Re: help about energy sucking antennas

Hi Benyuska,

That's an interesting question. I read the article at the link you provided. What I first noticed is that the author of the article seems to mix quantum mechanical phenomena and classical EM theory. The author treats atoms as very small antennas, which is not consistent with any of my schooling. If we leave QM out of the discussion, loop antennas can be explained effectively by classical electromagnetics alone, atoms are a different issue.

Most of the information I am providing can be found in any undergraduate or graduate level text covering electromagnetic radiation. Two reference which I use regularly are "Fields and Waves in Communication Electronics", Ramo, Whinnery, Van Duzer and "Antenna Theory, Analysis and Design", Balanis. Both of these texts contain excellent coverage of the subject and are not hard to follow if you have a good understand of vector calculus. The former is considered the definitive electromagnetics text used at many Universities. The latter covers antenna theory exclusively and is quite comprehensive.

Antennas convert electrical energy in the form of oscillating voltages and currents into electromagnetic radiation. They can also perform the reverse. The simplest antenna, which is commonly used as a starting point for analysis, is the infinitesmal dipole. It's basically two wires much shorter than the wavelength of interest. In the case of AM radio at about 1MHz, any dipole shorter than a few meters can be considered infinitesmal. What's interesting about this antenna, is that even though it is very small, it can radiate and receive considerable amounts of power. In fact, it is theoretically possible for such a small antenna to receive or transmit as much power as a significantly larger antenna. This does not mean that small antennas can replace large antennas in all cases. The problem with small antennas is that they are very inefficient. It is possible for a one meter antenna to radiate 50kW in the AM band, but it will take at least 100 times as much power to do it. There are also impedence matching problems due to reactive antenna impedences.

What's the source of inefficency in small antennas? To understand this, we need to talk about radiation resistance. Radiation resistance is an important figure inherent to any antenna. The value of radiation resistance depends on many factors, such as antenna geometry, frequency, nearby objects, etc. In the case of an ideal half wave dipole antenna, the radiation resistance is about 72 ohms. If a transmitter were to supply 1 Watt of power into an ideal dipole, exactly 1 Watt of power would be radiated. This is exactly equivalent to 1 Watt of power supplied to a 72 ohm resistor, but the resistor would only produce incoherent radiation as heat. An antenna is designed to produce coherent radiation at a specific frequency.

It turns out that real antennas also contain losses due to resistance in the antenna elements themselves. These losses appear in series with the radiation resistance and tend to be small, fractions of an ohm or so. This is no problem if the radation resistance is high, like 72 ohms. The problem with small antennas, those significantly smaller than a wavelength, is that the radiation resistance gets smaller as the antenna gets smaller. At some point, more power is lost in heat due to losses, than in radiation. For low power applications, such as walkie-talkies, this might be OK, but for high power transmitters, this is not acceptable.

Now let's turn our attention to the receiving end. An antenna can turn incident EM waves into electrical power. As you might have guessed, this is not perfect either. Antenna losses cause problems for receivers as well as transmitters. The antenna loss resistance burns up some of the power from the incident radiation, which means a weaker signal from the antenna terminals. We could try and compensate with more sensitive radio receivers, but there are limits due to noise. During the early days of television, TV receivers in the UHF band were not very good. TV stations compensated by blasting megawatts of power. To this day, some of the highest power transmitters are local TV stations.

Now back to your question. The specific antenna you were interested in is a loop antenna. Loop antennas work just like wire antennas, they exhibit a radiation resistance and a loss resistance. Loop antennas with electrically small circumferences exhibit low radiation resistance, just like an small dipoles. Even though a small loop might receive a lot of power, most of it will be lost before we can get out of the terminals. To increase the radiation resistance, we could make a bigger antenna, but this is not practical for AM radios with a wavelength of several hundred meters! Another way is to use a multiturn loop with a ferrite core. The ferrite core concentrates the magnetic field intensity because it has a high permeability. This increases the radiation resistance and improves efficiency. In addition, small ferrite loop antennas exhibit a series inductance because of the many turns of wire and high permeability core. To get the most signal power out of the antenna, it's best to tune out the inductance with a capacitor in parallel with the antenna terminals. This makes the antenna look like a tuned circuit, which has reasonable selectivity in the AM band. The effect of tuning increases the output voltage, but does not increase the power.

The article you mention touches on the related subject of active antennas. The idea of an active antenna is to cancel out some (or all) of the loss resistance in an antenna using an active circuit simulating a negative resistance. Conceptually, the negative resistance circuit adds power to the antenna exactly in phase with the incoming signal. This can be tricky, too much negative resistance will cause the circuit to oscillate. One of the earliest radio receivers invented by Edwin Armstrong operates on this principle, it is known as a regenerative receiver. You can find discussions and schematics of regenerative receivers all over the Web. They are easy and fun to build, but they do not violate any of the laws of physics as we know them today.

The circuit described by Sutton & Spaniol at this web page contains a negative resistance source, not entirely different from that of Armstrong. As an incredible co-incidence, I met John Sutton during my high school days as a summer intern at NASA Goddard more than 20 years ago! Although regenerative receivers are simple and effective, they are not practical in many applications. They take more effort to tune when compared to heterodyne receivers (also pioneered by Armstrong). As negative resistance is increased, the Q of the circuit increases, and the bandwidth decreases. The limit is zero total resistance, zero bandwidth and infinite Q. At that point, you have an oscillator. There's an old engineering adage: "If you want an oscillator, build an amplifier".

The ferrite loop with a tuning capacitor in parallel looks a lot like the work of Nikola Tesla (minus the ferrite). Of course, Telsa was attempting to transmit significant power over long distances, not just signals. Tesla's goal has been realized using microwaves, frequencies much higher than were possible in the late 19th century. This is usually accomplished with high gain, narrow beam antennas, such as reflective dishes. Researchers have developed small aircraft powered by microwaves capable of indefinite flight time. Doing the same thing with loop antennas is much less efficient.

I hope I have answered your questions, even though I rambled quite a bit! If you have any further questions, feel free to contact MadSci again with a follow-on question.