• Afu4BandXF-LPF-HF

Vorläufig / Preliminary, work in progress

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SW 4 Band Low Pass Filter

This web page documents the commercial Short Wave 4 Band 100 W Low Pass Filter, labeled XF-LPF-HF.

It is offered on www.ebay.de with the ID number 182097596845 for about 38 EUR. See the picture on the right.

Technical Data:

• HF LPF，3.5Mhz-30Mhz, 100W.
• Band: 80m 40m 20-17m 15-10m
• DC: 12 V
• LED for showing selected filter
• Version 1.1
• Size: 50 x 120 x 20 mm

When you click on a picture or diagram it will be shown in the original size for better reading.

Connections

• J1: RF input @ 50 Ohm

• J2: RF output @ 50 Ohm

• J3: 12 V, 0 V

• J4: Jumper to Pin 5: 0 V, Pin 1: 80m, Pin 2: 40m, Pin 3: 17-20m, Pin 4: 15.10m

The activated band will be shown with a lighting LED.

Filter Transmission Loss

The following diagrams show the Transmission Loss of the 4 Low Pass Filters. It was measure with YanaVNA-BT and visualized with program VNAJ.

The markers in the diagram are set to the following frequencies:

• Marker 1: Start of SW band

• Marker 2: Start of SW band * 2

• Marker 3: Start of SW band * 3

• Marker 4: Start of SW band * 5

if not otherwise set in the higher frequency range, because the used measurement device YanaVNA-BT does operate up to 65 MHz only.

For amateur use, FCC §97.307(d) requires that harmonics be suppressed at least 43 dB for operation below 30 MHz.

In a well designed solid-state push-pull short wave amplifier, the second harmonic is 30 to 40 dB below the fundamental (first harmonic) if the balance is good, but the third harmonic is only down by about 13 dB.

It can be seen, that the most dominant third harmonic is in all cases suppressed more than 40 dB.

The pass band loss is below 0.2 dB with the exception of the 10 m band (0.4 dB).

Standing Wave Ratio

The Standing Wave Ratio (SWR) was measured with YanaVNA-BT and a SWR bridge. In this case I had to photograph the display.

The first two diagrams on the right show the 80 m and 40 m band.

The marker Tune shows the start of the Amateur Radio band.

The markers L and R (Left and Right vertical green lines) usually show the Amateur Radio band limits.

Below 20 MHz the SWR is below 1:1.1, a Return Loss greater 21 dB, see ReturnLossBridge.

Above 20 MHz the SWR is below 1:1.2, a Return Loss greater 18 dB, see ReturnLossBridge.

The next three diagrams below show the 20 m, 15 m and 10 m band.

Band fault detection

In order to protect the Power Amplifier against Overvoltage and Overheating if a wrong short wave band was selected, or the RF output is not terminated, or the SWR of the antenna is very poor, two protection circuits can be added. In my case the SW band signals from my tranceiver are used for automatic switching.

Links

1. Vpp - dBm Calculator

2. dBm to Volt table

3. circuit analysis program LTspice, overview

4. Elsie, Win32 program for electrical filter design and network analysis

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-- RudolfReuter 2018-04-11 15:29:51

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