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The Radiola 20 – A Restoration

The RCA Radiola 20

Part 1 – Who Are We?

As one might guess, I love radio. I grew up surrounded by vacuum tube equipment and I love old vacuum tube radios. When my friend, Todd, mentioned that a 1925 vintage RCA Radiola 20 was coming into his possession, and he wanted to restore it, I was more than a little interested. Todd had known this radio since childhood, but had never seen it in operation.

Todd is a broadcast engineer with decades of experience operating, repairing, installing, and upgrading AM and FM broadcast radio transmitters. Radio is in his blood.

From 1973 to 1978, I ran a Hammond Organ repair business in Los Angeles, which found me constantly working on vacuum tube equipment, sometimes dating back to the mid-1930s. The Radiola 20 is ten years older than that. Technology improved very quickly in those days. The tubes used in the Radiola in 1925 were already obsolete by 1931. Since the Radiola operated on primary batteries (dry cells), it was designed for minimum power consumption. This radio will not help warm your sitting room, it runs cold. There was much to be learned here and we were both very excited.

While we were waiting for the radio to ship and arrive, we did research on every aspect of the radio we could find. It took a lot of digging on the Internet and we assembled pretty much everything there is to know and every bit of documentation and literature ever published about the Radiola 20. We found the owners manual and setup guide, RCA’s schematics, technical notes, service notes, and troubleshooting guide for radio repairmen, plus more service notes published by Gernsback. We also drew a modern style schematic using KiCAD. When the radio arrived, Todd found it to be in near pristine condition, like new, and all the original paperwork that came with it was there, including the warranty card. This was a museum quality radio perfect for taking modern color photos of the internals.

Original warranty registration card, never sent in, and inspection tag.

With all this information we collected on our hands, we decided to share it here in one place for others to use. A blog is not the ideal medium for this but it has the advantage of stability and permanance. While researching this information, we often ran into items that had been published on personal web sites many years before that no longer existed, dead links, and photos that were no longer hosted. It’s sad when that happens. Publishing on Blogger and a private blog should avoid this problem for as long as possible. With luck, one of them will be captured in the Internet Archive.

Under the lid is the nameplate and battery connection chart.

Part 2 – The Radiola 20 is Special

Lots of different kinds of radios were being made in 1924, 1925. The RCA Radiola 20 is special because it was the first radio that was both reasonably priced and easy to use. Early radios were difficult to tune and wouldn’t stay on frequency. They required constant fiddling with the controls. At this time, the first superheterodyne radios were appearing. They were easy to use but were large, required a lot of parts, a lot of power, and were priced from $500 to $700 — the price of a new automobile. That’s $7,300 to $10,220 in today’s dollars. Only the wealthy could afford that kind of money for a radio. The Radiola 20 sold for about $100 or $1,460 in today’s dollars. Still, a sizeable purchase, but a lot less than ten grand.

The Radiola 20 is a major engineering achievement and results from the melding of several aspects of radio technology. To appreciate this radio requires a basic grasp of those technologies. This understanding does not require technical knowledge, nor math. Since I don’t know the reader’s knowledge of these things, I will delve briefly into all the background and history leading up to radio, without getting too technical. I’ll also discuss some of the fascinating personalities involved in the development of radio — the birthing ground and foundation of modern electronics.

View down into the radio with the top lid open before any work was done.

If you already know the history and different radio design types, you can skip the rest of this section and go to Part 3. If you don’t know, I will cover more detail here than is absolutely necessary, but it will give you a more complete picture.

What I find interesting is how quickly technology develops when consumer demand appears. This is easily seen in the development of radio. Since practical radio and the birth of modern electronics are really the same thing, and both depend on the vacuum tube, I’ll begin with that.

Vacuum tubes are based on a phenomenon called thermionic emission. When an object is heated to incandescence (glowing hot), electrons become free to move and easily leave the surface of the material. This effect becomes noticeable above 1,300 degrees Fahrenheit and was first noted by Bequerel in 1853. If you put an electric charge on an object in dry air, it will hold the charge for a long time. If you then heat the object to 1,300F, it loses the charge. Bequerel observed this but didn’t know why it happened.

Over the next 30 years, the thermionic effect was repeatedly forgotten and rediscovered by different researchers. It was rediscovered again by Edison in 1880 when he was trying to discover why his incandescent lamps seemed sensitive to polarity when run on direct current. When they failed, the positive end of the filament was darkened more than the negative end. The answer was that not all of the current was traveling through the entire filament. Some of the current was somehow traveling through the vacuum in the light bulb and striking the most positive end of the filament. He made special bulbs with an extra electrode inside to measure the effect and it’s named after him: the Edison Effect. He filed a patent for a voltage regulator that used the effect. Edison didn’t see any practical use for it, nor did he understand why it happened. By the way, this was the very first US patent for an electronic (not electrical) device.

Some experimentation with thermionic emission took place over the next 20 years, but not much. The next step was taken by Ambrose Fleming. Since the Edison Effect resulted in conduction in just one direction, a diode, he surmised this might be useful for the detection of radio waves. He was right. It worked better than a crystal, and vastly better than the mechanical cohering detectors used to detect spark transmitter signals in the early 1900s. He patented the diode in 1904. It consisted of a small heated filament, as in a light bulb, and a plate electrode. The device looked liked a small light bulb.

It’s interesting that thermionic emission was not understood until the advent of quantum physics. It was the subject of the 1928 Nobel Prize in physics. Even today, in 2018, physicists still argue over certain fine points that underlie thermionic emission.

At this time, around 1904, the only way to transmit radio waves was with high-voltage spark transmitters or large high-frequency mechanical alternators. These could only be used with Morse Code. There was no speech or music, no audio. Radio was used by the military, by ships at sea, and by radio amateurs (hams). Transmissions were noisy and broad so often only one transmitter in a region could operate at a time because of interference, and there was deliberate jamming. It was chaos.

The next step was a giant one. Lee De Forest was a great promoter of radio with a long and tumultuous career that began in the 1890s. In 1905 and 1906, he was desperately trying to come up with a radio detector that worked well and that didn’t run afoul of the multitude of patents that already existed. He was working with Fleming type diode tubes and wondered what would happen if he inserted a grid of wires between the cathode and anode, and connected the antenna signal to the grid. It worked and he was granted a patent for his invention in 1908. De Forest wasn’t much of a scientist and didn’t understand how it worked, but he had invented the first electronic amplifying device. A small voltage on the control grid could control a much larger current flowing from the filament to the plate electrode.

It is impossible to overstate the importance of this invention — a device capable of power gain. He called his invention the Audion. De Forest himself and many others greatly underestimated the importance of this invention. De Forest thought it might be useful for a few military applications. In fact, it became the fundamental device at the heart of radio, telephone, television, radar, sonar, computers, and countless devices of the electronic age until the transistor was invented in 1947. It took 25 more years, into the late 1960s, for the transistor to largely supplant the vacuum tube. Even today, some applications are best handled by vacuum tubes.

An important detail is that De Forest’s tubes were not made using a “hard” vacuum, but included a small amount of gas. Little attention was paid to impurities inside the glass envelope. Irving Langmuir surmised that many of the problems with triode tubes of the time, such as non-linearity, instability, and limited frequency response resulted from impurities in the envelope. His development of tubes with a hard vacuum inside and scrubbed of all impurities solved the problems. Fleming diodes could only handle low voltages. Langmuir’s could handle hundreds of thousands of volts. Langmuir’s triodes were linear, with higher gain, much higher frequency response, and could handle high voltage. The true vacuum tube was born. This work occurred around 1913 and triggered the rapid adoption of vacuum tubes in long-distance telephone amplifiers. The first transcontinental phone call happened in 1915.

Typical vacuum tube for this radio. There were a few radio and electronics stores but parts were often found in existing stores like this tire shop. Note the phone number: “24”.

Early radio (the electrical era of radio) had many fathers, including Maxwell, Hertz, Tesla, Marconi, De Forest, and others. Modern radio (the electronic era) had just one father, Edwin Armstrong. Every type of radio receiver in use today was invented by Armstrong. In 2018, one can say that every radio device you’ve ever used, from AM, FM, shortwave radios, to cellphones, garage door openers, or wireless thermometers, was either a superheterodyne or superregenerative design. Armstrong was a brilliant scientist and inventor.

While growing up, Armstrong had experimented with the flawed and gassy De Forest audion tubes and desired to gain a full scientific understanding of how they worked, which was unknown. By 1912 there was a basic scientific understanding of vacuum tubes, and it was around this time that Armstrong made his breakthrough invention of “regeneration”.

Audion tubes and early vacuum tubes were primitive and had low performance compared to later vacuum tubes. They had low gain (they amplified, but not by much) and had high interelectrode capacitance, which greatly limited performance. Armstrong discovered that using positive feedback resulted in stunning increases in gain. Instead of an amplifier stage producing a gain of 8 or 10, Armstrong obtained gains of 10,000 and more in a single stage, while using the mediocre tubes of the time. In 1913, Armstrong prepared demonstrations of his invention, scientific papers, and he applied for a patent. The patent issued in October of 1914.

In 1914, Armstrong was an undergraduate at Columbia University, studying electrical engineering. There, he presented the first scientific paper that fully characterized De Forest’s audion tube, complete with oscillographs showing the “characteristic curves”. It amazing that it took six years for someone to do that work. It’s also amazing that the manner in which Armstrong presented the data is the same way we illustrate device data for transistors today.

De Forest discounted Armstrong’s invention and filed a series of competing patents that essentially copied Armstrong’s claims, stating that he discovered regeneration first. Obviously, De Forest realized the importance of the invention. Competing claims were also filed by Alexander Meissner of Germany and Langmuir at General Electric. This was the beginning of many court battles that continued into the 1930s, with lawsuits, and countersuits, and two cases before the US Supreme Court. It seems that whenever there’s an important invention, this happens. In one of the early court cases, Armstrong and De Forest argued face to face. Armstrong easily demonstrated to the court that De Forest hadn’t the faintest idea how his audion tubes worked or how regeneration worked. Yet, the courts finally found in favor of De Forest, which today, is just stunning. The entire engineering community was shocked and appalled, but that’s how it ended up.

Another example of this kind of injustice was Marconi winning the patent battles and the title “Inventor of Radio”. He even won the 1909 Nobel Prize. Fortunately, this miscarriage of justice had a just but too-late outcome. It took 40 years for the courts to finally overturn the previous decisions in favor of Marconi and award inventorship to Nikola Tesla. After all, Tesla was using radio for remote control purposes when Marconi was but a child. However, it was too late and Tesla died penniless.

There are many more examples of such injustices. The takeaway is that the showman always wins. The person who wins is not the smarter person, but the person who is the better promoter, the better politician, the better financier, and the better people-person.

Armstrong’s invention of the regenerative radio was the right invention at the right time. One vacuum tube and a few parts, some of which could be made by hand, resulted in a high-performance radio. Anyone who was interested could afford to make one. What do I mean by high-performance? High-performance means a radio that can capture signals all the way down to the noise floor, the limit of what is possible. (There are modern digital techniques that can get quite a way below the noise floor using computers, but none of that was even dreamt of in 1914.)

What is the noise floor? As you make a radio receiver more and more sensitive, you run into an electrical noise limit that cannot be avoided. Natural and man-made electrical noise sets this limit. In the AM radio band, which is what we’re discussing, this noise floor is quite high. There are 15,000 lightning strokes on Earth every second, and each contributes to the noise. Cars, power lines, and electrical equipment radiate noise. Thermal noise in the antenna adds more. You can’t get around physics. The regenerative radio readily reaches this noise limit. You can’t do any better in terms of sensitivity no matter what kind of radio you design.

But, there are no regenerative radios made today, except by hobbyists for fun. This is because the regen has some major drawbacks. It takes skill to tune a regen. To tune in a signal, you have to operate two controls at once. Once it’s tuned, it’s unstable. Changes in temperature, drafts, wind moving the antenna wire, even your hands near the radio cause the tuning to drift. A regen needs constant readjustment. It was great for pioneer enthusiasts, but it’s not a good consumer product that can be set once and left to play for hours.

In addition to sensitivity, a radio must be selective. It must have a narrow enough bandwidth that you only receive the desired signal and not several others at the same time. If you’ve used a crystal radio in an urban environment with lots of radio stations, you know the problem. No matter what you do, you hear more than one station at the same time. If there is only one strong signal it’s okay. The regenerative radio achieves both high gain and high selectivity using almost no parts. This is great, but it has the instability problems mentioned above and other problems of a technical nature that I won’t get into here.

In 1916, Alexanderson patented the TRF radio (Tuned Radio Frequency). Without regeneration, tubes didn’t provide enough gain for a tuned circuit to be sufficiently selective to receive just one station. His idea was to cascade several stages of tuning and amplification — each feeding the next. This actually works well enough if you cascade five to seven such stages. The problem with the TRF is each stage must be independently tuned to the same frequency or you get almost no signal out the end of the chain. Tuning is extremely challenging and requires great patience. Tuning for a certain signal is a process of successive approximation. TRF radios were equipped with precision dials, so that the settings could be written down and found again later. The TRF works. It’s stable. Once tuned, it stays put. But it’s very user-unfriendly.

Many attempts were made to gang-tune the stages of a TRF. In other words, to drive all the tuning capacitors from a common shaft and a single knob. But it couldn’t be done. The mechanics and precise matching of the capacitors was impossible.

Part 3 – The Radiola 20

Which brings us to the Radiola 20. I haven’t explained the superheterodyne radio, also invented by Armstrong, because it’s not necessary for this story. All consumer AM and FM radios made since 1935 are superhets. The superhet solved all of the above problems, but required a lot of parts, a lot of power, and cost a fortune. In 1924, RCA wanted to come up with a radio that was consumer friendly and didn’t cost a fortune. Armstrong was on the RCA staff when the Radiola 20 was designed, so he surely had a hand in it. RCA came up with an efficient hybrid design that used only three stages of TRF plus a small amount of regeneration. Precision manufacturing of the tuning capacitors and mechanics enabled RCA to achieve usable gang-tuning of three stages of TRF using a single dial. This provided enough selectivity, but not enough gain, so the third TRF stage includes some regeneration. This trick was possible because they had the inventor of regeneration, Armstrong, along with his patents, on staff. Melding these two technologies was clever and effective.

At top are the three TRF tuning capacitors, gang-tuned by a single dial. Below are the three inductors that resonate with the capacitors above. The black panel across the middle contains the vacuum tube sockets. Construction of this radio is unusual, with just a steel frame supporting all the components, and point-to-point wiring.

The result is a radio using only four low-power X-99 tubes, that just sips battery power, that tunes frequency with a single dial. The second dial, called “Amplification” is the regeneration control. Since the tuning is TRF-style, the radio is stable. Tune it once and it plays all day. A fifth vacuum tube is an audio power amplifier that delivers nearly one watt of room-filling audio to the optional speaker. If you’re listening in headphones, you can unplug the fifth tube and save power. The Radiola 20 sold for $115 at introduction in 1925, dropping to $102.50. RCA sold 135,121 of these radios. That’s about $225 million in sales in 2018 dollars.

The Radiola 20 was a radio for the masses. It was affordable and anyone could learn to use it.

This article will be followed by others that include the information we gathered and more photos that may help someone who wants to own a piece of electronic history.

Another vacuum tube. Note the original sale date of December 26, 1936. This is one of the replacement tubes that were acquired because a couple of the original tubes in the radio had lost emission. The only option for replacement is used or rejuvenated tubes.

KW2P Beverage Antenna Designs

In a perfect world we’d have full-size 160m Beverage antennas fanning out like the spokes of a wheel from a centrally located shack, and the feedpoints would all be located near the shack. Most of us don’t have the necessary 80 acres of land so the feedpoints to our Beverages often end up far away and must be fed through long runs of coaxial cable.

For example, let’s say I want to install a unidirectional Beverage aimed northeast and the shack is located in the northeast corner of the property. The Beverage wire must extend 800 feet towards the southwest of the shack, the termination resistor must be located at the shack end and the feed is all the way at the southwest end. I have to run the 800 foot Beverage wire plus 800 feet of coax to bring the signal to the shack. You can’t do anything to change the geometry of this problem but I’ll show here how the coax can serve both as the feedline and the Beverage wire.

Sometimes we build reversible Beverage antennas that require long runs of coax plus distant relay boxes to perform the required switching. The coax is often buried, making it susceptible to physical damage, especially on farmland, and subject to contamination from constant exposure to moisture. Buried coax can be punctured by nearby lightning strikes. Locating the damage and making repairs can mean replacing the entire run of coax.

Some time ago I developed construction and feed methods for Beverage antennas that eliminate or at least reduce some of these problems. Recently I searched all the antenna reference materials I could find, assuming that someone else must have also developed the same or similar designs, but found nothing. So I’m publishing the designs here. Hopefully they’ll be of use to others.

There are three embodiments of these ideas discussed below. These designs make use of coaxial cable for both functions: both as the feedline and as the Beverage antenna element. These designs take explicit advantage of the fundamental characteristic of coaxial cable–that RF traveling on the outer skin of the shield has no bearing on or interaction with RF traveling inside the coax. The signal propagation modes that occur on the inside and outside of the coax are also completely different and propagate at different speeds. Due to the skin effect, the the RF currents traveling on the outer shield travel on the very outside surface and do not even appear on the inside surface of the shield.

Each of the designs are shown using 75 ohm coax. The designs can be adjusted for 50 ohm coax simply by changing the turns ratio of the matching transformers. CATV distribution cable such as RG-6-messenger is ideal for these designs because it includes a steel messenger wire running parallel to the coax and molded into a weatherproof jacket. The steel messenger wire allows the cable to be pulled to much higher tension than would be possible with coax alone, allowing longer spans between supports. RG-6 is also a non-contaminating coax so it will last a long time exposed to the weather or buried. It’s tempting to use RG-59 because it’s cheaper and it weighs about a third of what RG-6 weighs but it has a contaminating jacket.

The designs discussed focus on 160m but the same principles can be applied for any band.

CLASSICAL BEVERAGE ANTENNA

The first diagram shows a typical unidirectional Beverage antenna with the terminating resistor at one end and feedline at the opposite end. The feedline is connected through a transformer that transforms the impedance of the feedline to an impedance closer to that of the antenna to improve coupling and minimize loss.

Drawing 1, Classical Beverage Antenna.

Drawing 1, Classical Beverage Antenna. Click to view larger.

EMBODIMENT ONE: The basic idea.

The basic design concept is shown in the following diagram. The additional embodiments below use the same technique described here. The coaxial cable is suspended above the ground and the outer skin of the coax shield serves as the “wire” of a classical Beverage antenna. The tiny currents induced in the antenna wire (the outer shield of the coax) are referenced to earth ground and are presented to the 450 ohm primary of matching transformer T-1, exactly as in the classical Beverage shown above. T-1’s secondary connects to the shield and center conductor of the coaxial cable feedline, which happens to be the same coax that forms the active element of the antenna. The RF signal injected by T-1 propagates inside the coax to the opposite end (the terminating resistor end) of the antenna. Note that T-1 is an isolation transformer with two independent windings.

Drawing 2, KW2P Coax Beverage with End Feed.

Drawing 2, KW2P Coax Beverage with End Feed. Click to view larger.

At the terminating resistor end of the antenna, we are faced with the problem of extracting the signal we want, which is propagating inside the coax, while preventing the RF currents traveling on the outer surface of the coax shield from flowing beyond this point. This is a common problem in antennas that is solved by means of a balun (L1). However the problem in this case is bigger than we usually face with ham antennas. In the case of a Beverage antenna we are likely working at 1.8 MHz, which means the inductances required are large. We are also working with an impedance that is 10 times higher than what we normally work with so the required inductances are that much higher still. (Remember that the balun is concerned with blocking the outside surface currents at the 500 ohm impedance of the Beverage. The 75 or 50 ohm internal impedance of the coax is irrelevant as far as the balun is concerned.)

The rule of thumb for baluns is to present an inductive reactance that is 10 times the impedance we’re working with. For 50 ohm coax you aim at 500 ohms. In this case, the impedance of the Beverage wire is 500 ohms so we’d like to see the balun present 5000 ohms of reactance. At 1.8 MHz, this is a relatively huge amount of inductance–about 450 uH. However, working in our favor is the fact that losses at the terminating resistor end of a Beverage have somewhat less effect on signal output than losses at the feed end of the wire so we can fudge down on the 5000 ohm requirement and call it 2500 ohms. But even so, we are still looking at 225 uH. Suitable baluns are discussed at the end of the article.

EMBODIMENT TWO: Feed it anywhere.

It’s probably obvious to some readers that since the coaxial cable (in terms of the signal traveling on the inside) is untuned, its length does not matter. The coax does not have to continue for the full length of the Beverage antenna as shown above and the feedline can be brought off at any point. The advantages of this are clear. Instead of worrying about where the endpoints of the antenna are with respect to the shack, all the antenna has to do is pass nearby the shack and the feed is brought off at the nearest point. Several Beverages covering different directions can be installed and as long as they pass near the shack at some point the feedlines can all be very short.

Drawing 3, KW2P Coax Beverage with Middle Feed.

Drawing 3, KW2P Coax Beverage with Middle Feed. Click to view larger.

EMBODIMENT THREE: Reversible KW2P Beverage

This variation may also be obvious to some readers. Note that I have never built and tested this variation but I have no doubt that it would work fine. I’m hoping to find and acquire a piece of land large enough to try this out.

Reversible Beverages invariably have relay boxes at the far ends of the antenna to switch between feedline and terminating resistor in order to reverse the antenna pattern. The concepts shown above in embodiment two demonstrate bringing the feedline off at any point along the antenna’s length. The same method can be employed to bring the terminating resistor to certain points along the antenna or all the way to the opposite end. Directional switching can take place in a single box located at either end of the antenna or at certain points along the antenna’s length. Switching directions is simply a matter of swapping the feedline for the resistor at L1 and L2.

Drawing 4, KW2P Coax Beverage with Reversible Feed.

Drawing 4, KW2P Coax Beverage with Reversible Feed. Click to view larger.

Now comes a question: Note that in the first two embodiments, the length of the coaxial cable(s) did not matter. In this third embodiment, I assume that the lengths of coax are halfwave multiples (electrical length), taking into account the velocity factor of the coax (inside). The reason for the 1/2 wavelength multiples is to ensure that the resistance of the termination resistor is reflected accurately at the other end of the coax as a pure resistance. However, if the impedances of the Beverage wire / matching transformers / coaxial cables are all matched closely enough that SWR inside the coax is low, the lengths should not matter and it should not be necessary to hold to 1/2 wavelength multiples. This remains to be tested empirically. For now I show 1/2 wavelengths because I know it will work.

Lightning Survivability

One thing to consider when building Beverages is ease of construction and low cost of components like transformers and baluns because these components are frequently destroyed by lightning. A Beverage is a very long wire so lightning strikes hundreds of feet away can still induce plenty of current to vaporize baluns and transformers, puncture insulation, etc. Spark gaps at strategic locations are inexpensive, low-tech, and well worth the effort. Each support should be equipped with a ground rod and spark gap. Nothing will save you from a direct or very close hit but spark gaps will protect against most of the nearby hits. It is also a good idea to frequently inspect the antenna and spark gaps. A spark gap that was vaporized by yesterday’s storm won’t protect you today.

Matching transformers cannot be made lightning resistant but fortunately they are cheap and easy to make. If you’re going to make one, make several at a time. You’ll need them.

Suitable Baluns

There are several ways to build a suitable balun for this antenna. The type of balun needed in these designs is actually an unun (unbalanced-unbalanced), also called a “current balun”, or a Collins balun. What the balun is doing is it simply uses inductance to choke off or block the antenna currents traveling on the outside of the coax, keeping them “up on the antenna wire” and not traveling down the feedline into the shack.

Construction parameters to consider are:

1) Keep inter-turn capacitance as low as possible to keep the self-resonant frequency of the balun as high as possible so the antenna can be used on 80m and even 40m if desired. If operation on bands other than 160m is not planned then self-resonant frequency is not that important as long as f0 is well above 2.0 MHz.

2) Baluns can be and should be designed with lightning surges in mind. A direct hit near the balun will likely destroy it no matter how it’s constructed but most lightning surges will come from induced current from nearby strikes that a properly made balun can withstand. The more surge resistant you make the balun, the less often you will have to replace it. Making the balun more lightning resistant mainly consists of making the balun physically large and distributing the surge voltage across the whole balun so that it doesn’t arc over between turns. The best way to achieve this is with a long solenoid-shaped coil that keeps the first and last turns as far away from each other as possible and distributes the voltage evenly. This also minimizes inter-turn capacitance mentioned above.

The Numbers for Some Solenoid Baluns

Here are some numbers for suitable baluns

RG-6

Form Dia. Coil Len. Turns uH Coax Len Ft
4.5 in 56 in** 168 246 198
12 in 11 in 33 239 104
12 in 18 in 54 448 169
18 in 7.3 in 22 254 104
18 in 11 in 33 461 155

 

RG-8X, RG-59

Form
Dia.
Coil
Len.
Turns uH Coax
Len Ft
4.5 in 30 in 124 243 146
12 in 7 in 29 244 91
12 in 11 in 46 464 144
18 in 5 in 20 247 94
18 in 7 in 29 451 136

 

RG-58

Form
Dia.
Coil
Len.
Turns uH Coax
Len Ft
4.5 in 21 in 107 251 126
4.5 in 36 in 185 456 218
12 in 5.3 in 27 246 85
12 in 8 in 41 452 129
18 in 3.7 19 248 89
18 in 5.3 27 441 127

 

RG-174

Form
Dia.
Coil
Len.
Turns uH Coax
Len Ft
4.5 in 6.5 in 65 251 76
4.5 in 10.5 105 446 124
12 in 2.3 23 247 72
12 in 3.3 33 451 104
18 in 1.8 18 265 84
18 in 2.4 24 444 113
** A single layer of RG-6 on a 4.5 inch form is impractical. One way to cut down on coil length and reduce the amount of coax in the balun is to wind a 2-layer coil. A conventional 2-layer winding that runs to one end, then reverses back over the first layer gives a neat-looking result but is a bad idea. It brings the first turn very close to the last turn and defeats the main reason for winding a solenoid instead of a toroidal or Collins balun: high arc-over resistance. However there is a quasi-scramble way to wind these that keeps most of the benefits while cutting the length of the coil and the amount of wire required nearly in half for the same inductance. Wind two or three turns on the coil form, then cross back and wind two or three turns in a second layer over the first three turns. Then put three more turns on the form, cross back and put a second layer on those, and so on. It’s not as neat looking but it creates a 2-layer coil that keeps almost all the benefit of a single-layer coil.

Coax losses at 2.0 MHz are shown below. Values are for 150 feet and SWR of 1.5:1. Except for RG-174 miniature coax, losses at 2 MHz are so low they can be ignored.

 

Type Imp Loss
RG-6A/U 75 0.604 dB
RG-8X 50 0.631 dB
RG-58 50 0.788 dB
RG-59 75 0.678 dB
RG-59B 75 0.734 dB
RG-174 50 1.74 dB

 

Collins Baluns

A Collins balun is very easy to make and consumes about half the amount of coax as a solenoid. However, unlike a single-layer solenoid it offers no special resistance to arc-over from lightning.

To make a Collins balun, simply wind the coax lightly on a round form, all in one spot, then slide the coax off the form and tape or wire-tie it into a ring. Unless the coax is very limp and pliable, an extra pair of hands from a helper is useful for this step. A slightly tapered form can be handy for sliding the coax off. Small plastic wastebaskets or barrels often have a good shape.

30 turns of RG-6, 12 inches in diameter, yields about 450uH and consumes 94 feet of coax (in contrast to 170 feet for the same inductance on a solenoid)

Toroidal Baluns

Toroidal baluns can also be used with these antennas. These antennas are receive-only so there are no issues of saturation or power handling. The target inductance is 450 uH so we want to use the highest permeability ferrite that will handle the frequencies of interest. Good old Type 43 is probably the best choice. It has a permeability around 850+ and handles 160, 80, and 40 meter frequencies. An FT-240-43 core yields 450 uH with 20 turns. The problem is the window of this core is 1.4 inches in diameter so it will only accommodate 12 turns of RG-6 (0.332″ dia.). Stacking two cores reduces the required number of turns to 15, but that’s still too many. Since the balun is really just an inductor, and inductors in series add together, we can get even closer to the goal with two 225 uH baluns in series. This gets the number of turns required down to 13 on each core which is still more than the 12 turns that will fit, but close enough.

On the other hand, building the balun with RG-59 (0.242″ dia.) coax solves the window area problem because 19 turns fits easily through a 1.4 inch window. And only one core is needed instead of two, which may be a concern because FT-240-43 cores cost about 10 dollars apiece.

I hope these Beverage designs can be of use to other hams, make your installations easier to build, easier to maintain, cheaper, and more reliable.