fm antenna from power cord

[scknight]

I just have a quick question that I was hoping someone could answer. I have a cd player/fm radio and the manual says that the fm antenna is built into the power cord. It's just a normal two prong U.S. power cable. When I look at the power supply board I see the two connections from the power cord, what I'm wondering is how does it also get used as an antenna? I guess I was thinking perhaps there was another cable in the cord.

[Psi]

Probably similar to how cellphones use the headphone cable for the fm radio antenna.

I think they just non-electrically couple one of the wires to the fm chip using something that will pass FM frequencies but block dc.
Not sure if it's capacitive or magnetic though.

Its not a tuned antenna that's for sure. Which is why they're usually shit and keep dropping reception

[Rascal]

This isn’t a new idea, you sometimes found this on vintage valve radios. A small value capacitor (condenser / filter) would isolate the mains voltage from the set (tuning circuit). It works quite well on FM obviously MW LW reception wasn’t too good.

[SeanB]

Basically they couple the antenna to the one side of the mains cable, normally by either wrapping an insulated wire around it a few turns or by using a 10pF 1kV ceramic disc capacitor. The metre or so of mains cable before the wall outlet then is the antenna, with the board forming the other half of the monopole antenna, or really a very poor half of a dipole. Can work well for FM in high signal strength areas, but has poor performance that is affected by other equipment in the house plugged into the same outlet.

[scknight]

Thanks everyone! After looking over the power supply board more when I got home yesterday I did notice where they had a small ceramic capacitor running off of the mains voltage and being used as the antenna. Additionally I found this page in case anyone finds it interesting. http://www.arthropodsystems.com/LiveWireAntenna/LiveWireAntenna.html

[Rascal]

I believe the capacitors used are classed as 'Y capacitors' - for applications where failure could lead to electric shock. I understand they are design not to fail (short circuit)

Never too keen on the idea myself!

[Norm_N_Tam]

Please have a look here for info re. how to make a Power Cord Antenna:

https://web.archive.org/web/20040914073658/http://www.angelfire.com/ak/egel/cord.html

[joseph nicholas]

I have one of those Sanyo clock radios with the wire wraped around the power cord.  It seems to work best on fm stations in the area.  Kind of a cool idea but does not pull in signals from any distance with quality, but then its just a radio to get you up and may be some news and weather.  That sort of thing.