Ferrite Antennas for the Ultra-Light Radio

As sensitive as the Sony SRF-59 is without an external antenna, it can benefit immensely from the boost received from an inductively or magnetically coupled external loop antenna. In this paper I will describe the construction and use of a compact external loop stick antenna that can be used “stand alone” with an Ultra-light or which can also be used to couple a random wire antenna to the Ultra-light.

Using Owen Pool’s “decibels for dummies” method of comparing power ratios of sound heard by the ear, my experience in using the external tuner compared with the barefoot SRF59 on a weak daytime signal from Miami FL, 450 miles from my home in Savannah, Georgia, the gain is somewhere between 8 and 10 dB. (see Owen’s article at the end of this paper) Compared with my Quantum Loop, an amplified loop made by Gerry Thomas of RadioPlus, the Quantum Loop is superior. With the axis of the tuner and radio’s ferrites aligned or parallel, the use of my external loop stick does not seem to lessen the null capability of the SRF59 which is excellent. I have not attempted to tilt the turntable to take advantage of differences in signal polarization.

As a crystal radio fanatic, I had on hand the following components to make the device:

4” x .5” ferrite rod Bytemark part no. R-050400-61 http://www.bytemark.com/
12 feet of 165/46 litz Wire from Kerrigan-Lewis Wire Co. of Chicago (also from Dave Schmarder) K-E no website. Phone: (773) 772-7208 Dave Schmarder: http://www.1n34a.com/catalog/
Single gang Jackson Brothers mini-air variable capacitor 17-390 pF Bill Turner: http://www.dialcover.com/components.html
Radio Shack plastic enclosure box 2”x5”x2.5”
Extras: Masking tape, hot glue gun Radio Shack banana clip jack. Thin card stock.

After first wrapping the ferrite rod with two layers of thin card stock and securing it with masking tape, I wrapped the center two inches of the rod with a close wound coil of the litz wire, securing the wire with masking tape and then covering the wire with masking tape to shield it from hot glue used in securing the rod to the enclosure. I didn’t count the actual number of turns, but this size litz is approximately 35 turns per inch with the total number of turns approximately 70. The wire is centered on the rod, covering two inches. This yields ample inductance to match the capacitor so that I was able to tune the external tuner to 530 kHz with a little room to spare. Upper range is no problem. I first soldered one end of the litz wire to the stator terminal of the air variable and the other end to the frame (rotor) of the air variable capacitor. An additional wire was soldered to the stator and soldered to the banana clip jack for connection of optional random wire antenna. As blind luck would have it, the project box was just barely wide enough between the internal screw posts to accommodate the full length of the ferrite rod. It actually wedges slightly between them. I hot glued the rod into the box for further security.


Performance: On weak signals, the use of the external loopstick makes the difference between barely audible, undecipherable signals and clearly audible, decipherable signals on the SRF59. In using the tuner, the peak is very sharp and optimum performance is gained by readjusting on each frequency. Just before it peaks, the signal will dampen as the tuner is acting as a notch filter.

A random wire further increases performance but with loss of directivity sacrificing the ability to null pests. In the below photos, my SRF59 sits atop a plastic lazy susan turntable from Bed Bath and Beyond. I hot glued a water bottle cap to the dial of the radio and then hot glued the water bottle cap to a larger plastic jar lid. The water bottle cap provided clearance from the components. Pencil marks indicate observed frequencies along the larger dial’s paper insert. Do this during the day so that bandwidth of distant signals is narrowest. I indexed marks against the edge of the plastic enclosure. The larger dial is easier to use than the standard dial, offering vernier-like performance. Remember, this is a $15 radio so I didn’t feel like a vandal doing this mod..




Components: The components used in my project yield higher Q than one would observe with solid wire and a lesser quality air variable capacitor. Crystal Radio nuts (including me) have extensively tested components on HP 4246A Q meters. The combination of the Jackson Brothers air variable capacitor and Bytemark ferrite rod with 165/46 litz wire has yielded raw Q readings of 800-1000 Q across the MW range. Despite the inherent lossiness of cardboard, the Q is higher with the slight separation of wire from the ferrite rod than if the wire had been wrapped directly on the rod. The tiny air variable has low loss ceramic standoffs in contrast with the usual lossy phenolic insulators in most air variables.

Solid wire and lesser avc’s rarely top 200-300 Q. However, the Q of my external loopstick will not yield the same Q as was tested in conditions with the components suspended on Styrofoam blocks above the Q meter with no hand contact during the readings.

Is high Q important? Some don’t like extremely high Q in an antenna tuner because it yields narrow peaks and can sometimes clip sidebands. I have not noticed this affect in my tuner. What I have observed in litz air coils versus solid wire coils is that the magnetic coupling field is stronger and wider in the Litz.
I can only assume that would be true in a quality ferrite rod/litz combination. Even the tiny internal ferrite rod in the SRF59 is wound with low count litz wire. It does make a difference, but the cost is not cheap. Many people I know buy minimum orders of litz wire from Kerrigan-Lewis in Chicago. The wire is special ordered and made up per order.

Crystal radio experimenters have determined that #46 litz wire performs best in the MW range. This wire is smaller in diameter than a human hair. 165/46 litz contains 165 strands, each insulated by an enamel coating. Dave Schmarder sells litz on his website in smaller quantities than the approximate $200 minimum order of 2 lbs. of wire required by Kerrigan Lewis. Dave currently offers 100 feet of 165/46 litz for $38. One important caveat with litz is that it should be heat-strippable with a solder blob. If it is not heat-strippable, it is difficult to strip the surface of the insulating enamel. There is also a range of quality with ferrite rods. Mix 61 is the preferred mix.
Quality differences are the result of annealing technique. New stock ferrites are hit and miss. The Russian ferrite rods available from Eastern Europe have uneven quality.
The Bytemark ferrite rod ($5) specified above is of good quality. There are superior ferrites, but these are usually NOS and difficult to find. My external loopstick is a simple unamplified L/C circuit. In such a circuit, resulting Q is always lower than the Q of the worst component in the circuit. In other words, if the Q of the air variable is lower than the Q of the inductor, the total Q will be less than the Q of the air variable, and vice versa.
External ferrite loops made from components other than what I used will perform very well. Gerry Thomas of RadioPlus has corresponded with me and stated that the polyvaricon variable capacitors he uses offer high Q. He would know. He uses them on his excellent products, including his Q Stick. He also uses litz wire and quality ferrites. The little English made Jackson Brothers mini-air variable is still made by Jackson Brothers, but because of the weaker dollar it is no longer available for $10. I have seen them available in limited quantity on Bill Turner’s website for $25 delivered.

From Owen Pool’s Crystal Radio Resources website: http://bellsouthpwp2.net/w/u/wuggy/testing.htm

Decibels for Dummies (how to sound like a tekkie) “The ear is a marvelous device which can process a wide range of noise levels. At a frequency of 1000 Hz, the weakest sound detectable is at a power of about 1 picowatt per square meter, normal speech is about at 1 microwatt, hearing damage starts at about 1 milliwatt (smoky bar with a loud band), and physical pain is felt at 1 watt. Since the ear doesn't respond to absolute power changes in a linear manner, and to cut this trillion to one ratio down to size, we use a logarithmic scale instead. The common logarithm of a number is the exponent or the power to which 10 must be raised in order to obtain the given number. E.g. the logarithm of 100 (10 to the second power) is 2, of 1000 is 3, and so on. Power ratios can be expressed as a logarithm, known as a Bel (after Alexander Graham), but more commonly as a deciBel, obtained by multiplying the logarithm of the power ratio by 10. This gives us a range of hearing loudness from 0 to 120 decibels, relative to the threshhold of hearing (1 picowatt per square meter), a much easier, and more useful set of numbers to bandy about. When working with your crystal set, you normally want to know how much better (or worse) a change to it is, and can express it in deciBels (dB). Say that you are taking voltage measurements across a resistor as I do. If the voltage goes from 20 to 40 milliVolts, the ratio of voltages is 2; but wait. Power varies as the square of voltage, so the power ratio is 2 squared, or 4. The logarithm of 4 is 0.6, and 10 times that is 6, or 6 dB. Incidentally, if the measured voltage had gone down from 20 to 10, you would have gone down by 6 dB - it works in both directions. So, all you have to do is divide the larger voltage (or current) by the smaller, square the result (this is your power ratio), and multiply the logarithm of that number by 10 to get the change in dB. Round your answer to the nearest whole number. If this seems like too much work, here is a short table of common power ratios, the corresponding dB change, and what it means to your ear:



Power ratio dB Effect
2 3 barely detectable change
4 6 noticeable
5 7 a little better than 6
10 10 oh yeah
20 13 a smidgen better than 10
100 20 oh yeah again -
200 23 (just to show you the effect of multiplying two numbers by adding their logs)
note: to the ear, an increase of 10 dB, followed by an increase to 100 dB above the original power level (another 10 dB increase) would be sensed as two equal step increases in loudness. Similarly, two 3 dB steps would be sensed as "equal" changes in loudness. “