Inductive Coupling and the Select-A-Tenna

selectatenna.com - Feb 2008

For a couple years I've have had these two inter-related topics I wanted to write about. First there is the poorly understood concept of electromagnetic inductive coupling; then there is the Select-A-Tenna a passive radio antenna you can use at home with almost any radio. (They are not a sponsor, that product is actually no longer made) Inductive coupling is poorly understood because beyond a surface understanding of the concept it's best understood as a set of mathematical statements. Frankly that's lost on the non-engineers so I'll do my best not to do that. 

So I'll try to use the Select-A-Tenna as a real world application of inductive coupling and not talk math at you. This antenna wildly improves the reception of my little GE radio (model P2870A). The Select-A-Tenna literally "pulls-in" completely inaudible signals and renders them as listenable as local stations. There is a great video here demonstrating it's function here.  

The Select-A-Tenna is 11 inches in diameter and 2.5 inches deep.The chassis is PVC plastic, I suspect the early models may be metal but I've never actually seen one. The chassis design is completely consistent over it's entire production run so far as I'm aware.The bottom of mine bears a sticker which reads "Made in USA by Intensitronics Corp. P.O. Box 28, Ashland, WI 54806". 

Most people are at least passingly familiar with Electromagnetic or magnetic induction. If an electric current flows through a wire it will produce a magnetic field around it. If you wrap that wire into a coil, say around a ferrite rod, the magnetic field will be intensified. If you wrap additional layers of wire are around that coil, with the same current flowing through them, the magnetic field strength will be further increased. What I'm describing is a static magnetic field. 

Michael Faraday was performing experiments like these around 1830. He is usually credited with the discovery of electromagnetic induction. James Clerk Maxwell codified Faraday  as the father of induction (among other things) 30 years later in his works on electromagnetic theory. Maxwell referred to his discovery as Faraday's law of induction, making the case that his formulas were a direct extension of Faraday's discoveries.  This was not true but it's how Maxwell framed it. Thus this "law" takes the form of a set of formulas making two statements:

1.  The Maxwell–Faraday law, which states that when a magnetic field is incident on a coil of conductor, the magnitude of the electromotive force (EMF) induced in the coil is directly proportional to the rate of change in the inducing magnetic field and dot product between the field direction and the axis of the coil.

2.  The Faraday–Lenz law, which states when a magnetic field induces a current in a conducting coil, the induced current also generates its own magnetic field that points opposite to the inducing magnetic field.

In some ways Lenz law is the more direct extension of Faraday. His discoveries were only 3 years after Faraday and validates Faraday proving that it obeys Newton’s third law on the conservation of energy. More here. I've mentioned some of that before in other articles such as my 2013 series on induction coils. [SOURCE]

Here I wanted to focus on a small part of electromagnetic theory that describes inductive coupling. Just as is defined in Faraday's law of induction, energy in the tuned antenna circuit is transferred through induction to the receiver, which is just another other tuned circuit. Because the tuner (your radio) is a closed circuit, it takes advantage of all the energy in the circuit. i.e. it benefits asymmetrically from the net total energy in the coupled circuit.  The common inductance transfers that energy via indirect magnetic coupling. It's immaterial that the antenna is passive. Faraday's law applies to the fields themselves and does not even require the presence of a physical circuit. This is physics, not engineering. 

Do I fully understand this area of physics? No definitely not. To paraphrase a Reddit comment I once read: There is nothing about the physical world that is 100% understood. Engineers work with phenomena that are less than 100% understood all the time. The rest is "magic".

 Radio frequency amplification, theory and practice - Kenneth Harkness

Way back in 2011 I found a website with an article which went deep into the function and performance of different models of proximate loop antennas. These antennas exploited inductive coupling to improve radio reception. One such antenna, the Select-A-Tenna is in my personal kit and it's one I've been very impressed with for decades. So I was very interested in the model comparisons.

That website was radiointel.com and no longer exists. I eventually found it's author, Jay Allen. He gave me permission to quote his work and pointed me at the new online home for the article here. You should absolutely read it. Jay went very deep in his testing with multiple antenna models and provided context and information even the manufacturers did not share. 

But I wanted to include two quotes in particular which get into some technical particulars you might struggle to figure out yourself:

"One bit of advice: Initial setup up and testing of these antennas can be confusing if done at night. During nighttime reception, there are many more strong signals than during the daytime. When you increase the level still further with an external antenna, the radio’s AGC circuits reduce the gain to compensate, so you may not hear a difference. This is often miss-construed to mean that these loops don’t do much at night, but believe me, nothing could be further from the truth. The improvements gained through their use at night is a bit different than what you may get during the day but sometimes will be very helpful...

 If your radio has no internal ferrite rod antenna (such as the Eton E-1 or vintage radios with wire-wound loops on their back panels), inductive coupling won’t work so you will have to use the direct connection. Both of the mini-jack equipped models (the SAT Model 541 M and the Terk) come with an adapter cable terminating in bare wires. Neither of these antennas furnish much information about output impedance, but when I tested them on a radio equipped with both high and low impedance antenna inputs (nominally 50 ohms and 500 ohms), both of the loops showed slightly higher signal levels with the high impedance connection, although they worked acceptably into either impedance so matching your particular radio’s input shouldn’t be a problem. However each antenna showed markedly stronger signal levels when used with inductive coupling so you may want to experiment to see which setup works best for you."

Back in 2011 when Jay Allen's article was published C.Crane was still selling the Select-A-Tenna. They no longer do. It was discontinued around 2015. They do still carry a Terk antenna which operates on the same premise. In the Terk antenna description today they state "C. Crane was started with this type of antenna 40 years ago in 1983." This is possibly true about the Terk, but the Select-A-Tenna is at least a decade older than that. 

This is how I first learned about the Select-A-Tenna —kids science catalogs and publications. Back then it was sold by the Edmund Scientific Co. of Barrington NJ.  Edmund Scientific was founded back in 1942 and it still exists today but re-focused on optical lenses. It's interesting to note that the debut of the Select-A-Tenna coincides with the retirement of Norman W. Edmund in 1975, when he left company operations to his son, Robert. What I remembered from those ads was the statement "Currently in use on Alaska's north slope where radio reception is very difficult." The smaller ad from 1988 I included specifically plugs 710 WOR-AM. I don't know why that particular Class A station would pay for that mention but presumably there was a deal with RKO general, then group owner.  Radiomuseum.com puts the start of manufacturing at Intensitronics to 1971. [SOURCE]  They're probably using the selectatenna.com website for that start date.

The ads from the Fall of 1975 are the earliest appearances of the Select-A-Tenna I am aware of. they ran in Popular Electronics, Radio Electronics, and Popular Science.  Back in 1976 the Select-A-Tenna came in two models 72095N and 72147N for $15.95 and 22.95 respectively. The difference is unclear. The latter model is merely referred to as the "ultra." In most ads only the 72147N is listed. By 1988 the price was $39.95 and by 1990 it was $49.95. By 1995 it was 59.95. I know it was reviewed in an issue of CQ around 1988 but I've never found that copy.  Sometime around 2005 another model debuted a 541S which took a 9v battery and provided "regenerative variable gain."  That unit went for $179.95 by 2011 and in the waning days of the series, $199.95.  I've never seen one for sale on the used market. More here and here. 

It appears in color in the Edmund Scientific catalog of 1990. [SOURCE]  It's in 1991 that I found the first C.Crane ad which specifically includes a Select-A-Tenna, but puts it beside other antennas and radios by Sangean, Grundig, Dymek and others. This was targeting radio hobbyists. I thought Crane bought the rights to the antenna around this time or otherwise negotiated exclusive distribution from Intensitronics. But in 2005 I found ads where Intensitronics was directly marketing the Select-A-Tenna themselves, but that may instead reflect declining sales and/or a loss of interest by C.Crane in exclusivity.

Edmund closed their factory store in 2001. The science-themed toys business was sold off to Science Kit and Boreal Laboratories in Tonawanda, NY.  Intensitronics operated their selectatenna.com website from  2000 through 2008 probably representing the time window where they directly marketed the Select-A-Tenna. 

There Is No Aether Either

Science achieves progress largely through consensus. That consensus is never that we are exactly right but that our assumption is probable. We proceed with it as an accepted theory and dominant paradigm. It's something that the religious right has difficulty with— the lack of absolute certainty and static stability.  But it's been a pretty good system for the last several centuries and brought our species a great number of technological advances. But very often to step forward, we must discard an erroneous assumption. In 1887 the Michelson–Morley experiment proved that there was no aether. Hertz discovered radio waves in 1888. the timing was no coincidence. More here.

In the case of "luminiferous aether" this was a medieval, pseudo-scientific alchemists idea that had less evidence behind it than dark matter. In the 17th century they called it æther, ether, aether wind, and even an "aethereal Medium." The æther was descended from the idea of the classical elements like earth, air, water and fire. It was referred to in Plato's dialogs "there is the most translucent kind which is called by the name of aether." He considered it a sub-type of air. But that was more than 400 B.C.!

Sadly we were still in the depths of that aether idea more than two thousand years later. In the 1880s Sir Oliver Lodge and James Clerk Maxwell were waxing poetic about the damn aether.  Maxwell believed that the propagation of electric and magnetic waves required a physical medium like waves in water. He discussed his scientific basis for aether in two papers in particular. In 1856, he published the paper On Faraday's Lines of Force and in 1862 he followed it up with On Physical Lines of Force which suggested that light, electric fields and magnetic fields could be explained in a unified electromagnetic theory.  In 1864 he wrote a paper debuting that theory: A Dynamical Theory of the Electromagnetic Field. He downplayed the aether in the third one and focused on the waves movement through space.

Michelson and Morley put it to rest in 1887. If the aether was real  it was thought that it should be possible to measure it's effects or motion. But since it was assumed to move at the speed of light it was also assumed it's perturbations would be also impossible to measure. Maxwell pointed out in 1878 that it would have to be inferred from second order effects. In 1881 Albert Michelson used a half-silvered mirror to split a light beam  and recombined them in an eyepiece to examine transverse displacement. He assumed that a beam reflecting parallel to the flow of aether would take longer than a beam reflecting perpendicular to the aether. His results were negative. He took this as a confirmation of Stokes' hypothesis of complete aether dragging.  There is a lot more detail on this in the book The Ethereal Aether by Loyd Swenson.

In 1885 Michelson began working with Ed Morley at Western Reserve University to improve on the 1881 experiment. A similar experiment was constructed but with extreme efforts applied to reduce vibration. it sat on a block of sandstone in a trough of mercury (above). They rotated the interferometer trying to find the flow of the aether. There was nothing. Instead of reporting the null result Michaelson reported the observation of one-fortieth of the expected displacement. but more importantly that this was within the range of experimental error that would allow the speed to be zero. Others confirmed the null result and thus radio waves were free to travel through a vacuum when Hertz discovered them the following year. 

Hz

Today Google reminded us of the importance of Heinrich Rudolf Hertz. Today was his 155th Birthday. Hertz (Hz) are defined in the International System of Units (SI)  as a frequency measured as the number of cycles per second of a periodic phenomenon. They are named, for Heinrich Rudolf Hertz...



Despite all the times I've written about past developments in radio and electromagnetic research, and made a reference to Hertz, I've never written of Hertz directly, and that's a damn shame. Today I correct that oversight.

He came from a family of means and attended private school and in 1875 at the age of 18 he decided to pursue engineering. He backed out within a year and left Dresden Polytechnic to join the military. He returned and then enrolled in the University of Munich with a more research focused path. A Professor Hermann Helmholtz took an interest in hertz and steered him toward physics. His doctorate thesis was "Über die Induction in rotirenden Kugeln" a paper on electromagnetic induction. Upon graduation in 1880 he became an assistant to Helmholtz. More here.

In 1885 Hertz became a professor at Karlsruhe Institute of Technology. It was there that he made real history. He did an experiment that related back to his doctorate. He used an open circuit to demonstrate to his students a condenser discharge. This was a simple spark gap circuit with a single coil a battery and a Leyden jar (an early capacitor). While opening and closing the circuit to make sparks he found that A nearby loop responded to the action with it's own sympathetic sparks across an air gap. This was a primitive spark gap transmitter and a very simple proof of the reception of that signal. He is widely believed to be the first person to send and receive radio waves. More here.

In 1873  James Clerk Maxwell's had theorizedan equivalence between light and electromagnetic propagation. Hertz made that connection, proving that electromagnetic waves could be transmitted and received as waves. He was just 28 years old. His findings were first published in the journal Annalen der Physik in 1887. He died of vasculitis at the age of 36 in 1894, the same year Marconi finally began his own experiments.

Radio in a vacuum

I was asked recently how radio waves travel in a vacuum.  I said "It's simple..."  and I stopped right there. it's not actually simple. It's actually a huge conceptual problem.  I understand it, but it's something that requires an understanding of physics.My biggest problem is trying to explain it in a concise way.  I decided to contact Paul over at the Blog Engineering Radio. Somtimes every engineer needs a consult. (thanks Paul)  Here was his response.

"Light, RF and all other EM energy travel through vacuums because they depend on the electromagnetic field, which is universally present. One might say that the EM field ends at the edge of the universe, or so the theory goes, because no one really knows for sure. Since space is mostly a vacuum, this allows us to see stars, transmit telemetry to space craft using radio waves, and listen for extra terrestrials since EM energy can transit through those area with little or no degradation."

Paul is right of course. If you read his blog you'd expect nothing less. So let's dig into some of the details. First the basic facts: In a vacuum, radio waves travel at the speed of light, or roughly 186,000 miles per second. The speed of light in a vacuum is C = 3.00 x 10⁸ m/. It's the top speed except in science fiction. This is the speed which all electromagnetic radiation travels at in a vacuum: light, radio, microwaves etc. In a vacuum all electromagnetic waves obey the inverse-square law.  It's power density is proportional to the inverse of the square of the distance from the source. But if the radio waves are propagating through a medium other than the vacuum, their speed will be less than C and vary to the medium. But the medium always diminishes it.

The problem after that is most lay people imagine it to be like a sound wave.  This is easier to grasp but is somewhat inaccurate. Sound waves move through matter. They vibrate air and are also travel only at the speed of sound which is actually variable. It travels faster in water than in air, that's 1,484 m/s compared to 343 m/s. That's a 70% different. But that vibration is traveling through matter and is bound by it. This was  first predicted by mathematical work done in 1865 by James Clerk Maxwell. I wrote about him previously.  Even then he predicted that it moved like a light wave. In 1887 Heinrich Hertz generated those waves in a laboratory confirming Maxwell's theory.

According to Einstein the photon is the basic "unit" of light and all other forms of electromagnetic radiation and is also the force carrier for the electromagnetic force. He believed mostly in a particle-based model. Photons have no mass, do not decay, carry no energy but spin and can be polarized. Their momentum is proportional to the frequency of their vibration. This is contrary to the wave-model. I am much more comfortable with the wave model, as are most despite it's short comings. Modern quantum mechanics fail to completely reconcile the two ideas.  Today they proceed with an idea called wave-particle duality. Isaac Newton originally described light particles as wave packets. He wasnt exactly right but we do consider photons to be "discrete bundles."  The energy of light is a discrete function of frequency or E = nhv. link

That is where it stops making sense for most people. Defining a vacuum is difficult. we've struggled with it since the ancient Greeks. Just because a vacuum contains no matter does not mean that it's truly a vacant space. It's full of electromagnetic fields, gravitational effects, radiation pressure, vacuum energy etc.  After that it's all Poynting vectors and the Lorentz Invariant. It becomes quite literally philosophical.  It is a wave or a particle?  Do waves exist or are they an artifact of the of the relative movement of the observer?  Is it just an excitation of the vacuum state?  In truth I don't think we have a definitive answer. I recently read that radio waves can be made to travel faster than light. link And that does not make this any easier to understand. In the end being succinct of being complete eon this specific topic.

James Maxwell's EM Wave

"I have the capacity of being more wicked than any example that man could set me"
-James Clerk Maxwell [1853]

He was a brilliant electromagnetic theorist a apparently very naughty man. Maxwell calculated that the speed of propagation of an electromagnetic field is approximately that of the speed of light. He proposed that the phenomenon of light is therefore an electromagnetic in origin. Because charges can oscillate with any frequency, Maxwell also concluded that visible light forms only a small part of the entire spectrum of possible electromagnetic radiation. (see post on frequency allocations 6/15)

The theoretical basis for the propagation of electromagnetic waves was first described in 1873 by James Maxwell in his paper to the Royal society in London A dynamical theory of the electromagnetic field, which followed his work between 1861 and 1865.

Maxwell had unified the work of previous electromagnetic and optical experiments reducing their results into a set of equations. These equations with his own work have become the basis for transmitting. he produced the actual laws of electromagnetism most importantly electromagnetic radiation. The equations are fundamental to the broadcast of radio and television.

bio here: http://www.phy.hr/~dpaar/fizicari/xmaxwell.html

He was also a notorious late-night carouser. When informed at Cambridge that cumpulsory church services began at 6:00 am he relpied "Aye, I suppose I could stay up that late." Spoken like a true radio man.