小中大sorry for using English, no Chinese typing on this computer.
sounds a very interesting, useful work.
my understanding is that you are detecting the earth magnetic field by using the ESR technique. If this is right, the principle should be like this; DPPH radical has an unpaired electron that has a spin. The magnetic moment of this spin is precessing about the magnetic field it experiences (it is the earth field <B_earth> in your case), and the frequency is called Larmor frequency, given by, v = g * u_B * B_earth, where u_B is a constant, and g is a tensor, depending on the geometry. In the presence of a microwave of the same frequency v, the electron spin can exchange energy with its surroundings, which manifest itself as a derivative characteristic on this called ESR (electron spin resonance) condition. So knowing this frequency v, one can infer the magnetic field.
As for your thing, I think what you need is a microwave synthesizer, (g * u_B = 1.76e11 rad/sT, B_earth ~= 20Gauss, roughly, v = 5.6MHz, double check me on this estimate). you need one whose frequency can be continuously adjusted, and you can get this guy simply by buying one. you also need something to detect at what frequency it resonances, but i dont know how to achieve this,
in NMR, it is just using the same Rf coil that delivers the microwave to detect the electrical signal that is induced by the nuclear spin, for your ESR, check it out by yourself.
Plus, you also need some complimentary stuffs like amplifiers, filters, ADCs, etc.
