[RFI] Noise puzzler

[Lee STRAHAN]

This sounds to me very suspicious of the apparent digital electronics in the PDU switch and not the fans. Try an incandescent light bulb as a PDU load and check for the noise. 
Lee  K7TJR

-----Original Message-----
From: RFI <rfi-bounces+k7tjr=msn.com at contesting.com> On Behalf Of nlsa at nlsa.com
Sent: Tuesday, January 10, 2023 10:51 AM
To: RFI at contesting.com
Subject: [RFI] Noise puzzler

Dear friends,

 

I have noticed something that puzzles me.  When I turn ON a pair of AC fans, my VHF noise floor *drops.*

 

Background: My EME station is remotely operated.  Everything is mounted at the antenna, about 100m from my house.  All the equipment is contained within a fiberglass enclosure that is part of the antenna base.  220VAC and a fiber optic control line are the only cables to/from the enclosure.  You can see a photo and some description on my QRZ page https://www.qrz.com/db/W9IP .

The enclosure has two 170mm 110VAC muffin fans for ventilation.  I can turn the fans on/off remotely using a Ethernet PDU
<https://www.apc.com/us/en/product/AP7901B/rack-pdu-switched-1u-20a-120v-852
0/>  without switching anything else in the system.   My location is rural
and the closest neighbor is at least 200m away.  I do have some strong FM broadcast stations a few km away but their overload effects are completely
(?) mitigated by helical filters mounted at the antenna feed.

Noise floor is observed with two separate receivers: IQ+ SDR and Anan Angelia SDR; both tell the same story.

 

Observations: When I turn the fans ON, my broadband noise floor drops by about 1.5 dB.  OFF and the noise floor rises to its previous level.  This is repeatable and does not vary noticeably with antenna azimuth or elevation unless I'm pointed at an unusually strong RFI source.  The noise change is immediate with the switch-on/off.  Aside from some minor birdies, the ambient noise has no structure that might identify it as generated by AC- or ignition sources.

Here is a screen capture from Map-65 with my antenna pointed toward reasonably quiet sky  https://www.nlsa.com/Fan%20On-OFF.jpg 

 

Interpretation: I see no indication of desense or other signs of overload.
Plenty of strong amateur 2m signals come and go without issue.  The noise decrease isn't related to temperature because it occurs immediately, well before any significant air has moved.  Yes, I have double-checked the actual fan state.

 

I'm open to ideas.  If none are forthcoming, perhaps I'll sell my magic noise-reducing fans to the highest bidder :)

73,

W9IP

 

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