project also appears in the
2009 ARRL Handbook Chapter 12.
software available to radio amateurs at no cost
Low cost - most
parts available at hardware stores
with common hand tools
high power - uses low self inductance Teflon capacitors
Works on all amateur
bands from 1.8 MHz through 54 MHz.
transmitter low pass filter design project was started with
goals of low insertion loss, broad SWR bandwidth, mechanical
simplicity, easy construction, 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
test equipment is necessary for practical alignment. Primarily
intended for coverage of the six-meter band, this filter has
low insertion loss and presents excellent SWR characteristics
for all HF bands.
Although harmonic attenuation at low VHF frequencies near TV
channels 2, 3 and 4 does not compare to filters designed only
for HF operation, the use of this filter on HF is a bonus to
six-meter operators that also use the regular HF bands. Six-meter
operators may easily tune this filter for low insertion loss
and SWR in any favorite band segment, including the higher frequency
FM portion of the band.
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,
W4ENE of tonnesoftware.com 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.
Program documentation and example data files are included.
data filename for this filter is DC54.lct
the ELSIE data file
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 used a low pass
filter design with 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
or PSpice. The Elsie software has 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 adding an Elsie chart
into a document already using a standardized plotting format.
For most uses, the Elsie internal video screen and hard copy
printer outputs are fine.
--- 235.68 nH Wind with 1/8" OD soft copper tubing, 5 turns,
.75" dia form, 1.75 inches long, ¼ inch lead length for soldering.
L1,L3 --- 178.9 nH Wind with 1/8" OD soft copper tubing, 3.5
turns, .75" dia form, .625 inches long, ¼ inch lead length for
soldering to brass plate, other lead length to RF connector
C1,C2 --- 74.1pF 2" by 2.65" brass plate sandwiched with .03125"
thick Teflon sheet. The metal enclosure is the remaining grounded
terminal of this capacitor.
Design, Assembly, and Construction
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.
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 ¼ 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.
.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:
k=dielectric constant of Teflon®
A=surface area of one plate in square inches
d=thickness of insulator
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
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.
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.
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 go to Step Two.
Two: If you can't measure the 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 at
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.
ea Miniature brass strip, 1" wide, 12" length .032"
thick (variable tuning cap)
1 ea Miniature brass strip, 2" wide, 12" length .064"
thick (main filter capacitors)
5 feet length of 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, .062" 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, or "Nutsert"
1 ea 1" long x .375" dia. Nylon spacer. ID to be smaller
than .25".(used for variable capacitor plunger)
4 ea nylon spacer, .875" OD, .25 to .34" ID, approximately
.065" or greater thickness (used to attach brass capacitor
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".
.03125" thick Teflon sheet is available from McMaster-Carr
Supply Co. Item # 8545K21 is available as a 12" x 12"
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.
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
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.
filter measured performance
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,
W4ENE 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.
to Steve Hageman for
measuring the actual filter characteristics on an Agilent 8753C
single page detailed assembly drawing in Adobe format
The ARRL Handbook, 72 Edition (Newington:ARRL, 1995), pg 6.9.