Home > Antennas, Theory > Antenna Puzzle #2

Antenna Puzzle #2

Here’s a simple but interesting antenna puzzle to think about…

Let’s say you use a wire dipole for the 20M band. You decide you want to boost your gain by adding a second dipole (end-to-end, i.e. colinear) and combine their feeds (in phase, of course.)

The two antennas receive the incoming signal (depicted in green below) which gets combined to double its strength at the receiver. Here’s the diagram:

Simple dipole array (receive)

So, this simple array provides 3 dBd of gain. It makes sense because each antenna is receiving an independent field of energy from the distant station.

Now, you use the same setup to transmit 20 watts on CW. Of course, your power gets divided in half between the two dipoles. Each antenna gets 10 watts.

Simple dipole array (transmit)

But, wait… if you simply put that 20 watts into a single dipole wouldn’t that be the same thing as two antennas at 10 watts?

The principle of antenna reciprocity dictates that an antenna must behave the same for both receive and transmit. It has the same gain, directivity, and pattern. So where’s your 3 dB of gain on transmit?

Perhaps you’ll answer that the two antenna fields significantly overlap. But, even if they add together perfectly, won’t the distant station still see the same signal as one antenna at 20 watts? Where’s the 3 dB? (The output should be equivalent to 40 watts.)

So, there you go, a puzzle. What’s your solution? Post it here as a comment.

Answer now posted here: Power vs Signal Strength 

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  1. Steve Turner
    2011/04/19 at 4:13 PM

    With speaker cones, connecting two drivers in parallel also increases the effective radiated power because for the same amount of power, more air is moved. I can not with any knowledge base extend that to the photons being emitted from the radiators but that is what my “gut” tells me is happening. What’s the answer Carl?

  2. 2011/04/24 at 5:11 PM

    I think that’s pretty accurate. I’ll write and post the answer now.

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