Why so many antennas in a laptop? How to reuse them for RF tinkering?

[RoGeorge]

The TL;DR - found a laptop lid with 6 antennas

Went to a brick and mortar electronics shop and bought this small $2 IPEX MHF (U.FL) to SMA adapter.



The plan was to fool around with an old router (which has a PCB antenna and an unused onboard U.FL receptacle) and an ADALM-PLUTO SDR.

On the way home, I spotted by mistake on the sidewalk, near a dumpster bin a few feet away, some wires on the ground.  They were looking like they were terminated with IPEX connectors, or something.   

Kept walking while thinking "Yeah, right, never seen a U.FL cable before, and now, just after buying a small U.FL pigtail, microwave cables are growing near dumpsters waiting to be picked up.  You wish." 

But they were looking like the U.FL I just bought, maybe I should go back and take a look.  Went back and they were indeed terminated with U.FL connectors.  One of those WTF moments when all you wished for was a potato to buy, bought one, but life keeps throwing free truffles at you!   

The cables were attached to a slab of Al that once was the cover lid of a laptop (no display, just the metal top cover).  Six cables attached to two groups of laptop antennas:











One cable had some mud on it on all its 3 connectors, but managed to clean it good enough.  The other cable looked pristine.







At home, took a closer look to the lid and realized that there is a bonus on it:  a small 3.2MP webcam module, CN-04P5V9, with two I2C digital microphones and a SMB ambient light sensor (ALS) bundled with the camera:



 

The facts:
By the webcam part number, the lid seems to be from a former Dell Precision M6500 laptop, year 2010.

Dell's M6500 service manual shows 3 RF boards:  WWAN, WLAN, WPAN

• WWAN card, 2 antennas - Wireless Wide Area Network, roughly this is for Internet from a mobile phone provider.  Requires a SIM card plugged into laptop, and usually a data-plan contract with the mobile phone provider, too.
• WLAN card, 3 antennas - Wireless Local Area Network, aka Wi-Fi (IEEE 802.11)
• WPAN card, 1 connector - Wireless Personal Area Network, never heard of WPAN before.  Internet says this is a generic name for any low range wireless like Bluetooth, IrDA, ZigBee, etc.
  
  From the Dell's service manual "NOTE: WPAN is a generic name for Ultra Wide Band (UWB) and Bluetooth® (BT). Insert a WPAN card only into the slot labeled WPAN/UWB/FCM"
  
  Never heard of UWB either.  Wikipedia says it's weak RF pulses with a very wide RF spectrum (>500MHz bandwidth), somewhere in the band between 3.1GHz-10.6GHz.  Sometimes called UWB (Ultra Wide Bandwidth) or ultraband, https://en.wikipedia.org/wiki/Ultra-wideband , requires no license and has a lot of neat applications, from radar imaging to data transfer.

In total 6 antennas, as in the pics.


The Questions:

• Why so many antennas in a single laptop?  Can not understand why 6 of them.  Let's say 1 for GPS, 1 for BT, 2 for Wi-Fi (maybe has both 2.4GHz and 5GHz bands?) and we are only at 4, not 6.
  
• I've seen where each antenna connector is plugged, but how to identify what for is each antenna?
• In case of reusing the antennas in an SDR or a Wi-Fi router, is it necessary to keep the whole metal lid as ground plane?  (the 6 antennas are grouped in 2 groups https://www.pchub.com/dell-precision-m6500-wireless-antenna-cable-dq643139700-dq643139701-p60927 and they come with auto adhesive Cu and Al foils to be taped on the laptop's lid, which is an Al alloy)

[ataradov]

1. https://en.wikipedia.org/wiki/MIMO
2. Trace them to respective components. It should nod be that hard, RF traces are typically kept short and on the outside layers.
3. No idea.

[David Hess]

Multiple antennas for the same radio may also be used for diversity reception.

[coppercone2]

because the type of antenna required with those lobes in that form factor probably made a super computer shit itself

[RoGeorge]

I still don't know which is which.  Need to take a closer look to each radio card (but I don't have them).  The laptop has had 3 RF cards, one with 3 antennas, one with 2 antennas, and one with 1 antenna.  Will lookup for documentation about them.

So far I found the schematic for Dell M6500, but it's only for the motherboard, no RF board schematic.

As a side note, the webcam is functional!
https://www.eevblog.com/forum/projects/second-life-for-a-webcam-from-a-laptop-lid-(cn-04p5v9-from-dell-precision-m6500)/msg2518353/#msg2518353



 

[ejeffrey]

I still don't know which is which.  Need to take a closer look to each radio card (but I don't have them).  The laptop has had 3 RF cards, one with 3 antennas, one with 2 antennas, and one with 1 antenna.  Will lookup for documentation about them.

Wasn't that in your first post?

Dell's M6500 service manual shows 3 RF boards:  WWAN, WLAN, WPAN

    WWAN card, 2 antennas - Wireless Wide Area Network, roughly this is for Internet from a mobile phone provider.  Requires a SIM card plugged into laptop, and usually a data-plan contract with the mobile phone provider, too.
    WLAN card, 3 antennas - Wireless Local Area Network, aka Wi-Fi (IEEE 802.11)
    WPAN card, 1 connector - Wireless Personal Area Network, never heard of WPAN before.  Internet says this is a generic name for any low range wireless like Bluetooth, IrDA, ZigBee, etc.

That is pretty clear.  The cell network card has 2, the WiFi network card has 3, and the bluetooth interface has 1.

[RoGeorge]

Yes, but what is the usage for each antenna?  As an example, for now my assumption is that the WWAN card has two antennas because one is for GSM Rx/Tx, and the other one is for GPS Rx.  Not sure if this is true.

WLAN has 3 antennas.  Are all 3 for 2.4GHz, or maybe one is for 2.4GHz Wi-Fi and the remaining two are a phase array for the 5GHz Wi-Fi?  Again, no idea, just assumptions.

That is what I would like to identify.

For now, the only antenna that is certain is the one from the WPAN card, because the WPAN card has only one RF connector, and the service manual state it clear the card is for Bluetooth (2.4GHz) and Ultra Wide Band (3.1GHz ... 10.6 GHz) - but is this is even possible to work at the same time?

[ataradov]

Read my link about MIMO. They all are used for RX/TX. Depending on how smart the frontend is, only one antenna may be used at a time, or multiple antennas may be combined in attempt to mitigate multipath propagation effects.