Interpreting an unknown signal seen at 147.18MHz

Hi Pranav,

First, welcome to amateur radio—and good on you for diving in with SDR. Your setup using GQRX, an RTL-SDR, and a dipole is a solid start, and your question shows you’re already thinking critically about what you’re seeing.

You mentioned picking up a strong signal at 147.18 MHz, along with visible harmonics, which fade as you shift antenna orientation. A few possibilities come to mind.

While 147.18 MHz falls within the 2-meter amateur band (144–148 MHz), it is not typically used for repeaters in Germany. In North America, yes—this frequency is often assigned to repeater outputs. But in your region, repeaters tend to cluster closer to 145.6–145.8 MHz. That does not rule out amateur activity altogether, but it does make it less likely that this is a standard ham signal.

More probable is that you are observing a case of strong local interference or front-end overload. The RTL-SDR is prone to both, particularly indoors. Nearby electronics—LED lighting, USB power supplies, your own computer, even the dongle itself—can all emit strong broadband noise or spurious emissions. When your antenna picks that up in the near field, the result can look exactly like what you describe: a strong carrier with visible harmonics that vary as you rotate the antenna.

Also worth noting: RTL-SDRs are easily overwhelmed by nearby FM and broadcast signals. This can generate intermodulation products—phantom signals that show up where they should not. Adding a band-pass filter or attenuator can sometimes help. In some cases, just repositioning the antenna or switching USB ports can make a difference.

Here’s one simple test: swap your dipole for a 50-ohm dummy load. If the signal remains, it’s coming from something local—very likely in your immediate setup. If it disappears, you’re looking at a signal that your antenna is picking up externally.

So, in short: I doubt this is an actual amateur transmission. More likely, you’re seeing a strong local artifact—common when starting out with SDR, and a useful learning opportunity. Keep experimenting. You’re on the right track.

73

Jim / K3MRI

I like Jim’s answer, but you might try inserting a step attenuator in between the SDR and the antenna. That way you can gradually reduce the outside signals.  There are a few kits, or you can study attenuator circuits and build one from scratch. Here is an affordable kit made in the USA.

https://www.qrpkits.com/attenuator.html

Here is another one out of the USA

AT1_AT1 41 dB Step Attenuator

The circuit is simple enough. It just helps having a proper printed circuit board layout so the signal on the input does not sneak over to the output without first going through the attenuation circuit.

It would be very interesting to find out if there is a transmitter at 147.18 MHz. This has happened before. A company can make a product this does not follow the legal standards. They don’t imagine that someone might notice.

Andy WA3LTJ