A Complete DoItYourself Kit ....... with just a few
simple calculations.
Although by their very nature, QRP transmitters radiate less power, the
output from such a transmitter does require adequate filtering. Usually
to keep the circuit compact, these transmitters have a final stage run
in Class C and being driven hard with RF power. Of itself, this is a recipe
for high harmonic output and a well designed low pass filter is essential.
For many years I have used low pass filters calculated from a series of
figures produced by Ed Wetherhold, W3NQN, (a G QRP Club member) and published
in two articles in the UK Short Wave Magazine in December 1983 and January
1984.
Ed Wetherhold has been the ARRL adviser on passive
filters for several years and published many fine articles on audio and
radio frequency passive filtering. I believe that the two articles in
Short Wave Magazine still represent the best source of information for
the design of good low pass filters for RF amplifiers.
The articles are comprehensive but here I just
want to share enough of the information to enable readers to build useful
filters to add to their home made transmitters. There is very little mathematics
 about 4 pushes of a calculator is the most required to produce information
for a buildable filter. I will also give a chart for “off the shelf”
low pass filters, which can handle up to 10 watts or RF power, suitable
for every HF amateur band.
The W3NQN designs are based upon a seven elements
: four capacitors and three inductors. They are designed for 50 ohms input
and output impedance and use standard capacitor values. This is very useful
because many calculations and computer programs for filter design give
very odd values of capacitance which have to be made up from series and
parallel values. Figure 1 shows a Seven Element Low Pass Filter. Now lets
look at some numbers.
Recommended Values
Table 1 is a very short extract from a large list
of filter parameters in the original W3NQN articles. I have taken the
practical values for the nine HF amateur bands which have given me the
best results over the years. Alongside each band are values for the seven
elements in the filters with values on pF for capacitors and uH for inductors.
The characteristics of each filter are described in terms of the ripple
cutoff frequency (Fco) and the frequencies of the 3dB (F  3dB) and
30dB (F  30dB) attenuation levels. The capacitors are all easy values.
I generally use polystyrene capacitors for my filter building.
The Inductors
The inductors are all wound on toroidal cores in
the popular Micrometals range. Translating the inductance value to practical
inductors is very simple. The formula is given to calculate the number
of turns. It does require knowledge of the inductance at 10 turns for
the required core. These values are given in Table 2. Again I have reduced
the W3NQN information to the 2 mix and 6 mix toroids, the ones that are
of most use for this application. The formula is easily executed with
a pocket calculator and the resultant figure is rounded to the nearest
complete number of turns. The wire gauge is not critical. Simply use the
gauge that will fit well on the core. The target is to wind an even coil
on the core to occupy about threequarters of the available space. If
the opposite ends of the winding are too close this will introduce extra
capacitance.
Power Levels
Table 3 shows the smallest core that may be used
for particular RF power levels. It is interesting because for transmitters
of 10 watts or less, T37 cores are suitable, making the filters very compact.
Also notice that larger cores are required for the lower frequency bands.
This again is an extract from the W3NQN data which used a very conservative
maximum AC flux density to determine the minimum core size. So use this
table to choose a core suitable for the required power handling of the
filter.
Practical Examples
Table 4 gives practical designs for a series of
low pass filters over the 9 HF amateur bands for transmitters of 10 watts
power output and less. The constructor simply has to read off the values
and make up the filters. All of these are filters that I have used to
good effect in the past. Should you require filters for use with higher
powers, take the information from the tables to choose a suitable core
and work out the appropriate number of turns for that core. A complete
DoItYourself filter design kit !
I keep a range of low pass filters in the shack,
each one mounted in a small tin, for testing purposes. So when playing
with transmitter circuits, I have a low pass filter I can put into use
for testing the output. The more frugal constructor could use such a set
of filters for several transmitters and not build filters into each of
them.
W3NQN 7 ELEMENT STANDARD
VALUE CAPACITOR LOW PASS FILTERS 

TABLE 1 : Recommended Values 
Band
MHz 
Fco
MHz 
F  3dB
MHz 
F30dB
MHz 
C1,7
pF 
C3,5
pF 
L2,6
uH 
L4
uH 
1.8 
2.16 
2.76 
4.0 
820 
2200 
4.442 
5.608 
3.5 
4.125 
5.11 
7.3 
470 
1200 
2.434 
3.012 
7.0 
7.36 
9.04 
12.9 
270 
680 
1.380

1.698

10.1 
10.37

11.62

15.8 
270 
560 
1.090 
1.257 
14.0 
14.40 
16.41 
22.5 
180 
390 
.773 
.904 
18.068 
18.93 
22.89 
32.3 
110 
270 
.548 
.668 
21.0 
21.55 
27.62 
39.9 
82 
220 
.444 
.561 
24.98 
25.24 
28.94 
39.8 
100 
220 
.438 
.515 
28  30 
31.66 
40.52 
58.5 
56 
150 
.303 
.382 
CALCULATING NUMBER OF TURNS REQUIRED ON A TOROID FOR
A GIVEN INDUCTANCE
N = 10 x SQUAREROOT ( L / L10)
N = Number turns
L = Required inductance
L10 = Inductance at 10 Turns
TABLE 2 : INDUCTANCE AT 10 TURNS
FOR MICROMETALS TOROIDS 



Inductance (uH) at 10 turns
 Core Size Prefixes  

Core Mix 
Colour 
T37 
T44 
T50 
T68 
T80 
Range
MHz 
 2 
Red 
.40 
.52 
.49 
.57 
.55 
17 
 6 
Yellow 
.30 
.42 
.40 
.47 
.45 
7 + 
Note:
1] Inductance values have a tolerance of 5% and are based upon a single
layer winding.
2] The core prefix gives the nominal outside core diameter in hundredth
of an inch
3] For example : a T372 core has a nominal outside diameter or 0.37 inches
and an inductance of 0.40uH at 10 turns
TABLE 3 : SMALLEST USABLE TOROIDAL
CORE FOR OUTPUT POWERS 



Designation of Smallest Usable Toroidal Core 


Power Level Range (Watts RMS) 
Core 
Colour 
<10 
1025 
2550 
50100 
100200 
 2 
Red 
T37 
T44 
T68 
T68 
T80 
 6 
Yellow 
T37 
T37 
T37 
T44 
T50 
TABLE 4 : Practical Examples for
Transmitters Under 10 watts RF Output 

Band
MHz 
C1,7
pF 
C3,5
pF 
L2,6
turns 
L4
turns 
Core 
Wire
swg 
1.8 
820 
2200 
30 
34 
T502 
30 
3.5 
470 
1200 
25 
27 
T372 
28 
7.0 
270 
680 
19 
21 
T376 
26 
10.1 
270 
560 
19 
20 
T376 
26 
14.0 
180 
390 
16 
17 
T376 
24 
18.068 
110 
270 
13 
15 
T376 
24 
21.0 
82 
220 
12 
14 
T376 
24 
24.98 
100 
220 
12 
13 
T376 
22 
28  30 
56 
150 
10 
11 
T376 
22 
Note :
Wire gauge is not critical.
Use size to comfortably fill the core about threequarters of full
circumference.
The number of turns has be rounded to the nearest whole number from
the calculated value. 
