26 September 2023

Emission of HLK-LD1115H as seen with a CDM324

In the picture below we can witness two radar modules looking at each other.

A CDM324 basic radar module was connected to an RTL-SDR to check the emission of a HLK-LD1115H. The greatest fear was that the HLK carried too much noise requiring an extremely wide modulation, but the picture shows a situation that is not worse than 2x CDM324 talking to each other. And we should consider that with this setup it is impossible to tell which radar is noisier.

The short-term frequency instability is probably caused by CDM-324 sensitivity to temperature variations, as it was fully exposed to ambient air.

The transmitting radar shows about -15 MHz drift at warm up in the first couple of minutes. I could also confirm that the transmission is continuous and not modulated.

On the second HLK module I own I pulled to ground the TXON control pin. Curiously on the serial port output it would report detection of movement and presence, alas with a lower intensity than the stock counterpart. With the receiving setup I could confirm that disabling TX introduces about 40 dB attenuation in the RF output.

I like the fact that the HLK-LD1115H can detect a movement 40 dB down, as it gives some headroom for completing a QSO at "some" distance. My current "record" with 2x CDM324 is 70 meters, without parabolic dish(es). 

What's next? Either trying to inject a modulation or pull the frequency closer to the other HLK and repeat the receiving experiment.

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