After adding the sponge under the lid of two CDM-324 radar modules (probably clones on InnoSent IPM-165) I repeated the transmission-reception test over a range of 4 and 10 metres.
While the SNR does not deteriorate of a visible amount with the increased distance, I noticed few things about these radar modules.
They are very sensitive to power supply voltage variations. Any noise on the power supply line is reflected on the output frequency. I opted to use a 78L05 linear regulator and a battery power source.
The IF output(/input) has low impedance. I tried modulating via a 1+1 Mohm divider but no signal was available at the IF pin of the radar module and no modulation could be detected on the receiving end. I will try to lower the impedance of my source, or will move to Vcc modulation.
Sensitivity to surrounding objects is less than 50 cm. When moving closer than 50 cm to the CDM-324 there is noticeable frequency drift. When the obstacle is removed the frequency returns more-or-less where it was before. This effect could be used to generate a physical FSK modulation for slow visual CW, for example.
They are sensitive to temperature. This is not a surprise, as HB100 on 10 GHz feels temperature variations too. That's intrinsic to DROs. But on 24 GHz the effect is 2.4 times higher. As long as receiving and transmitting end are subject to the same delta_T, their frequency difference remains constant.
Given all the causes of interference to frequency output are minimised, the CW radar-to-radar reception looks like this:
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I think the current SNR should allow the reception over a 300 metres distance, which I will try once I manage to modulate them. That will tell some more information about the antenna pattern.
