Well, the attenuator is one of those things you can do, or not, if you which. The reason that I do it following:

• If I power my QCX kit with 12.5 Volt DC (the Voltage of a car battery if properly loaded), the RF-output is about 2.0 Watt. That is a bit to enthusiastic for driving the amplifier. I think a proper input power would be about 1 to 1.5 watt. So I need about 3 dB attenuation. 
• A nice input impedance is good for the QCX kit, and the output filter of it.
• It helps for stability of both kit and amplifier.

To design the input attenuator I used the following file PowerAtt.xls. Thanks Bert, PA1B

It can give you a detailed set of resistors of which you can make your attenuator. Anyway, I simplified it to only three R's. The attenuation is not exactly 3dB, but I don't care.We start our testing with these R's, in and output R are 330 Ohm (one watt). The middle R, I picked a 27 ohm resistor of 3 watt. 1Watt for the R would be ok, but I don't like to sit at the edge,.....

About the input capacitor, basically it is not needed, because the TX-RX control circuitry has at the output also a C. Idem dito the output C. Not needed, because it is connected to the input of the transformer (O volt DC). So the attenuator ends up to be a very simple R-network