Terminators

          

These are usually just a resistor to ground.  When a 50W terminator is placed at the end of a 50W coaxial cable, any signal coming down the cable is totally absorbed (ideally) and there are no reflections.  Actually, there sometimes are reflections but placing terminators at both ends of the signal cable can help to reduce their effects.  The feedthrough type (as shown in the picture) are nice because one end can be connected to an oscilloscope and the other to the coaxial signal cable.  Note that because the resistor is usually only rated for ~1/4 Watt, there is a limit to how much DC voltage or pulsed power can be applied without burnout.  These can be checked simply by measuring resistance between inner and outer conductors (should be e.g. 50W at both ends).

Attenuators

      

These are usually placed between the end of the signal cable and the terminator (you must use a terminator to get the labeled attenuation).  The resistor values are derived by:

R1 = R3

R1 = Z₀*(1+2/(k-1))

R2 = (Z₀/2)*(k-1/k)

Where Z₀ is the cable/terminator impedance (e.g. 50W) and k is the attenuation factor (ratio of output voltage to input voltage).  Note that the attenuation is often given in dB.  The relationship between k and dB is 

20 log₁₀ k = Voltage attenuation in dB

Values for usual attenuations with Z₀ = 50W are as follows:

k	dB	R1	R2
2	6	150	37.5
5	14	75	120
10	20	61.1	247.5
100	40	51	2500

Attenuators are also often damaged by too much power.  These can be checked by measuring resistance between inner conductors on either end.  The resistance should be 2*R1*R2/(2*R1+R2) as given in the following table:

k	dB	Rcheck
2	6	33.3
5	14	66.7
10	20	81.8
100	40	98.0

Any large variation between the table value and the measured value probably indicates a bad resistor internally.

Notes: Sources:  Ron Boller and various.