Saturday, December 15, 2018

200 W 20/30 dB attenuator with 100 Ohm / 30w resistors

Between buying a 200 W probe/head for the power meter, a pre-made 30dB attenuator or making this project I chose the last option, it will not have the same frequency range but it's a lot more fun and a little cheaper . This attenuator is a nice way of extending the range of my existing 2 W power head URY-Z2 until the 200W mark. I wanted 200 W because in the future I need to test the FT-102 capable of that value.

The end result for the impatient:

The main schematic for the 20dB part is bellow. I added a after a 10 dB attenuator on the output, that was made with smaller resistors since it just needs to handle part of the final 1.7W (see further down the power distribution in the attenuator).

Keep in mind that the previous schematic only handles around 80 W if using 30W resistors. Most of the power is burnt in the first shunt.
A simple test layout diagram:

 The attenuator is only composed of 100 Ohm resistors (cheaper than the 50 Ohm ones on the flea bay) with the two terminals isolated from ground, so it was nice exercise to calculate and attenuator under this limitation.

 In previous setup and at 50Mhz, the attenuation obtained:

Not bad since the calculated value it's not exactly 20 dB.

The power dissipation calculation:

(addendum: just realized the power on the first shunt is miscalculated, should be 100W on the 100Ohm resistor and a total of 66W on the series 100 ohm with the 2 parallel 100 ohm)
The X marks the resistors that will be under power at an input of 200 W so the 100 Ohm (on the input, left side of the first shunt) taking 80w (100W is the correct value, see addendum) is now composed of 4 resistors, capable now 120W dissipation in total.

..and are the 4 ones diagonally oriented on the left top.

For the ground plane I used copper plate with 0.6mm tick and it had to be partially soldered in the oven, the iron just could not make it, then on the final assembly the iron just had to do the final touch.

Final assembly inside view:
The end 10dB attenuator added is the "mesh" on the lower side of the picture/box.

For the final 10dB also a PI attenuator was chosen with these values: on the first and on the end shunt: 4 x 470 Ohm all in parallel with 1x 560 Ohm . For the series element: 3 x 220 Ohm all in parallel with 1x 2K7 Ohm resistor.

I include glued to the box a chart with the output value in function of the final attenuation, the measured one at 50Mhz (29.39 dB) and the calculated 30dB, this way if I don't set on the power meter the correction I just look at the table:

The VNA pass:

..still need to validate with more accurate power meter than the AD8307  from my homemade "VNA" and with a proper reference set, never the less looks OK for the type of construction involved.

So far I only tested with 100 W on the input, gets only mild warm for quick tests, for extended periods it might need to spread the resistors a little more and possibly include some thermal paste and better heat sink.

Approx cost of the project was as follows:

- N plugs: 3.20 €
- 30W Resistors (18) + 1/4W resistors : 12.00 €

- Box: 7.00 €
- Copper plate: 3.00 €
- Solder/paper/ink: 1.00 €

# Total #: 26.20 €

Have a great day!

The schematic changed

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