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Author Topic: "Q and the Energy Stored Around Antennas" in Feb QST: Interesting & Puzzling  (Read 1190 times)
VE7TIT
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Posts: 62




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« on: January 20, 2013, 10:45:46 AM »

A dipole with a Q of 20 at 100 watts has (100x20)/2*PI VA (i.e. V*A, as in "watts") of reactive power (320 VA) in the stored fields surrounding the antenna. (Yes, I know watts and VA are not identical.)
A magloop with Q=800 and 100 watts has (100*800)/2*PI VA of reactive power (13000 VA) in the stored fields surrounding the antenna.
"The stored energy exceeds radiated energy out to a half wavelength from the loop. No wonder the small loop has a reputation for coupling to everything in sight that isn't nailed down!"

I've read this article a few times now and all the math makes sense, but I still don't really understand this idea of "reactive power being stored in the near field." Can someone who understands this please discuss it in more detail or from another angle? (The all-copper, all-soldered-connections magloop in my living room has a theoretical bandwidth of 20 kHz but a measured bandwidth of 200 kHz, so this article has real-world relevance for me in addition to just being interesting.)

Thanks for any light you can shed. (The article in on page 37 of this month's QST.)
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G8HQP
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« Reply #1 on: January 20, 2013, 01:47:55 PM »

I haven't seen the article, but there was something vaguely similar in one of the IEEE magazines a few months ago.

Think of an antenna (and the fields in its immediate surrounding space) as a somewhat leaky tuned circuit. The lower the Q the more leaky it is, so energy escapes more easily. The energy which escapes is the radiation field. So for a given amount of tuned circuit energy the lower the Q the more energy which gets radiated. That in turn means that for a given amount of radiated energy the lower the Q the less energy you need in the tuned circuit.

Then remember that a tuned circuit consists of a capacitor and an inductor. These store energy in electric and magnetic fields respectively. For an ordinary capacitor almost all this energy is between the plates. Similarly for an inductor; the energy is stored mainly between the coils. An antenna is like an 'opened up' capacitor/inductor, so the energy is not concentrated but spread out around the antenna. Most of the energy simply gets converted from electric (potential) to magnetic (kinetic) energy every half-cycle - exactly like any tuned circuit. However, an antenna is a leaky tuned circuit so some of the energy leaks off to infinity.
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VE7TIT
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« Reply #2 on: January 20, 2013, 03:27:32 PM »

That helps, especially "The lower the Q the more leaky it is, so energy escapes more easily." Thank you. Smiley
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