Rain fade at 1420 MHz?

[Eduard Mol]

Hi all, 

What- if any- impact does rain have at 1420 MHz? I was doing drift scans of M31 at 1420 MHz and noticed that the spectrum from last evening was lower and had a worse SNR than my observations from earlier this week. The only important difference I can think of is that it was raining yesterday.

[Alex P]

Hi Eduard,

I wonder if the rain drops reflect/refract some of the surrounding Earth temp noise back into the antenna beam ?

I have noticed a few instances where small cumulus clouds quickly passing  between the Sun and the main beam

cause momentary shifts in the overall signal by a 1 dB or more ! 

Regards,

Alex

[Eduard Mol]

Small cumulus clouds would not have much of an effect at 1420 MHz- the cloud droplets are just so much smaller than the wavelength. What I think you were seeing is gain change die to the temperature change. When a cumulus cloud passes over, the LNA and other components are no longer heated by the sun and cool down, causing a gain increase. I noticed that SDRs are particularly sensitive to temperature. That’s why I keep my SDRs inside mounted on heavy blocks of aluminium with thermal paste, to keep the temperature stable. 

But with rain, I don’t know if there is an effect or not…

Op vr 14 nov 2025 om 10:54 schreef 'Alex P' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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[b alex pettit jr]

The change was pretty fast ..One time, I could watch the clouds and see the shift as I had the PC outside  ( but in the shade ). 

In this recording, the PC & SDR were inside . 

A shift from thermal changes in electronics would not shift then stabilize as quickly ??

[b alex pettit jr]

It also goes the wrong way : the noise is Higher During the cloud cover

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[Eduard Mol]

Yes but that’s exact what you would expect- cloud passes over -> electronics cool down -> gain increases -> background noise level goes up.

Op vr 14 nov 2025 om 11:18 schreef 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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[b alex pettit jr]

Processed w/ HL3D to remove the shifts

RAW

CORRECTED

[fasleitung3]

The rain attenuation is absolutely negliable at 1420 MHz, see https://www.itu.int/dms_pubrec/itu-r/rec/p/r-rec-p.838-3-200503-i!!pdf-e.pdf

According to this, the attenuation at a rain rate of 100 mm/hr is about 0.0048 dB/km. So even if your line of sight passes through 10 km of heavy rain you will not see any impact.

Different story for water maser observations: At 22 GHz and the same rain rate it is 12 dB/km.

Best regards,

Wolfgang

Am Freitag, den 14.11.2025, 09:05 +0100 schrieb Eduard Mol:

Hi all, 

What- if any- impact does rain have at 1420 MHz? I was doing drift scans of M31 at 1420 MHz and noticed that the spectrum from last evening was lower and had a worse SNR than my observations from earlier this week. The only important difference I can think of is that it was raining yesterday.

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[Eduard Mol]

Okay, thanks Wolfgang. Then there is something else going on…

Op vr 14 nov 2025 om 14:38 schreef 'fasleitung3' via Society of Amateur Radio Astronomers <sara...@googlegroups.com>

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[Shaik Samad Pasha]

I have observed H1 in heavy rain using my horn antenna, i covered it with a plastic cover and observed it - the only issue I found was due to water accumulated on top of the antenna - when I cleared rain water on top it was receiving good signal, I had to repeatedly clear water on top. Later while clearing, some water entered the horn and things got bad and stopped the observation.

you can see the discontinuity due to water accumulation

after corrections

In the next day observation - it was raining moderately

Only saw issues during rain water accumulated on top of the horn. once rain stopped i was getting good one

corrected one

- Shaik

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Shaik Samad Pasha

[Don Latham]

OK, guys!  It's a dense array of water drops at near freezing. Your receive setup is a radiometer, remember? the drop size is a red herring.

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From: "sara" <sara...@googlegroups.com>
To: "sara" <sara...@googlegroups.com>
Sent: Friday, November 14, 2025 3:18:41 AM
Subject: Re: [SARA] Re: Rain fade at 1420 MHz?

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Don Latham
PO Box 404,
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406-626-4304

[Don Latham]

I refer here to a cloud. Rain may or may not be dense enough to show its presence.

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From: "Don Latham" <d...@montana.com>
To: "sara" <sara...@googlegroups.com>
Sent: Friday, November 14, 2025 1:37:33 PM

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[Eduard Mol]

Thanks @Shaik- Nice data by the way! 

My feedhorn is looking down and the feed assemblage is also covered by a plastic box, so no rainwater should get directly in front of or in the feedhorn. 

Did a new driftscan yesterday and compared the local HI peak from our own galaxy between the runs from the last few days. There is very little difference regardless of the weather. On the other hand there is some low level RFI at my location and fully subtracting the bandpass response of the system is quite difficult especially at the 0.1K level, so in hindsight seeing some day to day variation in the results is not that surprising. 

Op vr 14 nov 2025 om 17:45 schreef Shaik Samad Pasha <sksamad...@gmail.com>

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