Can You Identify This Mysterious RF Signal at Night?

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    Rf Signal
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Discussion Overview

The discussion revolves around identifying a mysterious RF signal detected at night using an RF spectrum analyzer and an HF antenna. Participants explore the characteristics of the signal, its potential sources, and the technical setup used for detection, including the implications of using an off-resonant antenna.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes observing a pulse across the RF spectrum, occurring in bursts approximately every 30 seconds, and hypothesizes it may be related to Chinese OTHR technology.
  • Another participant questions the frequency range of the observed signal and suggests that varying the scanning rate might affect the rate of change of frequency.
  • Concerns are raised about the pollution of the spectrum making it difficult to identify pulse-like transmissions without reliable equipment.
  • Participants request detailed information about the setup, including the receiver model, frequency tuning, and the configuration of the IF output.
  • There is a discussion about the difference between using an oscilloscope and a spectrum analyzer, with emphasis on the importance of the latter for frequency analysis.
  • One participant mentions that the signal resembles a navigation beacon but is not identifiable as any known source.
  • Another participant suggests that the signal may be affected by interference from the receiver and recommends connecting the spectrum analyzer directly to the antenna.
  • Some participants propose that the signal could be similar to CODAR, which is used for oceanographic radar, although the characteristics differ.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the signal and the effectiveness of the current setup. There is no consensus on the source of the signal or the best approach to identify it, with multiple competing hypotheses and technical challenges remaining unresolved.

Contextual Notes

Participants note limitations in the information provided, such as the need for diagrams and detailed descriptions of the equipment used. There are also discussions about the assumptions regarding the antenna's resonance and the potential for interference affecting the readings.

houlahound
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Looked at a RF spectrum that came from the IF output on a radio so it was not tuned thru the radio filters, just whatever the HF antenna was capable of.

A pulse went across the entire spectrum slow enough to see it sweep across the screen.

The pulses were not regularly spaced but came in bursts on average prolly 1 every 30 seconds.

Lots if people in my circle know it.

The hypothesis is Chinese OTHR, purpose unknown.

How can one identify such a thing.

I can video the computer screen this Friday if that helps.
 
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What is the frequency range of the frequency spike?
Does its rate of change of frequency change if you scan at different rates? That may or may not apply to different forms of frequency analysis.
If you could operate your receiver at two or more different sites, you could see if the level changes much. If not much then probably a distant source but if a lot then probably a local source.
I assume you have tried turning on and off any local possible sources of interference - before looking for the 'exotic' cause. Interesting, though.
 
Good questions I will follow up on, thanks.

Not sure what I am looking at given we have a fixed antenna that is working so far off resonance. Does that change how the pulse looks. Does an off resonant antenna work like a resonant antenna just "weakerly".

I will reference yr screen name live so you know my video is legit.
 
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The spectrum is so polluted now that it is very hard to identify pulse-like transmissions without really good detective work with known reliable equipment. On the face of it I can't see why an OTH would have such a slow PRF and transmit simple pulses.
 
you really have given far too little info to be helpful

describe you exact setup ( preferably with diagrams) starting at the antenna and going through
to what ever you have connected to the IF output

1) what is the receiver ?
2) what frequency was it tuned to
3) does it have more than 1 IF frequency ? if so what are they

houlahound said:
Looked at a RF spectrum that came from the IF output on a radio so it was not tuned thru the radio filters, just whatever the HF antenna was capable of.

what were you using to look at the spectrum ??

if you say an oscilloscope, then my response will be you should be using a spectrum analyser
an oscilloscope scans in time, a spectrum analyser scans in frequency

the pulse on the oscilloscope could be just indicating a burst of signal on the freq set by the receivers IF
it tells you nothing about the frequency of the signalDave
 
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We used an RF spectrum analyzer and a HF antenna. The frequency of the pulse went across the entire spectrum.

Sort of like a navigation beacon but we know all them so something artificial but not known yet.
 
houlahound said:
We used an RF spectrum analyzer and a HF antenna.

and what else ?
you commented on an IF
your description of a pulse didn't match using a spectrum analyser
 
houlahound said:
The hypothesis is Chinese OTHR, purpose unknown.
the Chinese radar sits on one freq for a period of time before freq hopping to a different freq with better propagation
the old Russian OTHR did the dame thing ... unaffectionally known as the "woodpecker"
 
davenn said:
and what else ?
you commented on an IF
your description of a pulse didn't match using a spectrum analyser
If you have a spare few hours I will tell all about the station. What else would you like to know specifically?
 
  • #10
davenn said:
you really have given far too little info to be helpful

describe you exact setup ( preferably with diagrams) starting at the antenna and going through
to what ever you have connected to the IF output

1) what is the receiver ?
2) what frequency was it tuned to
3) does it have more than 1 IF frequency ? if so what are they
what were you using to look at the spectrum ??

if you say an oscilloscope, then my response will be you should be using a spectrum analyser
an oscilloscope scans in time, a spectrum scans in frequency

the pulse on the oscilloscope could be just indicating a burst of signal on the freq set by the receivers IF
it tells you nothing about the frequency of the signalDave


Will post a video if I get permission. You won't see many better stations in civvy or non commercial world.
 
  • #11
houlahound said:
What else would you like to know specifically?

have already asked twice ... what was the IF thing you spoke of
an IF assumes a receiver, and I asked about that too

IF you are really using a spec an, then it alone should be connected to the antenna ... NOT via a receiver
 
  • #12
Wrong, the IF internal port bypasses the receiver filters. The spec is connected to the antenna and so is the radio to the same antenna.
 
  • #13
houlahound said:
Wrong, the IF internal port bypasses the receiver filters.

not normally wrong

so AGAIN, WHAT is the receiver ??

PLEASE stop making it difficult for people to help you and answer the Q's when asked
dragging every bit of info out of you tiny bit at a time is a pain in the butt

WHAT is the IF freq?
 
  • #14
You mean model/ brand??
 
  • #15
houlahound said:
You mean model/ brand??
YES!
 
  • #16
houlahound said:
A pulse went across the entire spectrum slow enough to see it sweep across the screen.
The word "pulse" implies a short burst in time. A spectrum analyser shows the frequency spectrum. The frequency spectrum of a pulse is not a single peak of frequency. What your trace is showing is a signal of narrow bandwidth with a centre frequency that's sweeping about slowly.
 
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  • #17
sophiecentaur said:
. What your trace is showing is a signal of narrow bandwidth with a centre frequency that's sweeping about slowly.

Yes that is what I am seeing and pulse seemed a more efficient way of saying it.

Dave it's a Kenwood, will find out the model.
 
  • #18
houlahound said:
Yes that is what I am seeing and pulse seemed a more efficient way of saying it.

Dave it's a Kenwood, will find out the model.
"Spike" might be less confusing, perhaps. That has no 'temporal' connotations - as in time domain.
 
  • #19
Try turning off the Kenwood. It may be scanning for signals and leaking interference back to the antenna.
 
  • #20
houlahound said:
Wrong, the IF internal port bypasses the receiver filters. The spec is connected to the antenna and so is the radio to the same antenna.

why have the radio there ? ... it's only confusing the issue and isn't doing anything to help the situation

Tom.G said:
Try turning off the Kenwood. It may be scanning for signals and leaking interference back to the antenna.

yes, you could easily be seeing signals from the radio ... take it our of the equation and connect the spec an directly to the antenna coax
With NOTHING else connected to the same antenna
 
  • #21
davenn said:
why have the radio there ? ... it's only confusing the issue and isn't doing anything to help the situation
Yes.
In the absence of filtering, that signal could be getting mixed in from anywhere at a different frequency. You have to hope that the spekky annie can cope with what you're looking at or it will involve an extra home-built RF filter to make sure you know what's being looked at.
There are checks you can do, like seeing if the displayed signal actually drops by 10dB for a gain change of 10dB. My (fairly significant) experience of RF spectrum analysers is limited to high end analogue versions from the past but the 'rules' still apply to digital versions (plus extra stuff because of the sampling).
 
  • #22
There is a signal that I pick up on my SDR (a lot like a Analyzer) on HF. Looks rather like what you describe. Called CODAR. Do you live "near" the ocean?



Google CODAR.

DC
 
  • #23
Well, that was supposed to be a youtube link. Just check out CODAR on line plenty of info.
DC
 
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  • #24
Nice, my signal is not periodic like yours and scans a bit slower.

Link worked fine.
 
  • #25
houlahound said:
Will post a video if I get permission
houlahound said:
Nice, my signal is not periodic like yours and scans a bit slower.
Sorry if I missed it, but did you post your video, along with a block diagram of your setup and pictures of your antenna? That would help a lot...
 
  • #26
as mentioned my next planned visit to the station is this weekend. if I am allowed, and can't see why not, I will video the screen.
 
  • #27
houlahound said:
Looked at a RF spectrum that came from the IF output on a radio so it was not tuned thru the radio filters, just whatever the HF antenna was capable of.

A pulse went across the entire spectrum slow enough to see it sweep across the screen.

The pulses were not regularly spaced but came in bursts on average prolly 1 every 30 seconds.

Lots if people in my circle know it.

The hypothesis is Chinese OTHR, purpose unknown.

How can one identify such a thing.

I can video the computer screen this Friday if that helps.
Sounds like some variation on 'over the horizon' radar, using long term pulses to see things thousands of miles away on radar screens.
 
  • #28
litup said:
Sounds like some variation on 'over the horizon' radar, using long term pulses to see things thousands of miles away on radar screens.

it as said, could be any random noise from a number of sources, specially the radio he has hanging off the same antenna

lets await for him to make more posts on the setup before making lots of guesses
 
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  • #29
I plan to be on the 40m band this weekend some time at night.
 
  • #30
houlahound said:
this weekend some time at night.

at nite, where ?
 

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