16 January 2022

Notes from InnoSent Application Notes on 24 GHz RADAR

In the download area of the innosent.de website there are four application notes. They contain 24 GHz wisdom, but you need to read through them to notice it. 

Since I am still convinced that some HAM radio result can be achieved using those radar modules, I collected below the interesting information I picked from those A.N.'s.

For the IPM-165 and the CDM-324 clone we should have faith and believe that the maximum EIRP is +20 dBm, AKA 100 mW. Typically 16 dBm.

Frequency decreases with temperature -1 MHz/°C (IPM-165). This is one of the largest challenges of these radar modules we already faced on 10 GHz with HB-100. I insulate from fast variations (wind) and keep the module in the shade when operating outside (sun/cloud transitions).

Frequency increases with voltage +25 MHz/V (IPM-165). They work from 4.5V to 5.5V, then they blow up. Vcc can be used to frequency modulate the carrier, while it must be as clean as possible: I chose linear regulators.

The A.N. also mention that the I.F. output impedance is middle-ohmic. I'd put that in the 5k-10k range. They suggest to use a HI-Z amplifier. I will consider this improvement once I am sure some sort of "DX" is possible. Remember: there is no cheap way to receive 24 GHz!

The I.F. output bandwidth reaches 100 MHz. Good! This confirms that there is not a LPF in the I.F. output. I would stop at 85 MHz to stay away from the FM broadcast band.

Regarding the installation, the A.N. lists how much common materials adsorb. For my imaginary setup I need to know that "microwave penetrate plastic very well – 0.5 to 3 dB loss with optimized thickness and correct spacing." I would rather not loose even 0.5 dB, but that gives some form of thermal stability. More from the A.N.:

The following suites very well:
•  coverage with plastic materials (ABS, PVC etc.) as long as they aren’t in direct contact with the antenna patch structures and the correct thickness and spacing has been evaluated
•  foams like Styropor and similar materials, whose relative dielectric constant is close to 1, they can even be mounted in direct contact with the antenna surface.
For 24 GHz the following rule of thumb can be used:
•  thickness of the plastic material around 3mm or 0.12 inches
•  air spacing to antenna surface similar, around 6mm or 0.24 inches

I will try surrounding the module with styropor and put everything inside a thin plastic container.

The application notes also describe the theory behind a microwave radar, with some light maths too. I think the formula ignores free-space-loss, that even at 20 metres return (obstacle at 10m) accounts for 86 dB! Using that data and the observed SNR of my experiments, I think I can establish a module-to-module contact at 200 to 300 metres. Unless the IF circuitry is saturating/clipping therefore cutting the SNR visible on the SDR.