A Balanced Balanced Antenna Tuner

By Richard L. Measures, AG6K

 

The published version of this article appeared in the February 1990 issue of QST Magazine. This is the pre copy-edited version, updated in May, 2003.

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JPG of a remote controlled, balanced L-network tuner built by Mike, WA1SEO.

Antenna tuners are like shovels. It takes more than one kind of shovel to be able to efficiently perform a variety of jobs. For example, a snow shovel isn't suitable for digging holes in hard ground. A tiling spade could be used to shovel snow, but it wouldn't be very efficient. Similarly, no single antenna tuner circuit can do everything extremely well.

A balanced load tuner should be designed, from the ground up, for the job that it is intended to perform. This article describes a circuit that does a good job of feeding an open-wire transmission line [ladderline]. It is not intended to be used for unbalanced loads such as a coaxial transmission line or for end-fed, Marconi or Hertz antennas - although it seems to be capable of doing so.

Now that we have nine amateur radio bands below 30MHz, an open-wire transmission line, center-fed wire, all-band antenna system looks even more attractive than it did when it first came into popular use in in the 1930s. In those days, we had only five bands below 30MHz. Taking advantage of this versatile antenna system requires a box that will interface the 50ohm unbalanced output of today's transceivers to the highly variable impedance [Z] of the balanced feed points of the all-band antenna .

Contemporary antenna tuner circuits claim to be able to operate into an unbalanced load or a balanced load such as ladderline. In actual use, most of the contemporary "matches everything, balanced or unbalanced" antenna tuner circuits produce a semi-balanced output when used with a balanced load. Although the antenna will radiate in this situation, a semi-balanced output is like having a semi-balanced checking account. It is less than wonderful.

A look at the diagram for the contemporary "matches everything" antenna tuner circuits reveals that they are usually unbalanced, high-pass filter characteristic, T-Network circuits with an add-on balancing device hooked to the output of the unbalanced tuner circuit. This is a compromise design which, not surprisingly, also has compromise performance when used with a balanced load.

The imbalance in these "balanced" tuners can be easily confirmed with a RF voltmeter or RF amperemeter(s). When the actual current or voltage is measured at each output terminal, the observed imbalance gets progressively worse above about 7MHz. At 28MHz, it is not uncommon to have 50 more current or voltage in one of the legs than in the other leg.

Some may ask: "why not use the balanced tuner design that was in vogue in the 1930s?" As many of you old-timers know, the 1930s-era balanced tuner consisted of a resonant or near-resonant, center link-coupled, tank-circuit with movable-clip taps on the secondary. For each band change, the clip taps had to be moved and re-optimized, the total inductance changed and the tuning capacitor re-tuned. Changing bands was a labor intensive job. These ancient tuners were seldom built in enclosures because near constant access to the clip taps and the inductor(s) was a necessity for changing frequency. It was a common practice to build these tuners on a breadboard for maximum accessibility. Hence the name breadboarding.

In the 1950s, the E. F. Johnson Company came out with their Matchbox® series of balanced antenna tuners. These tuners used the same center link coupling arrangement as the earlier tuners, but they eliminated the movable clip arrangement by using a double-differential capacitive voltage divider across the tank inductor. A differential capacitor is the RF-equivalent of a potentiometer / DC voltage-divider. This allowed the operator to electrically increase and decrease the voltage fed to the antenna, without changing taps. The Johnson circuit worked, but the Z-matching range was severely limited. Frequently, the SWR could not be reduced to a satisfactorily low level.

The balanced tuner that is described in this article has two, front panel adjustments, one optional, hi-Z / low-Z switch, and no clips. It uses the rarely seen, balanced version of the familiar, unbalanced, L-network. Changing bands is a piece of cake with this balanced tuner and the matching range can be made very wide by using enough L and C to handle the job.

The Trouble With Baluns

On paper, an unbalanced tuner, feeding a balun, connected to a ladder-line fed antenna should work well. In practice, it does not work well. The reason for this lies in the balun.

As a rule of thumb, a balun should have about 4 times as many reactive ohms as the resistive ohms of the load. This means that for use with a 600ohm balanced load, the balun should have a secondary winding reactance of about 2400ohm. For 80 meter operation, this works out to be more than 100µH of balun inductance! To create this much inductance on an appropriate MF/HF-rated [µ=40] ferrite core, an impracticably large number of turns of wire would be required.

The use of a balun, in a high-impedance circuit, inevitably creates two, very sticky problems: More turns means more ampere-turns of magnetic flux in the balun's core, and high magnetic flux densities can cause the ferrite-core to saturate. This distorts the RF waveform and creates harmonics. These harmonics extend well into the UHF TV band. The remaining problem with using many turns of wire is that doing so increases the winding-capacitance of the balun. The high capacitance of the winding creates unwanted reactance and/or balun imbalance. This is especially true with the commonly used 4 to 1 bifilar-wound balun, which does not have an evenly distributed winding capacitance like the trifilar-wound balun. When enough turns are placed on the 4 to 1, bifilar balun for satisfactory 80 meter operation, the inherent capacitive imbalance in the balun causes a progressively greater imbalance in the output voltage of the balun as the operating frequency increases. This imbalance causes a differential RF-current to flow through the ground wire on the tuner. The name "4 to 1 balun" is a misnomer. They are much better suited for broadband, unbalanced-to-unbalanced 4 to 1 transformer service such as would be needed in the input circuit for a grid-driven Class-AB1 amplifier, whose grid terminating resistance was 200ohm.

There is a problem with a substantial current flowing in the ground wire on any tuner: Since ALL conductors, no matter how wide, have inductive reactance, the RF current that flows through the ground wire or strap can develop a large RF-voltage on the tuner-end of the ground wire. With 1000W on 21MHz, 24MHz or 28MHz, the RF voltage on the "matches everything" tuner chassis can brilliantly light a neon-lamp, make sparks with a graphite pencil and burn fingers.

The 1 to 1, trifilar-wound balun solves the capacitive imbalance problem of the 4 to 1 balun. Unfortunately, it does not solve the problem of high capacitance in the windings themselves. And, more importantly, it does not solve the problem of core saturation due to the high magnetic flux-density created by the large number of turns required for any high-impedance balun.

The bottom-line is: high-impedance baluns are a very likely source of GRIEF no matter how carefully they are engineered and constructed.

All of these problems are easily avoided. The solution is simple: don't put the balun in the highest-impedance part of the circuit. Instead, put the balun in the lowest-impedance part of the circuit, and build a balanced L-network tuner for the balanced output of the low-impedance balun. So, why have we been putting the balun in the wrong part of the circuit for these many years? Good question. In most cases, the lowest-impedance part of the circuit is the 50ohm coax input to the antenna-tuner.

An Inexpensive, High-Performance, Ugly 50ohm-Balun

Building a no-grief 1.8MHz to 30MHz 50ohm-balun is easy. No costly ferrite-cores are needed, just a short length of 3 to 5 inch size plastic pipe, about 25 feet of 50ohm coax plus some nylon cable ties. Solid-dielectric coax is best for this application because foam-dielectric has a tendency to allow a change in the conductor to conductor spacing over a period of time if it is bent into a tight circle. This can eventually result in voltage breakdown of the internal insulation. The required length of the plastic pipe depends on the diameter and length of the coax used and the diameter of the pipe. For RG-213/U coax, about one foot of 5 inch size pipe is needed for a 1.8MHz to 30MHz balun. For 3.5MHz to 30MHz coverage, about 18 to 20 feet of coax is needed. This length of coax is also adequate for most applications on 1.8MHz. The number of turns is not critical because the inductance depends more on the length of the wire (coax) than on the number of turns, which will vary depending on the diameter of the plastic pipe that is used. The coax is single-layer close-wound on the plastic pipe. The first and last turns of the coax are secured to the plastic pipe with nylon cable ties passed through small holes drilled in the plastic pipe. The coil winding must not be placed against a conductor. The name of this simple but effective device is a choke-balun.

Some people build choke-baluns, without a plastic coil-form, by scramble-winding the coax into a coil and taping it together. The problem with scramble-winding is that the first and last turns of the coax may touch each other. This creates two complications. The distributed-capacitance of the balun is increased and the RF-lossy vinyl jacket of the coax is subjected to a high RF-voltage. The single-layer winding on the plastic coil-form construction method solves these problems since it divides the RF-voltage and capacitance evenly across each turn of the balun.

A more compact, less ugly, 1 to 1 impedance-ratio, 50ohm trifilar-wound (with wire) ferrite-core balun could also be used but there would be some tradeoffs. Ferrite cores are not cheap. Also, the air-core of the coax-balun can't saturate like the ferrite-core and, unlike ferrite-core wire-wound baluns, single-layer wound coax-baluns almost never have an insulation breakdown problem. Also, a trifilar-wound balun does not like to work into anything but a perfectly balanced load. With an imperfectly balanced load, the coax-balun will not, as does the trifilar balun, generate a differential, third RF-current on the outside of the coax that brings the RF to the input of the tuner. The choke-balun is not fussy. It will work as well into a less than perfectly balanced load as it will into a perfectly balanced load, and do so without the possibility of creating a differential RF-current on the station ground and fricasseeing the operator's fingers.

The Versatile L-Network

The L-network is THE basic RF-resistance or reactance plus resistance (impedance) [Z] transformation tool. It is also used to build high-pass, low-pass and band-pass filters. The imbalanced L-network forms the basis for all of the antenna tuners that are currently being sold. All contemporary amplifiers use two, or more, L-networks in series to match the high, anode (plate)-resistance of the amplifier-tube to the low-resistance of the coaxial output. Most amplifiers use (2) L-networks in series, which is more commonly known as a Pi ( Pi )-network. A few amplifiers use (3) L-networks in series, which is called a Pi -L-network. "No tune-up/broadband" amplifiers may use 5 (or more) fixed L-networks in series for each bandswitch position. More L-networks means more harmonic-suppression in the output of the amplifier. The complex-appearing Butterworth and Chebyshev filters are nothing more than a series of basic L-networks.

When used for matching resistive loads, an L-network consists of one capacitor and one inductor. When the L-network is used for matching loads that contain both resistance and reactance (Rohm +/- jXohm = Z), the reactance of the load may partially or sometimes completely replace one of the reactances in the L-network. Thus, in rare cases, it is possible to build an L-network with only one component, but only for a specific frequency and load Z. In some situations, cancelling the load reactance will require the use of a larger reactive component in the L-network. In more extreme situations, the load may be so reactive that the L-network must be made from two capacitors or two inductors!

There are four ways to connect the capacitor and the inductor in an L-network. See Figure 1A. When inductance is used for the series-reactance and capacitance is used for the shunt-reactance, the L-network acts as a low-pass filter as well as a resistance matching device. When capacitance is used for the series-reactance and inductance is used for the shunt-reactance, the L-network acts as a high-pass filter and a resistance-transformation device.

The resistance-matching range of the L-network is remarkably wide. It can match 50ohm to a 1ohm or to a 10Kohm load with ease and good efficiency provided that a reasonably high-Q inductor is used.

When the L-network is used for stepping up the input R, the shunt-reactance is placed across the load. For stepping down the input R, the shunt-reactance is placed across the input of the L-network. Another way to look at it is: The shunt-reactance is always connected across the highest-resistance side of the L-network. This means that, for a 50ohm input, wide-range tuner, which will match loads of more than 50ohm and less of than 50ohm, a step-up/step-down [Z] switch must be provided so that the shunt-reactance can be switched to the input side for <50ohm loads and to the output side for >50ohm loads. The same result could be accomplished by reversing the input and output connections. The switch saves time.

The T-network eliminates the need for the step-up/step-down switch by using a clever tool from AC circuit-analysis. This tool is based on the fact that, for every R-X series circuit, an equivalent R-X parallel-circuit may be calculated and substituted for the series-circuit. The equivalent circuit will act exactly the same as the original circuit. This also works in reverse.

See Figure 1, B. An additional capacitor or inductor is placed in series with the load of a resistance step-up L-network which will not normally match a load resistance that is lower than the input resistance (usually 50ohm). If the added series capacitor or inductor is adjusted so that it has a sufficiently high reactance, the resistive component of the R-X series load's parallel-equivalent circuit will be above 50ohm and the step-up L-network will be able to match the load.

For example: Given: a Z step-up, series-L / shunt-C {low-pass filter characteristic}, L-network, that will only match load resistances that are greater than 50ohms, is connected to a 1ohm load. Problem: Obtain a Z-match.

One solution is to add a +j10ohm (inductive) reactance in series with the 1ohm load { Z = 1ohm + j10ohm} This series RX circuit is electrically equivalent to a parallel-circuit of R=101ohm in parallel with an inductor whose reactance is plus j10.1ohm. Since a load resistance of 101ohm is above 50ohm, a match could be achieved if minus j10.1ohm is added to the L-network's shunt capacitor in order to cancel the parallel equivalent circuit's +j10.1ohm.

Adding more capacitive reactance to a variable capacitor is easy: simply adjust the capacitance to a lower value. This is clearly a case where less [C] gives more [ohms]!

The Balanced L-Network

The L-networks shown in figures 1A and 1B is designed to work with an unbalanced input and an unbalanced load. The balanced L-network is similar to an unbalanced L-network. The difference is that the balanced L-network's series-reactance is divided into two equal parts and both ends of the shunt-reactance must be well insulated from ground instead of one end being grounded. See Figure 1, C. The balanced L-network must be fed from a balanced source. The same formulas are used for either network.

Mechanical Considerations For Balanced L-Networks:

The tuning shafts of the variable-inductors and the variable-capacitor are hot with RF voltage and must be insulated from each other and from the outside world. Also, the insulated frame of the variable-capacitor should be kept well away from any grounded surface. This requirement is much easier to meet if the balanced tuner is built in a polyurethane-varnished plywood-box, instead of in a metal box. The (RF-hot) frame of a single-section variable capacitor should be elevated on standoff-insulators. This helps to keep the circuit capacitively balanced.

If a split-stator variable capacitor is used, it won't be necessary to insulate its tuning shaft for high-voltage RF, or to keep the capacitor well away from ground. However, an insulated tuning knob with a well recessed set-screw, instead of an all-metal tuning knob, might prevent an unpleasant surprise to the operator's fingers if the split-stator capacitor proves to be less than perfectly balanced.

The variable-inductors must have equal inductances and be driven in synchronization by one tuning-shaft. It is possible to end to end-couple the two variable-inductors with an insulated coupling that can handle minor, axial, shaft misalignment, but this does not result in good electrical symmetry or optimum inductive de-coupling between the two variable-inductors unless a shaft extension is used. Good symmetry is probably a moot point for 80 meter operation but it is a consideration on the higher-frequency bands. If you decide to use end-to-end coupling, there is a material, which is available in stores that carry drip-irrigation materials, which is ideal for end-coupling RF-components. The material is called clear (actually translucent) 1/4 inch size,polyethylene drip-tubing. It is semi-rigid and solves the axial shaft alignment problem and RF insulation problems nicely. It is a very-tight fit over 1/4 inch shafts. It can be held in place with 5/16 inch size flat spring-clamps.

Another method of coupling the variable-inductors is to use a 3/8-inch plastic timing-belt and two plastic timing-belt pulleys like the type used on xerographic copiers. This allows the variable-inductors to be placed side-by-side which results in better layout symmetry. One single-flange timing-pulley and one double-flange timing-pulley should be used so that the belt can not slip off. Small flats can be ground into the variable-inductor's tuning shafts so that the pulley set-screws will stay put.

The ends of variable-inductors that have a coil-end contact that is electrically connected to their tuning-shafts should be connected to the lower-voltage, input side, of the balanced tuner. This minimizes the RF-voltage stress on the insulated parts that synchronize and drive the variable-inductors. The roller-contact must be shorted, across the un-used turns of the inductor, to the low-voltage end of a variable-inductor. This is done to stop the Tesla-coil transformer-effect which can cause spectacular HV RF-arcing at some L settings. The courser-turns-pitch ends of variable-turns-pitch variable-inductors is placed at the (higher voltage) output side, of the tuner.

Sometimes it is more convenient to put the balanced L-network antenna tuner in a remote location so that the ladderline does not need to be brought through the wall of the house. A simplified diagram of a remote-controlled, permanent-magnet {reversible}, DC motor-driven tuner is shown in Figure 2. This simplified diagram does not show the detailed wiring of the control cable and the remote indicator/control-box/power supply.

Limitations: The balanced L-network that is illustrated is designed to work with balanced or semi-balanced loads that have a resistance greater than 50ohm. The vast majority of open-wire transmission-line fed antennas fit into this category. Rarely, it is possible to have a situation where an open-wire transmission-line fed antenna would have a resistance of less than 50ohm. This would be the case with a half-wavelength dipole, less than one-quarter wavelength above ground, that is fed with a transmission line that contains an even-number of quarter-wavelengths.

If a load resistance of less than 50ohm is to be successfully matched, the variable-capacitor must be switched to the input side of the variable-inductors, or, as in a T-network, a matched-pair of appropriate reactances can be inserted in series with the load to obtain a match.

Since the actual feedpoint Z at the bottom of a ladder-line fed, multi-band antenna, can be almost anything imaginable, it is probably prudent to include a DPDT Z-step-up / Z-step-down switch in the design of a balanced L-network antenna tuner.

Calculating The Series And Shunt -Reactances

The formulas for calculating the total series-reactance [Xser] {in +/- j ohms} and the shunt-reactance [Xsh] {in +/- j ohms} are shown in Figure 3. These ohmic values can be converted into actual values of capacitance and inductance for a specific frequency by using the two formulas at the bottom of Figure 3. Since most hams do not have access to an RF Z-Bridge, the formulas are not widely used.

The reactance formulas give exact values only for non-reactive, purely-resistive loads. If the reactances of the L-network are adjustable, a wide range of load reactances can be cancelled by adjusting the L-network to create an equal and opposite reactance. This is accomplished by tuning the L-network for zero reflected power while using a minimum power level.

The shunt and series-reactances, in ohms, that are found with the formulas can be either inductive or capacitive, but they must always be opposites for resistive loads. If a high-pass filter characteristic is desired, as is frequently the case on 160 meters where a strong, local, broadcast-station would otherwise cause receiver overload problems, the series-reactance is capacitive and the shunt-reactance is inductive.

If an L-network is to be used on just one band, only one of the two reactances usually needs to be variable in order to obtain a low-SWR over the entire band. A good use for this technique is with a half-wave antenna for the 160 meter or 80 meter band. This antenna would otherwise have a low-SWR at the middle of the band and a 5 to 1 SWR at the band edges. With a single variable component L-network tuner, the SWR will usually be less than 1.1 to 1 at the band edges. Of course, if both components are made variable, the SWR will not exceed 1 to 1.

If a band-pass filter characteristic is needed for a specific single-band operation, a high-pass L-network can be coupled to a low-pass L-network with each network contributing about 1/2 of the total resistance transformation. When this is done, only one of the 2-stage L-network's four reactances usually needs to made variable to allow covering the entire band with a low-SWR. I use this circuit in a 160m tuner. It keeps the local broadcast stations from overloading my receiver, and it attenuates the second harmonic, on 80m, to -63db.

Selecting a Suitable Peak Voltage-Rating For The Capacitor:

The RF-peak-voltage that appears across the capacitor varies widely depending on the feed-line Z and the power level used. The voltage can be calculated if the impedance and power at the feed-point are known. The formula for calculating this is E= [P x Z]0.5 . Since P is measured in RMS-watts, the AC voltage (E) result will be in RMS-volts which can be converted to peak-volts (e) by multiplying the result by 20.5 . For example, if Z=600ohm and P=1500W, e= [1500W x 600ohm] *20.5 =1341v peak.

When using a vacuum variable-capacitor, it is important to realize that the maximum RF peak-voltage rating at 30MHz is usually 60% of the rated DC / 60Hz peak-voltage rating. Thus, a "5000V" rating translates to a 3000 peak RF-volt rating.

Used or new-surplus vacuum-variable capacitors should be tested before use because they may have developed a small air-leak that renders them useless.

Constructing Ladder-Line:

Commercial open-wire transmission-lines seem to use #18 gauge copper wire exclusively. Although #18 gauge copper wire is completely satisfactory for use in transmission-lines that do not carry a high SWR, it is not adequate for the combination of high-power and very high-SWR that is the norm for multi-band-use antenna systems.

Number 14 gauge solid-copper wire is better suited for this application. High-quality, #14 gauge, 2-wire transmission-line does not seem to be commercially available so it must be constructed by the user.

For use in an open-wire transmission-line, solid-wire is better than stranded-wire because it will not twist as easily as stranded-wire and short-out. The exception is the top few inches of the transmission-line, at the center-insulator, which should be fine-stranded for flexibility. Copper-weld wire is not only dangerous to eyes when it is being handled and cut, it is too springy and unruly for use in a transmission-line. It also prematurely becomes brittle due to normal wind movement. Number 14 gauge, TW, single-conductor solid-copper house wire is commonly available in 500 foot rolls where building materials are sold. It affords a good compromise between strength and stiffness. The insulation can be removed by fastening one end of the wire to a stationary object and horizontally pulling a sharp knife between the copper and the insulation of the stretched out wire. TW wire can also be used for the antenna itself if some support is provided to hold up the center-insulator and transmission-line of the antenna.

High RF-quality, lightweight, long-lived, inexpensive and easy to fasten feedline insulators can be made from ABS thermoplastic. If you can find it, 3/8-inch ABS rod-stock or, as a second choice, 3/8 inch ABS square-stock works well. Rod-stock has the advantage of having less wind resistance. These materials can be found or at some of the larger plastic-supply houses. If you have a color choice, black is usually the most UV-resistant color. If you can't find the rod or square stock, the insulators can also be made from commonly available ABS plumbing pipe.

ABS-pipe must first be heated and flattened. This is how: Cut the ABS pipe length-ways into halves or thirds with a table-saw. The lengths should be about the same as the width of a Teflon-coated cookie-sheet that will fit into your oven. Since ABS is a thermoplastic that melts at 150ºC, the oven temperature should be set to 180ºC [350ºF]. The 1/2 or 1/3 round sections of the pipe are baked, concave-side down, on the cookie-sheet until they begin to soften. The oven is opened and a sheet of plywood, weighted with a brick, is placed on top of the pipe sections. When the ABS is flat, it is set aside to cool-down.

ABS sheet is cut, on a table-saw, into strips about 3/8-inch wide and 3 inches to 6 inches long. Or, the rod-stock is cut into appropriate lengths. The ends of these insulators are notched to a depth of 1/4 inch with a hack-saw or band-saw whose saw-cut width is less than the width of the #14 gauge solid copper wire. The lengths of the insulators is not at all critical since the characteristic Z of the ladder-line will change only slightly between 3 inches and 6 inches spacing. The main consideration is that the wires do not short-out.

The space between insulators is normally about 5 to 10 times their length. For example, 6 inch length insulators would normally be spaced about 30 to 60 inches apart. No more insulators need to be used than are necessary to prevent feedline twisting and shorting-out.

The ABS insulators can be securely fastened to the bare copper wires thusly: Clamp both parallel wires, spaced appropriately, into a vise. Stretch the wires out straight and fasten the other end of the wires to a post. With a flame from a propane-torch, heat the wire at the place where an ABS-insulator is to be fastened. When the wire is hot enough to melt the ABS, press the wire into the notch on the insulator. The heated wire will melt its way to the bottom of the notch in the ABS. The wire should be held in this position for about 15 seconds while the thermoplastic cools-down and re-hardens, which traps the wire. A damp rag can be used to speed up the cool-down time.

The useful life of ladder-line in windy areas can be extended if a Dacron-cord or braided Dacron fishing line tether {or tethers} is fastened to an insulator about half-way up the ladder-line. The tether is pulled sideways to form an angle of 45º and fastened to a stationary object. The tether prevents the feedline from whipping around in the wind which would otherwise eventually cause the wires to break. Two or three tethers, fastened to the same feedline insulator, spread about 120º apart work better than one.

Parts

A source of variable-capacitors and variable-inductors is Cardwell-Multronics Corp. (631) 957-7200

"http://www.cardwellcondenser.com/index.html"http://www.cardwellcondenser.com/index.html

They manufacture the old, E. F. Johnson line of 5 ampere [229 series] and 10 ampere [226 series] variable-inductors. Their P/N for an 18µH, 5a, variable-inductor is 229-0202-1. The 28µH version's P/N is 229-0203-1. These P/Ns are the same as E. F. Johnson's old P/Ns.

Another source for variable-inductors is occasionally Fair Radio Sales, 419 227 6573. Fair Radio also has a few sizes of variable capacitors and a ceramic, HV, RF-switch that is capable of switching the variable-capacitor on the balanced L-network between the output of the variable-inductors, for high-resistance loads, and the input of the variable-inductors, for low-resistance loads. The P/N of this switch is 3Z9626 and the price is $2.50.

MAINTENANCE NOTE: Variable-inductor roller-bars and other sliding RF-contact surfaces should be routinely wiped clean with a lint-free cloth and re-lubricated about once a year. A suitable lubricant is GC Electronics Tunerlub, "high-frequency lubricant", Catalog #26-01. This material must be applied thinly with a lint-free cloth. More is not better.

Source of cogged belts and pulleys: Small Parts, Inc.,13980 NW 58 Court, Miami Lakes, FL 33014, telephone [305] 751 0856

The part numbers [p/n] for the 40-tooth pulleys are TBPN - 40S [single flange] and TBPN - 40/D.[double flange]. At least one of the two pulleys should be double flanged to prevent the belt from slipping off during use. The outside diameter of 40-t pulleys is 2.78". For 30-tooth pulleys, change the number after the dash from '40' to '30'. The outside diameter of a 30-t pulley is 2.14". Either 30-t or 40-t pulleys may be used but the belt does not have to bend as sharply on 40-t tooth pulleys.

The belt length equals the desired distance between the two roller coil drive shafts x 2, plus the circumference of one pulley. The belt-contact-surface circumference of the 40-t pulleys is 40/5=8" and the circumference of the 30t pulleys is 30/5=6". Thus, if the coils will be mounted 5" apart [shaft-shaft] and 40-t pulleys are used, the belt-length needed would be 5" x 2 = 10" plus 8" = 18" [p/n TB6-180]. The belt-length part of the p/n is the 2nd and 3rd digits from the right. Belt lengths from 6" to 26" are available in 1" increments.

The price of an 18" belt is $5.50 in my old Catalogue #10. The price of 40-t pulleys is under $7.00 each. The price of 30-t pulleys is under $6.00 each. I assume that the prices have probably increased.

Acknowledgements:

Congrats to Charles, W7XC, who provided valuable input during the editing of this article.

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If you would like to discuss this article with me, I can usually be reached at 805 386 3734.