# How does a receiving antenna work?

I thought something like this would be easy to find on the web, but apparently I was wrong. Or maybe I'm using the wrong search terms. I'm imagining some kind of resonance which amplifies only a specific frequency and lets the rest cancel themselves out, and then outputs an electric current that oscillates in that frequency - for example 2.4 billion times a second for a Bluetooth antenna. But I can't find the (presumably simple) mechanism for that.

So, in layman's terms - what is the simple mechanism for a receiving antenna?

## 2 Answers

What happens at the antenna is important, but you can't really understand the mechanism of radio reception until look look at the entire field-on-field interaction: the combination of the incident field from a distant transmitter, and the re-radiated field of the receiving antenna.

There is no reception, no absorption of power, unless current flows in the receiver. And when current flows in the receiver, it re-radiates as a transmitter. How is there a net absorption of power?

I have never seen this question clearly addressed anywhere except my own blogsite. Here is a link to my post, "The Crystal Radio". In the picture below, the parallel plane waves (coming from the left) representing the incident field, and the concentric circles representing the outgoing re-radiatede field:

The shadow region behind the antenna (seen from above) represents the area where destructive interference takes place, meaning that power is removed from the incident wave and gathered in the antenna.

The absorption of power is acheived by controlling the phase of the re-radiatede filed so that it matches the incident field for optimum effect: this is called tuning the antenna. The power is then maximized by controlling the current so that the re-radiated power OUTSIDE the shadow zone does not overpower the absorbed zone. That is called "impedance matching".

If people understood half of what I have explained here about antennas, there would be a lot less nonsense being spread about things like the photo-electric effect.

DISCLAIMER: I am a recognized crackpot on this website whose answers are routinely and massively downvoted by people who know a lot more than me.

• One upvote and one downvote so far. I get lots of downvotes but none of them ever tell me what's wrong with my physics. – Marty Green Feb 28 '17 at 4:20
• @ marty green, spot on explanation, also regarding photoelectric effect. Nobelist Willis Lamb had similar opinions, but unfortunately died before really getting started on the topic. Also see refs: amasci.com/tesla/dipole1.html – wbeaty Aug 24 '17 at 11:29
• thanks bill. but don't forget you're a bigger crackpot than even me. – Marty Green Aug 24 '17 at 13:33

The electromagnetic field radiated by the transmitter (and every other transmitter and noise source) causes current to flow in the antenna. This is filtered and amplified by the radio. The radio needs to weed out the signal it's looking for from all the rest of the junk coming in on the antenna.

Normal antennas aren't as selective as you are expecting. A resonant antenna can be somewhat selective, but it requires special construction. A ham antenna called a magnetic loop is about the most selective real antenna I know of. They are employed at tens of megahertz.

The degree of selectivity is referred to as $Q$.

• I believe that degree of selectivity is unwanted information in the answer. You could try adding more information about $Q$ or remove it completely. – Yashas Feb 28 '17 at 5:21
• Perhaps I'm mixing up two things, but as far as I "understand" we can use the Fourier transformation to break a complicated waveform into its components. Are you saying that all of the incoming waves picked up by the antenna are combined into one wave that we, then, need to apply the Fourier transformation to? And if so, then what is the tuning knob on a radio, isn't it to somehow "select" a certain frequency? – ispiro Feb 28 '17 at 13:05
• The OP seems to me, to be thinking that the antenna is far more frequency selective than it is. – user103218 Feb 28 '17 at 15:18
• The energy delivered from the antenna contains some of every signal that is illuminating the antenna. The "Tuning knob" may be altering parameters to a software algorithm, or actually moving inductors or capacitors. That's up to you. – user103218 Feb 28 '17 at 15:20
• I understand from your answer that these "inductors or capacitors" would somehow amplify only a certain wavelength. Is that correct? [It would also be beneficial if you could outline how they would do that, but only if it's not too technical as the point of this question is to get a layman's understanding.] – ispiro Feb 28 '17 at 18:33