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I've been just scanning through the HF band at night seeing what all I can pick up, and I'm trying to figure out a particular signal. I'm using a VHF discone, which isn't ideal, but it works well enough for simple reception with an RTL-SDR.

22.695mhz. When I tune in the evening (roughly midnight UTC) here in Baltimore, MD I receive an AM broadcast in Japanese that a friend tells me is NHK World Radio. The station goes off the air after a bit, but then came back a few hours later the other night broadcasting several hours of pro-Cuba and pro-North Korea propaganda in English.

What is also strange is that the specific frequency seems to deviate by a couple kHz over time.

The contents of the broadcast, the unusual frequency I'm receiving it on, and the slow shifting of the frequency has me very curious where this is coming from and why I'm receiving the signal where it is.

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  • $\begingroup$ I'd ask myself if they're the same station. $\endgroup$ – Duston Oct 8 at 16:22
  • $\begingroup$ Hello and welcome to ham.stackexchange.com! $\endgroup$ – rclocher3 Oct 12 at 17:17
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Since you said you're using an RTLSDR, it's very likely that you aren't receiving a station on 22,695 kHz, but rather one on 6,105 kHz. NHK World Radio does broadcast on that frequency (according to the schedule I'm looking at, they broadcast between 0200 and 0400 UTC from a transmitter in France). The RTLSDR "direct sampling" mode is a hack, and it can't distinguish between a signal at a frequency f and one at 28,800 kHz - f.

As for changes in content and variation in frequency... well, you're hearing different stations. It's common for there to be many unaffiliated stations around the world sharing a single frequency, and with shortwave's global propagation, it's possible for you to hear different ones at different times (or even at the same time). Long reach also makes shortwave a good medium for distributing propaganda, and some countries even run high-powered transmitters for the specific purpose of drowning out other stations that they don't want their citizens to be able to hear.

It also wouldn't be surprising for one of the transmitters to be a kHz or two off-frequency.

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    $\begingroup$ It also wouldn't be surprising if the receiver drifted a bit in frequency. It's an RTLSDR. $\endgroup$ – user253751 Oct 9 at 10:37
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As hobbs - KC2G says, there could easily be multiple stations transmitting.

Also, the frequency stability of the RTLSDR is quite poor. If you are seeing the frequency drift slowly over time (and not jump discretely) it's likely what you're observing is a side effect of the oscillator in the RTLSDR slowly changing as a function of temperature and other environmental variables.

Fun test: try deliberately changing the temperature of your RTLSDR and see what happens. A duster can held upside down to spray liquid can get it cold real fast. You could also pop it in the freezer for a minute, then turn it on and warm it with your hand or a blow dryer.

It's also technically possible a similar thing is happening at the transmitter site, though it's much less likely since transmitters are usually engineered to a higher standard.

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    $\begingroup$ Yeah, that's a very good thought. Wandering RTLSDR is definitely more likely than wandering broadcast stations :) $\endgroup$ – hobbs - KC2G Oct 9 at 2:31

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