A normal meter will measure the average voltage at the 555 output, it's the peak that is important. Because some of the time the voltage is zero, the average will always be below peak. To measure the voltage, use a CRO connected across the Gunn diode bias with the circuit under load, in othe words, check the 555 is able to deliver the Gunn current, you should get a waveform with fast rising and falling edges and a peak of about 7V. If the voltage is lower than that, switch off, it means you need more current capability than the 555 alone can provide.
The voltage at the mixer will be very small, only a few mV, it is better to measure the current by connecting your meter between the detector output and ground and using a uA range.
It is important to understand how these devices work: The Gunn diode and the cavity it sits in make an oscillator circuit, the cavity has a small hole in it which couples some of the signal into a waveguide (the rectangular tube and flange). Sitting inside the waveguide and offset to one side is the mixer diode. The diode sees two signals, one is directly from the Gunn cavity and the other is coupled thrugh the waveguide from outside. Being a mixer, it produces an output signal which is proportional to the difference between these signals. So under static conditions, the output will be very tiny, what makes the signal bigger is a difference between the signals. Now consider normal operation, the module has a small antenna attached to it so it's beam width and gain can be set, the Gunn signal is directed out of the antenna into the area in front of it and some of the signal will reflect back into the antenna again. The mixer therefore sees two signals, one is direct frm the oscillator, the other is reflected from objects ahaead of the antenna but they are both at the same frequency so mixing them produces little or no variation in output current. Now, if anything moves in front of the antenna the reflected signal will travel over a shorter or longer path and Doppler shift will occur, this shift the frequency of the reflected signal and it no longer exactly matches the original Gunn frequency. The difference in frequency causes a signal at the mixer output, in fact it will be a frequency proportional to the rate at which the object moves, hence their use in radar and speed traps.
A simple test will prove this. Connect the module as I described and monitor the mixer current, an analog meter is best for this. Place the module on a table with the waveguide running parralel to the top so the signal is aiming level over the tables surface. Now place an object with an upright edge in front of the flange, something dense works best. Note the mixer current, now slowly move the object away from the flange, keeping it in line with the waveguide, you should see the mixer current rise and fall as the distance changes. This is because the reflected wave will add or subtract from the direct Gunn signal depending on the distance and therefore phase it reaches the mixer. There is a neat trick you can try: place a sheet of paper on the table and repeat the test, at each peak of mixer current, mark the position of the object on the paper. Afterwards, measure the distance between the marks you made, they should be exactly one half wavelength apart (remember the signal had to travel there and back so it passed twice the distance of your marks) from this you can calculate the microwave frequency of the oscillator.
Brian.