluminum as far as you can see on the back – You Speak So Well Aug 15 '18 at 15:00
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Pretty good question of physics everyone should ask when thinking of the number of wireless interfaces there are in today's iPhone and Mac.

Fast answer

Apple tests different pieces, materials and antenna positions through mathematical models to compute the resulting outside signal obtained and get strenghest and most omni-directional one.

Physical reality

First thing to know in this field, we are not talking of electrostatic here, but of electromagnetism and moreover oe electromagnetic wave in the micro-waves spectrum (300 MHz - 300 GHz).

Consider a closed metallic box connected to the ground. If you apply to this box an electrostatic field with no variation, then the electrons within the metal of this box will move until they will reach one of the face of the metal and until the overall electrostatic field is fully equal to zero. Then the electrostatic field inside this metallic box will also be equal to 0 within 1 ps (10⁻¹²s) for a metal of 0.2 mm thickness. This will also be the case with a varying electrostatic field, but only if this field is varying staying in the same direction. This is the case with a lightning strike: the electrons within will be able to follow the high variation in intensity of this electrodynamic field so as to cancel it inside the box. This box is named a Faraday cage (Faraday cage on Wikipedia) as nearly everyone learned in school long before learning electromagnetism.

A Faraday cage blocks electrostatic fields

But in the electromagnetism field the things are completly different. The electrons won't have enough time to go from one face of the metal to the other the time the electromagnetic field will take to switch direction: 0.2 ps at 5 GHz (reminder: periode = 1/frequency). And in a metal the electrons will start to oscillate at the same frequency than the exterior electromangetic field. They won't be fast enough to follow the music and won't anymore cancel through equilibrium the external electromagnetic field. They will create their own electromagnetic field slightly phase shifted with the outside one (exactly like electrons in an antenna). The two electromagnetic fields will add together and create an electromagnetic field inside the metallic box.

A Faraday cage doesn't block electromagnetic waves

A common misconception is that a Faraday cage provides full blockage or attenuation of electromgnetic waves, this is not true. We should talk of the transparency of different metals or alloys to electromagnetic waves (more acurately of their "absorption spectrum"). Most metals are fully black to slow frequencies and transparent to high ones (look at the picture of your MBP when you pass the X-rays control at the airport 😎). On the other hand, concrete with irons regularly spaced every 25 cm or 50 cm if fully opaque to electromagnetic waves in the frequency around 2.4 GHz.

Here is a nice experiment to check this physical reality.

Find a plain steel small box like an old sugar box. Connect it to the ground with an electric cable (any ground pin on a wall electric outlet will be perfect). Put your mobile phone inside the box, close the box. Then call your prisoner of the Faraday cage. If you are in a normal reception area, your mobile phone will ring, and you will hear (a Faraday cage doesn't block sounds either).

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Apple cleverly places the WiFi antenna assembly adjacent to the cooling vent to seemingly "circumvent the laws of physics" and get around the Faraday cage problem.

Looking at the underside of the MacBook Pro (15" 2017), you can see a cooling vent that goes for about 80% of the width of the machine.

2017 MacBook Pro Bottom (Vent) View

2017 MacBook Pro Bottom Case View

The WiFi antenna assemblies are put on either side of the MacBook directly adjacent to the cooling vent. So, while it appears that the WiFi antenna is hidden behind an all aluminum case, it's actually out in the open which is how it receives WiFi signals in closed-clamshell mode.

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Apple seems reluctant to publish such information, and we must fall back to photos and videos published by technicians and repairmen to find any useful clues. This is such a video. The subject is an older Macbook, but I believe antennas on current Macbooks are in the same location. The video shows the antenna location clearly: behind the plastic bezel at the bottom of the display - where you see the name "MacBook Pro" (on mine at least).

When you close the lid, and inspect the case this is the only location that makes sense. As you've recognized, the case presents limited opportunities for rf signals to escape. In fact, the only opportunity (assuming the case material really is aluminum) is the slot at the rear; the same slot where heat is exhausted from the case. The plastic bezel will face the top of this slot when the lid is closed.

The MacBook case, and its packaging in general, are marvels of modern design and manufacturing I think. Apple is good, but even their rf signals must follow the laws of physics discovered by Michael Faraday nearly 200 years ago.

  • Nice find on the video! Very insightful answer. I totally see them putting the antennas still near the hinge at the bottom of the screen with plastic (transparent to RF). I still struggle to see how it works when the lid is closed. Technically there IS a small crack you can put RF out of, but knowing how hard RF engineering is, I'm baffled that they can get essentially omnidirectional connectivity with such constraints! – You Speak So Well Aug 15 '18 at 15:03
  • Also, the video appears to be a pretty old model. The latest models have a plastic bar with "MacBook Pro" written on it. That being said I still think the hinge is where the antennas must be. When they launched the 2016 model they were so proud that they pointed out it's "for the first time" an "all-aluminum enclosure" – You Speak So Well Aug 15 '18 at 15:10
  • Well... it's the only thing that could work really. And the small crack/slot was no doubt included in the antenna modeling that was done during design. The models today are impressively accurate. But that's not to say the antenna is omnidirectional; in fact I find the Bluetooth antenna to be very "finicky" about the macbook's orientation wrt the BT device. – Seamus Aug 15 '18 at 15:23
  • Yeah - mine is a 2016 model according to the"About This Mac" info, and you're correct: The plastic bezel is only at the bottom, but there is a black "border" surrounding the display. – Seamus Aug 15 '18 at 15:28
  • Gotcha. When I fully close the lid, all the border of the screen is basically blocked by the aluminum chassis itself. I suppose they figured out how to use the tiny crack under the bottom of the hinge when closed to do RF. And as you pointed it it probably does diminish the RF performance to some extent. Still I wish someone does an in-depth teardown or RF analysis on it. – You Speak So Well Aug 15 '18 at 15:39

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