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.