10GHz Attenuation by Heavy Rain

DK8KW · Jul 2nd 2020

Hello all,

Yesterday I had the opportunity to record the level of the CW beacon while a heavy rain shower came down.

The nominal value I adjust the beacon to is -73dBm. You can see from the picture that the lowest level during the rain is around -88dBm (only look at the upper values of the curve -- I tried to produce an envelope in Excel but that seems to be a challenge in itself).

So the maximum attenuation is 15dB. My uplink signal, adjusted to the beacon level, also had the same attenuation as the beacon, which means, that the 2.4GHz signal seems to be much lesse affected by the humidity in the air.

Did anyone else took similar measurments?

Best 73

Holger 'Geri', DK8KW

P.S.: I don't really know if this is relevant and if the attenuation is the same for different antenna sizes. My QO-100 antenna is a 1.8 Meter offset dish from Kathrein (CAS 180).

G0MJW · Jul 2nd 2020

If you are interested you can look at the models for rain attenuation in ITU-R Recommendation P.837. https://www.itu.int/rec/R-REC-P.837/en with specific attenuation described here https://www.itu.int/rec/R-REC-P.838/en and its application to satellite links here https://www.itu.int/rec/R-REC-P.618/en - I wrote a tutorial a while back http://www.mike-willis.com/Tutorial/PF10.htm - anyway if you look at the maths it's clear why 10 GHz is much more impacted than 2.4 GHz.

Specific attenuation γ = kR^α dB/km where R is the rain rate in mm/hr

DD0KP · Jul 2nd 2020

G0MJW Yes Mike that is true. But the impact is mostly only on the WB transponder. We had also rain like cats and dogs with strom yesterday. The impact on the NB was minimal, on the WB most signals were gone. For us this is an usual indicator when we are sitting in the living room and the ASTRA TV signal is gone. Then we know that there are only a few minutes left to bring the garden into a safe condition. Usual the clouds are comming south over our mountain 1245m high into the valley.

DL1GNM · Jul 2nd 2020

I've seen dropping the NB beacon 7-8dB lower compared to clear sky during heavy rain. WB: was gone of course. Uplink remained stable.

HB9SKA · Jul 3rd 2020

Was your LNB wet or dry ?

DK8KW · Jul 3rd 2020

HB9SKA: what my equipment is concerned, I don't believe in covers ... my LNB as well as the helical antenna are in the open environment, so the front end of the LNB surely was wet during the rain. However, when a "normal" rain occurs and the surface of LNB lens is wet I can measure no attenuation.

Vy 73

Holger 'Geri', DK8KW

DL1GNM · Jul 5th 2020

Quote from HB9SKA

Was your LNB wet or dry ?

Dry.

Dish wet.

Myself dry. Indoors.

HB9SKA · Jul 6th 2020

My intention to ask was to learn, if a wet LNB has an influence to the attenuation. If yes, then you can't measure it alone for heavy rain. But Geri answered my question.

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