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I'm not so sure there is much point in buying a special high power 20dB attenuator.
Even at 10 Watts, a 20dB attenuator would be rather expensive.

You probably already have a 50 ohm dummy load suitable for use with your transmitter.
If you plan to work on your transmitter at all, you should have a dummy load.??
A 12 Watt 50 ohm dummy load could be built with four 3 Watt 200 ohm resistors in parallel.
You can get ten such resistors for one dollar from Mouser,? part number 283-200-RC
Forty 2000 ohm resistors in parallel can dissipate 120 Watts,? Mouser 283-20K-RC

To reduce transmitter power by a power ratio of 100), must reduce the voltage by a factor of? sqrt(100) = 10.
So add a 450 ohm resistor from the top of your 50 ohm dummy load into your 50 ohm step attenuator,?
The step attenuator must be capable of dissipating 100 milliwatts if the transmitter is giving 10 Watts.

The 50 ohm step attenuator sees a voltage that is reduced by a factor equal to its 50 ohms divided by
the total resistance of that leg, or 50/(450+50) = 0.10, which is a factor of 10 as computed above.

The transmitter will see 50 ohms in parallel with (450+50) ohms, or 50*500/(50+500) = 45.5 ohms,
close enough for government work.? Could correct this 10% error by making your dummy load
look like 55.55 ohms instead of 50 ohms by adding a 1/8 Watt 5.55 ohm resistor in series with it.?
(The 5.55 ohm resistor only sees 1/10'th of the transmitter's voltage, so only 1/100'th of the power.)

Kees has a kit available for a step attenuator at a good price, kit number 13 here:
? ??

Jerry, KE7ER