I just disassembled an external FM antenna designed for an old Asus laptop. I expected to find some circuitry and a monopole or some other simple antenna. However, what I found was the antenna shown in the picture (bottom and top view). Honestly, it resembles somewhat a monopole because there is a ground plane and only one side of the wire is connected to the coaxial cable that exited from the case of the antenna. But I can't explain why it has that strange shape. I should add that the bottom of the pcb was in contact (or maybe just really close) to two magnets.

  • What kind of antenna is that and how does it work? (or where I can find how it works)
  • What was the purpose of the two magnets?
  • Why the is the wire shaped like that? And why is the ground plane shaped too? (see the bottom view)

enter image description here

enter image description here

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    \$\begingroup\$ The two magnets are probably there to stick it to a steel surface, and almost certainly have nothing to do with the operation of the actual antenna \$\endgroup\$ – C_Elegans Dec 9 '18 at 16:07
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    \$\begingroup\$ It looks like a folded loop antenna, but I can't really see if the end is terminated onto the ground plane or not. \$\endgroup\$ – isdi Dec 9 '18 at 17:08
  • \$\begingroup\$ There is a cut in the ground plane that separates the two ends of the antenna. It's not easy but it can be seen in the picture of the bottom view. \$\endgroup\$ – Elia Dec 9 '18 at 19:22
  • \$\begingroup\$ @isdi Yes, one of the ends is while the other one seems to be connected to some sort of a small diameter coaxial wire that's been cut (with its shielding soldered into the ground plane as well). \$\endgroup\$ – CoolKoon Jan 2 '19 at 21:06
  • \$\begingroup\$ How old is old? This could be a GSM or CDMA cellular antenna. \$\endgroup\$ – MadHatter Feb 3 '19 at 15:11

Personally I'm also almost certain that this is a folded loop antenna. It's really similar to this one (found at http://ok1ike.c-a-v.com/soubory/200anten.htm):

Capacity Tuned Folded Loop Antenna for 20m

The only differences between the two lie in the fact that yours doesn't use capacitive coupling for tuning (because it's obviously tuned for a fixed frequency) and the target frequencies are completely different too. In fact I'm almost certain that what you have is not an FM (nor VHF) antenna because it's way too small for that. My guess would be some early Wi-Fi, some GSM or (since you said that it comes from older equipment) even WiMAX design whose wavelengths could roughly match the dimensions of the antenna based on your picture. The ground plane could be there for extra shielding against interference, perhaps to improve reception (obviously this is not a TX antenna).

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