[Physics] Radio antennas that are much shorter than the wavelength

antennaselectromagnetic-radiationelectromagnetismradiowavelength

From my limited experience with ham radio when I was a kid, I expect transmitting and receiving antennas to have lengths that are on the same order of magnitude as the wavelength, and in fact I recall having to mess around to compensate for the fact that a given antenna wouldn't be properly resonant over an entire frequency band. This also seems to match up with what we see with musical instruments, where, e.g., a saxophone's tube is half a wavelength and a clarinet's is a quarter.

For commercial FM radio with a frequency of 100 MHz, the wavelength is about 3 m, so I can believe that some of the receiving antennas I've seen are a half-wave or quarter wave. But for AM radio at 1000 kHz, the wavelength is 300 m, which is obviously not a practical length for a receiving antenna.

Can anyone explain this in physics terms, hopefully without making me break out my copy of Jackson and wade through pages of spherical harmonics? Does AM reception suffer from the length mismatch, e.g., by being less efficient? Does it benefit from it because it's so far off resonance that the frequency response is even across the whole band? Is there a dipole approximation that's valid for AM only? For both AM and FM? If the sensitivity is suppressed for the too-short antenna, is there some simple way to estimate the suppression factor, e.g., by assuming a Breit-Wigner shape for a resonance?

This question touched on this issue, but only tangentially, and the answers actually seem inconsistent with the observed facts about AM. Also related but not identical: Radio communication and antennas

Best Answer

http://www.antenna-theory.com/antennas/shortdipole.php is a website with useful info., including formulas.
To oversimplify, it seems to say that once the antenna is a tenth or less of the wavelength, the exact ratios don't matter so much. The antenna is inefficient, but it works for both sending and receiving. If you can detect the signal, of course you can amplify it as much as you want.

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[Physics] How is extremely low frequency (ELF) radiation collected by a submarine antenna

There exist a wiki article on this, project elf.

The scaled-down system the Navy eventually constructed, called Project ELF, began testing in 1982 and became operational in 1989. It consisted of two transmitter facilities, one at Clam Lake, Wisconsin and one at Republic, Michigan. with a total of 84 miles[ of above-ground transmission line antenna. The two transmitters normally operated synchronized together as one antenna for greater range, but could also operate independently.

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In 2004 the Navy shut down both transmitters, with the explanation that very low frequency (VLF) communication systems had improved to the point that the ELF system was unnecessary

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The Russian Navy operates an ELF transmitter facility, named ZEVS ("Zeus"), to communicate with its submarines, located 30 km southeast of Murmansk on the Kola peninsula in northern Russia. Signals from it were detected in the 1990s at Stanford University and elsewhere. It normally operates at 82 Hz, using MSK (minimum shift keying) modulation. although it reportedly can cover the frequency range from 20 to 250 Hz. It reportedly consists of two parallel ground dipole antennas 60 km long, driven at currents of 200 to 300 amperes.[ Calculations from intercepted signals indicate it is 10 dB more powerful than the U.S. transmitters.Unlike them it is used for geophysical research in addition to military communications

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As fore receiving antennas, they do not have to be large.It is the generation of a frequency modulated with a signal that needs the full wavelength length. The receiver if the signal is strong works with smaller dimensions.

Ground dipoles are not needed for reception of ELF signals. The requirements for receiving antennas at ELF frequencies are less stringent than transmitting antennas, because the signal to noise ratio in ELF receivers is dominated by the large atmospheric noise in the band. The level of signal strength at the antenna is far above the noise in the receiver circuit, so small inefficient receiving antennas can be used. Various types of coil and ferrite loop antennas have been used for reception.

This might help somebody designing an antenna.

[Physics] How does the human body affect radio reception

Small pocket radios usually have fairly poor antennas in them, because of their small size. But by placing your hand nearby the antenna of that small radio, you are creating a capacitor in which one terminal is the radio chassis and the other is your hand. Any radio frequency signal induced in your skin by radio broadcasts will be capacitively coupled from your skin to the radio by means of that capacitor, and since your body is a lot larger than the antenna in the radio, the radio reception will be strengthened.

Depending on the frequency the radio is tuned to and how sweaty your skin is, the strengthening effect may be greater when you are actually touching the antenna in the radio.

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