1. Turn your mike gain down so that your xcvr output does not exceed ten watts, (if SSB). #
2. #If FM or AM modes are desired, then you are right and need an attenuator that is resistive. #A 3db splitter with one side going to a dummy load will work with your amp on the other side. (A Wilkinson combiner/splitter would work as a 3db splitter and can be made with two 1/4 wave length sections of coax in a "Y" configuration and a 100 ohm resistor across the top of the Y)
3. #Many xcvrs have an adjustment for output power.
4. #Attenuators can be designed in many ways such as resistors in a "T" configuration or an "PI" configuration. #The ARRL handbook has info on how to determine the resistor values. #You will need to parallel a number of carbon resistors so that they will be able to handle the power level desired. #Heat sinking them will increase the wattage capacity of the resistors.
Try this site for resistive attenuator calculations:
5. #you could also use "PIN" diodes to make a variable attenuator and adjust your xcvr output precisely. #Most diodes will perform somewhat as a pin diode. #Ordinary power rectifiers can be used at HF quite effectively in an RF attenuator or SS relay circuit and will handle considerable power.
A 3db pad will reduce your 25 watt radio to 12.5 watts output, so if 10 watts is the max desired, then design a 3.5 db pad. Also consider that your radio may put out more than 25 watts and need an even greater attenuation. Measure it before connecting to your amp.
Terry Graves, K7FE
Chief Editor, QRZ.COM
"Some people call CW a MODE but in
reality it is an autonomous LANGUAGE."