by Andrei – YO6TJJ | last updated: 26th of Jan. 2024

This article describes a reproduction of a LPF design by George Dobbs G3RJV [1] with our practical implementation (enclosure, connections, inductors construction, measurements…etc.).

7th Order Chebyshev Low Pass Filter Set

The purpose of this low pass filter set is to be a 9 reference filters that can be easily used to test QRP to medium power PA amplifiers, in an ergonomic design, small-sized, lightweight and completely shielded, with a BNC-female standard connection on both ports. The idea is to use these filters plug-and-play in any experimental / test setup or design. Both the mcHF designs and G3RJV are using 7th order Chebyshev LPF architectures…

1. Details and schematic explanations

Such a design comprises 3 inductive elements and 4 capacitive elements as shown in Fig.1 below, with shunt first or series first (only for exemplification, values not used in our design):

Fig.1 – Marki Microwave’s online filter calculation tool extract, for a 7th Order Chebyshev Filter example

In Table 1 below i have compiled both tables from G3RJV (Table 1 and Table 4) and also i have rounded from 3 decimals to only 2 (enough needed, from my point of view, at least for inductance values and cutt-off frequency etc.). The most important parameters that we will be looking at during this document, using nanoVNA and its associated Windows software (nanoVNAsaver) are [2]:

S21 – forward transmission, often name S21 Gain
S11 – input reflection coefficient (mathematically related to SWR) [3]
SWR – on input
complex impedance on input (on Smith Chart Diagram)

Table 1 – Practical examples and simulated values from G3RJV measurements and results

All values below starting Table2 were measured using Keysight U1733C Handheld LCR Meter. These are real values compared with expected values from table T1 above.

Beware that styroflex polystyrene (PS) capacitors that are thermally stressed (during long term soldering) can be short circuited very easily, it has happened to me two times during this filter construction!

There are markers set on interest frequencies on measurements below (for Fc, for 2nd harmonic, for 3rd harmonic and for F @ -3dB and F @ -30dB, in Fig.3, 5, 7, 9, 11).

The nanoVNA tool was calibrated for each interval of frequencies, for accurate measurements. If you need assistance how to do this, for start, you can follow:

For this filter i have used T37-2, T37-6 and T50-2 genuine Micrometals iron powder rings that can easily stand 15…25W RF power from 3…30MHz.

For a complete list of Magnetic Materials from roWaves click below:

Using an industrial printer for metal-finish labels i tried to make a summary on the most important parameters on the label, so the filter can be easily identified during its usage. Also, inside, on the back side of the back-cover there is the schematic and the values of the 7th order filter passive components.

2. Results of the measurements for each band

160m band:

Table 2 – 160m 7th order low pass filter – measured components on digital handheld LCR meter

Fig. 2 – L2, L6 and L4 measured inductors on T50-2, for 160m band © YO6TJJ

Table 3 – 160m LPF 7th Order Chebyshev measurement results on nanoVNA

Fig. 3 – S21 gain and S11 measured results for 160m LPF on nanoVNA © YO6TJJ

80m band:

Fig. 2 – L2, L6 and L4 measured inductors on T50-2, for 160m band © YO6TJJ

Fig. 4 – L2, L6 and L4 measured inductors on T37-2, for 80m band © YO6TJJ

Table 5 – 80m LPF 7th Order Chebyshev measurement results on nanoVNA

Fig. 5 – S21 gain and S11 measured results for 80m LPF on nanoVNA © YO6TJJ

40m band:

Table 6 – 40m 7th order low pass filter values of components after measurements

Fig. 6 – L2 and L4 measured inductors on T37-6, for 40m band © YO6TJJ

Table 7 – 40m LPF 7th Order Chebyshev measurement results on nanoVNA

Fig. 7 – S21 gain and S11 measured results for 40m LPF on nanoVNA © YO6TJJ

30m band:

Table 8 – 30m 7th order low pass filter values of components after measurements

Fig. 8 – L2, L6 and L4 measured inductors on T37-6, for 30m band © YO6TJJ

Table 9 – 30m LPF 7th Order Chebyshev measurement results on nanoVNA

Fig. 9 – S21 gain and S11 measured results for 30m LPF on nanoVNA © YO6TJJ

20m band:

Table 10 – 20m 7th order low pass filter Chebyshev values of components after measurements

Fig. 10 – L2, L6 and L4 measured inductors on T37-6, for 20m band © YO6TJJ

Table 11 – 20m LPF 7th Order Chebyshev measurement results on nanoVNA

Fig. 11 – S21 gain and S11 measured results for 20m LPF on nanoVNA © YO6TJJ

17m band:

Table 12 – 17m 7th order low pass filter measurement results on nanoVNA:

Fig. 12 – L2, L6 and L4 measured inductors on T37-6, for 17m band © YO6TJJ

Table 13 – 17m 7th order low pass filter Chebyshev measurement results on nanoVNA:

Fig. 13 – S21 gain and S11 measured results for 17m LPF on nanoVNA © YO6TJJ

15m band:

Table 14 – 15m LPF 7th Order Chebyshev measurement results on nanoVNA:

Fig. 14 – L2, L6 and L4 measured inductors on T37-6, for 15m band © YO6TJJ

Table 15 – 15m 7th order low pass filter Chebyshev measurement results on nanoVNA:

Fig. 15 – S21 gain and S11 measured results for 15m LPF on nanoVNA © YO6TJJ

12m band:

Table 16 – 12m LPF 7th Order Chebyshev measurement results on nanoVNA. *Remarks for L4 below

Table 17 – 12m LPF 7th Order Chebyshev measurement results on nanoVNA:

10m band:

Table 18 – 10m 7th order low pass filter measurement results on nanoVNA

Table 19 – 10m LPF 7th Order Chebyshev measurement results on nanoVNA

3. Practical implementation details

For enclosures for thsi low pass filter set i used (TME) code 402.16 metal cases, rectangular shape + U, top and bottom lids. On those cases i mounted BNC-females panel mount connectors. All the GND connections were soldered directly to the side walls of the cases. General recommendations:

each polystyrene (PS) capacitor (styroflex) has been measured individually for proper selectionc(closest) from a lot usually of 10…20 units
the inductors have been wounded tightly and using the recommended wire sizes
component’s leads and all connections and terminals should be kept as short as possible
please clean the leads of capacitors, if you are using NOS old styroflex capacitors! (oxidation level will create problems during soldering and thus an increased insertion loss in the signal path due to poor contact)

Each unit was properly labeled using a professional Zebra printer for metal labels (PET MATT) . The most important parameters have been written down to labels , in order to have a future reference during testing purposes. Calculations and all measurements have been performed with nanoVNA, including all the plots listed in this article.

* Please note that the L4 no. of turns from Table 16 (12m band) is the updated one, it is different than the no. of turns of L4 from Table 1 (so 12 turns instead of 13 turns, as in original article)

This article will receive further updates very soon. A change history with updates will be visible throughout the article. Further improvements of this low pass filter set:

one of the BNC-female connectors will be changed to BNC-male, panel mounted so the filter can be easily attached on a BNC-female PCB connector, without the need of an extra coax cable (BNC-male-to-BNC-male) – this has been done for a single filter box as a test and the results are just great (i hope i can plan to migrate all the filter boxes with this type of panel mount male BNC male connector)
anti-slip self-adhesive bumper / feets (that will prevent also surface scratches onto the labels)
etc. (looking forward for your comments with improvements….)

vy 73 de Andrei – YO6TJJ

4. References

[1] A Complete Do-It-Yourself Kit with just a few simple calculations, https://www.gqrp.com/harmonic_filters.pdf
[2] Alan, W2AEW YouTube Channel, #315: How to use the NanoVNA to measure a low-pass filter
https://www.youtube.com/watch?v=F17mN5uuzGY&t=299s
[3] return loss Vs. VSWR, https://www.minicircuits.com/app/DG03-111.pdf

7th Order Low Pass Filter Set for 160 -10m (WARC included), 20W