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Low Pass Filter/Dual Directional Detector

Update: There is a newer version of this board available now that will also cover 6 meters. You can jump to the setup table and schematic for this newer version by clicking the bookmark here:

I thought it might be nice to save some space and simplify construction of the kilowatt 70cm amplifier, so I designed this little critter. I got lucky, it also works quite well on 144 and 222MHz.

This 3-in-1 assembly filters the harmonics from the amplifier, and feeds the forward and reflected power signals to the control board and metering circuits. It can be configured for any one of the three bands mentioned by selecting the correct component values from the setup chart at the end of this description. Components for the 2m version are shown in this photo. A high resolution photo is here.

This photo shows an earlier prototype of this board (70cm), and how I mounted it into the amplifier

On 70cm, the capacitors in the filter are printed right onto the board substrate, and the inductors are made by winding #16 magnet wire to the appropriate size. This filter reduces the second harmonic by more than 40db, and the third by more than 60.

For the 2m and 222MHz filters, additional capacitance is provided by high-power metal mica capacitors. Different inductor values are also used.

The dual directional detector provides the signals for the SWR lockout, and also drives the LED forward and reverse power meter bar graph displays.

Additional details can be seen in the write up of the 70cm KW amplifier.
These next 2 photos show the component selections for the low pass filters at 70cm and 222MHz.

If you are building this assembly from a kit I supplied, disregard the positioning of the inductors as shown here. Originally, I had them placed as you see them here because the filters would not tune easily unless I did that. I later discovered this was due to inter-stage mutual coupling between adjacent inductors; I was able to neutralize this effect by winding each inductor in the opposite direction from it's neighbor, and that way was able to place them all in a straight line across the board.  Tuning the filter is best accomplished by spreading and compressing the coils for the lowest SWR reading at the input of the filter (terminate the other end with a good dummy load). I used a scalar analyzer to set mine up, but the former method mentioned will work fine. Performance data, schematics, and a setup chart are shown at the end of this page.

The components for the 2 detector areas consist of a chip resistor attenuator to set the correct signal level, a diode detector and an LC filter. The directional pickup traces can separate the forward and reflected signals by more than 30db (directivity is the commercial term for this specification). One is oriented to detect forward, the other oriented for reverse.

Reverse power should be detected 10db below the forward power level (equivalent to 2 to1 SWR). See the article on the stand-alone RF detector board for additional explanations of this technique.
 


 


 


 

And finally, here's the schematic and setup table.

The output from the detectors should be terminated with the resistances shown in the chart. If you are driving high impedance devices like the LED bar graph displays, a 5k chip resistor can be placed across the input of the FWD power display board. The swr input on the amplifier control board is already set at 5k for the reverse detector output, which is usually connected in parallel with the reverse power LED bar graph display. Because the load resistance is already at 5k in this situation, do not also place 5k at the input of the reverse LED display.

However, on 2m, the output of the reverse detector must be handled differently. The input trimmer on the control board should be changed from 5k to 20k. This will bring the signal level up, necessary for the reverse detector on this band, as the coupling is very loose (51db) at 2m.

Here is the setup table and schematic for the 4/2014 version of this board, which also covers 6 meters and 70 MHz:


Here's a look at component placement under the shield: