Antenna coupling question

In another thread, over there --> (somewhere), an explanation of antenna behavior was presented. Basically that an antenna as used for a transmitter (FM, Wi-Fi, CB, etc.) worked as, well, an antenna.

But the “antenna” on an RFID system is really just one half of a transformer. Two coils joined by a magnetic field. It seems obvious to me that power transfer occurs from one half to the other in such a system.

My question is:
Does a transmitting antenna (not RFID) couple to a receiving antenna? In a proper antenna sense, (FM, Wi-Fi, CB, etc.) Does power transfer occur?

If I had godlike power to suddenly switch on or off every receiving radio in a major city, all of which were tuned to a particular station, would I be able to see a power draw if I had sufficiently sensitive equipment?

Yes, this is pointless, but I’ve been thinking on it all day today.
I both love and hate my brain somedays.

1 Like

Not sure on the minutia but I think you could get power transfer but you’d not see a change in power draw. There is transmission but not coupling.

For example a torches power draw doesn’t change if you put a PV panel Infront of it afaik. I think regular radio transmissions behaves in the same way just at a different frequency from visible light obviously.

1 Like

Yes, power transfer occurs… but as @leumas95 said, it’s not necessarily like a coupling. Power transfer does occur because this is exactly how a crystal radio set works without any batteries of it’s own. It’s also how microwave ovens work to heat your food… energy is transferred.

The emitter is pushing out a certain amount of power… like a fountain pushes out water… either that water flys out, dissipating energy over time and distance as it arcs to the ground. The energy to push it out has already been expended. Now think of receivers in the area as bowls that catch the water before it hits the ground. They are getting a tiny fraction of the energy spent and doing something with it. In fact, the arc of some radiation patterns produced by certain types of transmitting antennas look an awful lot like a fountain actually.

Another way to think of it is sound… when you scream at someone’s face, a certain amount of energy is expended by your lungs to push the air and your vocal chords to modulate that air. How many ears that happen to be around don’t affect the amount of energy you expend screaming, but they do convert the energy received into mechanical movement of your ear drum. Ears that are further away get less energy because the distance means the energy is expanding, becoming less dense, losing power per cubed millimeter of free space… but this has no bearing on the energy expended to produce the soundwave.

Finally… if you know much about the CB radio antics of the 1980s, you’ll surely be aware of teenagers with CB rigs in their cars, connected to hopped up linear amplifiers that bump the output wattage so high, when they keyed up, the energy emitted would bleed over other amplification electronics in FM radios, television sets, telephones, and even kids toys… there was so much energy being pushed through the air and into any kind of unshielded conductors in the path of that energy that you COULD FORCIBLY SPEAK TO THE ENTIRE NEIGHBORHOOD LIKE A GOD and there was nothing anyone could really do about it.

2 Likes

You don’t have these plans by chance, you know, for science?

1 Like

Ya know, screw sensing magnets. I want to SEE magnetic fields. Damn that would be cool.

I did that when I was a kid, before I got my ham radio license: I had a Sirio Yago on a mast with a rotor in the backyard, and I was pushing 1000W through it. It didn’t take long before the gonio truck parked in front of the house, and my dad got a mighty fine - and I got a mighty earful about it for days :slight_smile:

2 Likes