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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