A low pass filter design project was undertaken with the goal of low insertion loss, broad SWR bandwidth, and operation on all HF amateur bands including six-meters. This filter easily handles legal limit amateur power levels. It was originally built as an accessory filter for the 1500 Watt Six-meter Amplifier described on this site.The FCC requires good harmonic attenuation for VHF transmitters. This filter is useful in reducing harmonic radiation in the VHF and higher frequency bands, and is made at home with low cost commonly available parts. No complicated sweep frequency display test equipment is required for practical alignment.

1.8 to 54 MHz Low Pass Filter Housed in Die Cast Enclosure

Electrical Design

The use of low self-inductance capacitors with Teflon dielectric easily allows legal limit high power operation and aids in the ultimate stop band attenuation of this filter. Capacitors with essentially zero lead length will not introduce significant series inductance that upsets filter operation. This filter also uses an adjustable LC choke that greatly attenuates second harmonic frequencies of the six-meter band.

A suitable software tool to design this low pass filter is named Elsie. Jim Tonne, WB6BLD of Trinity Software has made ELSIE filter design software available in a student/demo version at no charge. The program is a professional design tool aimed at engineers/technicians involved in filter design/network analysis. The student/demo version is limited to 7 stages. This limitation does not affect the usefulness of this program for many amateur radio filter requirements. In addition, there is no time limit on how long this student version will remain active on your computer.

This program may be downloaded from his site.

(On this site, go to Software, and then select ELSIE.ZIP). Program documentation and example data files are included.

The data filename for this filter is DC54.lct. Download the ELSIE data file

The Elsie menu options and intuitive program design make it relatively easy to get started. The user has a choice of manual filter design or design assisted by the computer. I chose a basic low pass filter design using inductor input and having five poles. After making other filter choices like design frequency, the program can calculate all performance parameters and display the predicted filter response. You can use keyboard arrow keys to select an item, tune it, and immediately see the result. A variety of program options are available for fine-tuning the initial design to allow specific design goals to be realized. The data files may be exported into other applications like Touchstone, PSpice, Super Compact, or ARRL Radio Designer.

The Elsie software has many auxiliary tools that help in filter design. These tools run within the program, and do not require exiting the software and then restarting again. I found that my existing external scientific graphing software could take advantage of Elsie standard two-column format export option for all charts. This helps when generating a document with a standardized plotting format. For most uses, the Elsie internal video screen and hard copy printer outputs are fine.

Component Values

L2       235.68 nH    Wind with 1/8" O.D. soft copper tubing, 5 turns, .75" dia form, 1.15 in. long, 1/4 in. lead length for soldering.

L1 & L3   178.9 nH    Wind with 1/8" O.D. soft copper tubing, 3.5 turns, .75" dia form, .625 in. long, 1/4 inch lead length for soldering to brass plate, other lead length to RF connector as required.

C1 & C2     74.1 pF     2" by 2.65" brass plate sandwiched with .03125" thick Teflon sheet. The metal enclosure is the remaining grounded terminal of this capacitor.

Mechanical Design, Assembly, and Construction

Download the detailed mechanical drawing of this filter.

One design goal of this filter was easy tuning with modest home test equipment. To realize this, build the coils carefully according to the component values table. The homemade coils solder directly to the top surface of the brass capacitor plates. The capacitors are made using a brass to Teflon to aluminum case sandwich. An easy to make variable capacitor is made from two pieces of .032-inch thick brass plate and a Teflon insulator. The filter inductors are mounted at right angles to each other to help maintain good stop band attenuation.

One Elsie software tool will calculate the details of each inductor. Coils L1 and L3 are designed with a half turn winding. This allows short connections to the brass capacitor plate and the RF connectors mounted on the enclosure walls. The coils are physically spaced with 1/4 inch lead lengths, and then soldered to the brass plates.

Many of the parts required to make this filter are available at hardware stores. In particular, the one and two inch wide brass strips (sold as Hobby or Miniature brass), 1/8 inch diameter soft copper tubing, nuts, bolts, and nylon spacers and washers are commonly available at low cost. It is important that the specified .03125-inch thickness of Teflon be used since another size will result in a different capacitor value. If you have another Teflon thickness available, you will need to calculate the specific capacitor values depending upon the new thickness and brass plate sizes. The opaque white color Teflon used here has a dielectric constant of 2.1. The clear varieties of Teflon typically have values less than this, and will result in different capacitor values for the same size brass plates.

The capacitance will decrease if the assembly bolts are loose, so be sure to have the bolts tightened. Make sure to use the .064 inch thick brass plate for the bolted down capacitors. When under compression, the thinner brass size used for the variable capacitor tends to flex more and doesn't fit as flat to the Teflon.

A separate Teflon sheet is also used in the variable capacitor, and is glued to the stationary vertical capacitor plate. This insulator is used to prevent a short circuit in case the tuning screw is tightened too much. Teflon is extremely slick, and doesn't glue well unless chemically prepared. One way to get acceptable glue joint performance between the brass support plate and the insulator is to scuff the Teflon and brass surfaces very well with 240 grit sandpaper. The intention is to increase the available surface area as much as possible, and provide more places for the glue to fasten to. Glue the Teflon in place with a bead of RTV or epoxy. After drying, the Teflon sheet can be intentionally peeled from the brass plate, but it appears to hold reasonably well. Special Teflon that has been treated to allow good adhesion is available, but the expense isn't justified for this simple application. This Teflon variable capacitor insulator sheet measures 1.5 inches wide by 1.75 inches tall and is larger than the two brass plates. This gives an outside edge insulation safety margin.

Calculating the capacitance of the plates.

The .064-inch thick brass capacitor plates have two .5-inch holes in them for the mounting bolts. The surface area of each hole is PI R squared, so the two holes combined have a total surface area of .3925 square inches. The brass plate size is 2 inches by 2.65 inches. This equals 5.3 square inches of surface area. Subtracting the area of the two holes gives a total surface area of 4.9 square inches. The formula for capacitance¹ is:

Where:
C = capacitance in pF
k = dielectric constant of Teflon®
A = surface area of one plate in square inches
d = thickness of insulator

The dielectric constant of the Teflon used here is 2.1, and the thickness used is .03125 inches. The calculated capacitance of each plate equals 74.1 pF. Measured values agree closely with this number. When built as described, the capacitor plates measured between 2% and 2.5% of the calculated value. This is acceptable for a practical filter. The brass sheet material acts like a large heat sink, so an adequate soldering iron is required. A large chisel point 125-watt iron will work well. The soldering heat does not affect the Teflon material. However, beware of the temptation to use a small propane torch. Two bolts in each capacitor hold the Teflon sheet and brass plates firmly together. The bolts are insulated from the brass plates by nylon spacers the same thickness as the brass. The nylon plunger for the tuning capacitor needs to be drilled and tapped to accept the 1/4 x 20 thread of the adjustment bolt. A threaded insert or PEM nut in the enclosure provides support for the tuning screw.

Tuning Capacitor and Input SWR Adjustment

The small variable capacitor is shunted across coil # two. This coil and capacitor combination acts like a tunable trap for second harmonic frequencies when operating in the six-meter band. After soldering into place, the flexible tuning plate of this capacitor is simply bent towards the adjustment screw. Brass of this thickness has a definite spring effect. Just bend the plate well towards the tuning screw, and then tighten the tuning bolt inward. This will result in a stable variable capacitor.

Six-meter alignment procedure.

If you are not concerned with six-meter operation, ignore this procedure. Simply set the variable capacitor plates .1-inches apart and disregard the following steps. If you wish to use this filter on the HF amateur bands from 1.8 to 30 MHz only, the adjustable tuning capacitor adjustment is not critical at all, and does not affect HF SWR performance. However, don't eliminate the capacitor entirely. The software predicts degraded VHF response with it missing. For use on the HF bands only, the tuning screw and associated nylon plunger may be omitted. Normally, tuning this filter would be an aggravating experience since three variables (with two interacting) are involved (L1, L2, and the variable capacitor). I realized that the Elsie software "Tune" mode held the answer. After studying what the software predicted, I generated this tuning procedure. My very first attempt to exactly tune this filter was successful, and was completed in just a few minutes. This method was predicted by software and then confirmed in practice. A common variable SWR analyzer is required. These steps may seem complicated, but are actually pretty straight forward once you get a feel for it. Read first before you start adjusting.

Step One: After the filter is constructed, adjust the variable capacitor until the top plate spacing is about .1 inches apart. Using a variable SWR analyzer, sweep the six-meter band area, searching for a very low SWR null anywhere in the vicinity of about 45 to 60 MHz or so. If a low SWR value (near 1:1 ratio) can be found, even though the frequency of the low SWR isn't where you want it, proceed to Step Two. Otherwise, adjust the input coil L1 by expanding or compressing the turns until a low SWR can be obtained anywhere in the range of about 45 to 60 MHz . Then, if you have a way to measure the notch response at 100.2 MHz, proceed to Step Two. Otherwise, proceed to Step Three.

Step Two: If you do not have a method to measure notch response at 100.2 MHz, proceed to Step Three. Now apply 100.2 MHz to the filter input. Adjust the variable capacitor until the six meter second harmonic at 100.2 MHz is nulled on the filter output. Then, hook up the SWR analyzer again, and sweep the six-meter band with the SWR analyzer. If the low SWR location is too low in frequency for you, adjust middle coil L2 for less inductance (expand turns apart), and then readjust the variable capacitor to bring the notch back on frequency. Continue these iterations until the SWR null is where you want, and the notch frequency is correctly set.

Alternately, if the desired SWR low spot is too high in frequency for you, adjust L2 for more inductance (compress the coil turns), and then readjust the variable capacitor for the second harmonic notch. Continue this until both the low SWR frequency location and the notch null are set where you want. You may need to unsolder one end of coil L2 to allow the adjustment for a longer or shorter coil length as you expand or compress turns. Just solder the end again after you make your length correction.

Note that you will probably need to install the enclosure lid during the very final tuning steps. I was able to reduce the second harmonic into the noise floor of an IFR-1200S spectrum display, but the lid needed to be installed. The lid also interacts some with the variable capacitor. Once the SWR and the notch frequency are set, the tuning process is complete and the filter is optimally adjusted. Do not perform Step Three below.

Step Three: This step is only performed if you don't have a way to generate the 100.2 MHz input signal, and then detect a null on the filter's output terminal. The variable capacitor will become your SWR adjustment to move the SWR null spot to the portion of the six-meter band you desire. If you run out of adjustment range on the variable capacitor (turned all the way in), just compress the L2 coil turns together, and try again. Alternately, if the variable capacitor is backed completely off, just expand the L2 coil turns, and try again. After your SWR is set, you are finished. Although the second harmonic notch probably isn't exactly on frequency, you will still have good (but not optimum) suppression since the notch is very deep.

Parts list

1 ea Miniature brass strip, 2" wide, 12" length .032" thick (variable tuning cap)
1 ea Miniature brass strip, 2" wide, 12" length .064" thick (main filter caps)
5 feet 1/8" diameter soft copper tubing
4 ea ¼ x 20 x ½ inch long hex head bolt
4 ea nylon washer, .5" OD, .25" ID, .0625" thick; Mouser 561-D2562 or equivalent
6 ea ¼ x 20 hex nut with integral tooth lock washer
1 ea ¼ x 20 x 4" long bolt
1 ea ¼ x 20 threaded nut insert, PEM nut, "Nutsert" or similar
1 ea 1" long x .375" dia. Nylon spacer. ID to be smaller than .25" (variable cap plunger)
4 ea nylon spacer, .875" OD, .25" to .34" ID, approx. .065" or more thick (to atch brass cap plates)


Aluminum die cast enclosure is available from Jameco Electronics, as their part number 11973. The box dimensions are 7.5" x 4.3" x 2.4".


The .03125" thick Teflon sheet is available from McMaster-Carr Supply Co., Item # 8545K21, and is available as a 12" x 12" sheet.

Performance Graphs Discussion

Assuming the six-meter SWR is set to a low value for a favorite part of the band, the worst case calculated forward filter loss is about .18 dB. The forward loss is better in the HF bands, with a calculated loss of only .05 dB from 1.8 through 30 MHz.

The filter cutoff frequency is about 56 MHz, and the filter response drops sharply above this. There are parasitic capacitors on coils L1 and L3. These are also included in this filter analysis. The calculated self-capacity of each coil is almost one pF even. These small capacitors are included on the schematic and are also included in the software for the model. These capacitors occur naturally, so do not solder a one-pF capacitor across each of the end coils in this filter. The capacitors have the effect of placing additional notches somewhere in the UHF region. The calculated self-resonant frequency of L1 and L3 is about 365 MHz.

Refer to the graph showing calculated filter response from 1 to 1000 MHz. The impressive notch in the 365 MHz vicinity is because of these inherent stray capacitances across each of the coils. Slight variations in each coil will make slightly different tuned traps. This will introduce a stagger-tuned effect that results in a broader notch width. These exact capacitance values are hard to predict because of variations in home made coil dimensions and exact placement of each coil inside the enclosure. The best way to determine their effect is to physically measure the UHF response of this filter. Using low self-inductance capacitors in a VHF filter helps to take advantage of predicted filter attenuation at extended frequencies.

The SWR across the HF bands and six-meters is shown in the graph showing SWR response from 1 to 55 MHz. A more detailed graph showing only the six-meter band SWR is also shown.

Calculated return loss of the filter across 1 to 200 MHz is shown in a separate graph. Notice that the ten-meter region has particularly good return loss. Component values in this filter were adjusted so that this return loss spike was moved from about 40 MHz to the vicinity of the 28-30 MHz amateur band.

Performance Graphs

Conclusion

This filter meets the original design objectives. Since I use six-meters as well as the regular HF bands, this project has produced a doubly useful station accessory. Low insertion loss on six-meters makes this filter useful for receiving applications also. The ELSIE filter software tool made the electrical design portion of this project fun. Thanks to Jim Tonne, WB6BLD for the Elsie design software and for his informal consultation and helpful comments about this filter. Jim suggested this filter topology and offered component values to consider.

Notes

1. The ARRL Handbook, 72 Edition (Newington:ARRL, 1995), pg 6.9.

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