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Hunting Spurious Emission at 154 MHz

 

I was exploring the VHF High public service bands (150 - 162 MHz) on my tinySA spectrum analyzer using the waterfall display. I noticed a very strong, very narrow, very clean unmodulated FM carrier at exactly 154MHz. I noticed the strength varied as I moved around the house but I could not locate a source on the end of the house with the strongest signal.
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So I swapped to a small rubber ducky antenna on the tinySA and headed outside. The emission was clearly coming from a neighbor's house. Neighbor is a great guy so I knocked and we went looking for the source and suspected an older Windows PC desktop. He shut it down and the emission went away. Likely a failed filtering capacitor or choke in the switching power supply.
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Don't know if he'll bother to fix or replace it, but I had fun hunting it down. I didn't think of it at the time but I may have been seeing a harmonic and probably should have checked 38.5 MHz using the tinySA MEASURE/HARMONIC tool as that is a more likely frequency from a failing switching power supply. Maybe next time he boots the PC...
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Peter
 

It is KHz it is likely SMPS noise. A signal in the MHz range you describe is more likely leakage from a reference crystal oscillator on the motherboard or a plug in card.

On Tue, 01 Apr 2025 19:22:28 -0700
"Peter Finch via groups.io" <peterlewisfinch@...> wrote:

I was exploring the VHF High public service bands (150 - 162 MHz) on my tinySA spectrum analyzer using the waterfall display. I noticed a very strong, very narrow, very clean unmodulated FM carrier at exactly 154MHz. I noticed the strength varied as I moved around the house but I could not locate a source on the end of the house with the strongest signal.

So I swapped to a small rubber ducky antenna on the tinySA and headed outside. The emission was clearly coming from a neighbor's house. Neighbor is a great guy so I knocked and we went looking for the source and suspected an older Windows PC desktop. He shut it down and the emission went away. Likely a failed filtering capacitor or choke in the switching power supply.

Don't know if he'll bother to fix or replace it, but I had fun hunting it down. I didn't think of it at the time but I may have been seeing a harmonic and probably should have checked 38.5 MHz using the tinySA MEASURE/HARMONIC tool as that is a more likely frequency from a failing switching power supply. Maybe next time he boots the PC...

Peter
--

73

-Jim
NU0C
 

I discovered a strange signal on 3.846 MHz in the 80 meter band. I asked my ham friends around town if they could hear it. They all answered no. ?So I went out with a large loop and DF’d the signal to a house 1050 feet away from mine. The signal is always present and appears continuously with no changes until night fall. Then the tone ?(listening on USB) changes its periodic modulation rate. ?I do not know the owners and they live in the adjacent gated neighborhood. ??

The house does not have solar panels on the roof.?

Mystery not yet solved. Just located.?

-Charlie
?W5CDT
?

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