I live in an apartment. I don't have control over all of the appliances but perhaps I can mitigate if I know where the noise is coming from and under what circumstances.

If I'm shopping for such a detection device, what are the keywords I should be using? What is the name of the device I am looking for? Could I make my own? How tough would that be?

Edit: It's occurred to me that the RFI at this location could be coming from more than just Common Mode. The massive radio tower near the National Voice of American Museum is nearly within sight from here and I can see a cell tower also. Not so much for power transmition cables. They all seem to be underground in the immediate area of this complex. Maybe I'm just too close to these?

National Voice Of America Museum

Nearby Radio Towers

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    $\begingroup$ If it were me I would want to learn how to track down RFI myself first, but I would like to mention that your local ham club may have people with equipment and experience in solving RFI problems who may be glad to help. $\endgroup$
    – rclocher3
    Jul 9, 2020 at 13:31
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    $\begingroup$ What does the noise sound like? If you put your radio in AM mode, does it sound like a buzz that occurs over a wide band rather than a single frequency? Listen to the sound posted on this question; does yours sound like that? If so, it is rather doubtful that it is coming from those towers you mentioned. $\endgroup$ Jul 10, 2020 at 19:58
  • $\begingroup$ One problem is that I'm a fairly new Ham and I've never been at a location other than this one so I'm having a tough time being certain I know what a baseline for noise should look like. Today, I tried reaching my brother in MI and that did not go so well but we could both hear someone in Tennessee. $\endgroup$ Jul 12, 2020 at 5:55

3 Answers 3


How do you know the appliances are noisy? Are you hearing noise in your radio? If so, you already have the necessary detection equipment.

You can do OK simply unplugging things or flipping circuit breakers to see what makes a difference, and locating the noise source by process of elimination. It can help to have a battery to power your radio: then you can turn off your main breaker and know if the noise sources are even in your apartment.

A battery also means you can move around with the radio, and you can find the noise source like playing a game of hot and cold.

A portable radio and/or directional antenna can also be useful. There is an activity called fox hunting where the participants try to locate a hidden radio beacon. This is essentially the same problem as finding a noise source, and the same equipment is applicable.

  • $\begingroup$ I'm having a hard time hearing much of anything. I've tried several antennas and I have tuner so I don't think that is the issue. I've also tried just a tuned wire. This is one of those out the window type angled antennas so the angle could be the issue. It's been hard to nail down. I've added ferrite cores and noted improvement in some areas. $\endgroup$ Jul 10, 2020 at 2:42
  • $\begingroup$ I started with one of those MFJ deals but removed the coils when I added the tuner. I can just barely hear my brother in Michigan at times but I'm on the Ohio River near Cincy. I can't make out what he's saying but I know it's him because we check it against a phone call. $\endgroup$ Jul 10, 2020 at 2:44
  • $\begingroup$ Same radio performed well at field day. So it's my location or the power in my apartment, I guess. $\endgroup$ Jul 10, 2020 at 2:45
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    $\begingroup$ @NonYaBidnezz yeah, getting the antenna even just a little bit away from a building, with all the switching power supplies and electrical cable running everywhere, can make a really big difference. I'd say first step is to get a battery that can run your radio just on receive, then flip off the main breaker to your apartment. Then you have a best case scenario for what could be achieved by reducing noise coming from appliances in your apartment. If that's still not good enough, look for a better place for the antenna. $\endgroup$ Jul 10, 2020 at 4:38
  • $\begingroup$ What I can try is to deploy to the lawn outside with my super-antenna, I guess. Can't shut down the house for 2 days as the freeze dryer is running. $\endgroup$ Jul 10, 2020 at 11:00

For first-pass broadband EMI fox-hunting, I use an old pocket-sized transistor AM radio with a ferrite loopstick antenna. You can get them as electronic kits, which makes for a nice educational construction project for the kids. When constructing such a kit, I left the loopstick antenna exposed as an LF/MF EM sniffer probe.

I tune between stations, so that I only hear hiss when away from anything powered. The old AM radio will start buzzing loudly when I get roughly on the order of a meter or so from the offending (LED lamp, USB wall wart, appliance, etc.) EMI emitter.

This is several orders of magnitude cheaper than a spectrum analyzer, and vastly more portable as well, for finding what to try unplugging. Then recheck with your HF/VHF radio to see if the noise floor looks better.


If I'm shopping for such a detection device, what are the keywords I should be using? What is the name of the device I am looking for?

Now, I'm interepreting this as

I'm looking for sources of radio-frequency noise, and I need to find them, for reasons that are professional and not limited to a specific interference ocurrence.

The device that you want to use for that purpose is a Spectrum Analyzer. It is a measurement device that shows at which frequencies there is power "in the air". It typically works over a large range of frequencies, e.g. from 800 kHz up to 8 GHz.

You'll also want a set of antennas that work on a very wide band, so you don't have to switch antennas for every single band you test.

Just like the me measurement device, if you want these power readings to be physically meaningful, your antennas need to be calibrated, if you want the output to actually represent the power that is in the air. This is kind of important, considering "off by a factor of 100" not unusual for antenna properties for the same antenna at different frequencies.

Such (entry-level) devices look like this

Rohde und Schwarz handheld spectrum analyzer

or this

Keysight Desktop spectrum analyzer

You'll pretty quickly notice the price of these things is in the region of "car" if you have modest requirements. (There's literally no limit upwards – at some point you get into the range where Keysight will offer you a custom solution, but then you're in the ballpark of megaeuros.) The price very much scales with the range of frequencies that can be observed, so in the interest of your savings account, you will need to first understand what you're trying to measure ("noise" isn't really telling us anything), and then define requirements for your device from that.

There are things on amazon that are scams, but try to take money from people worried about "noise" with no understanding of what that is. Don't buy them, they are a scam. I've covered that before.

Could I make my own?

Technically: yes. A spectrum analyzer is a technical device, and given theoretically unlimited resources, anything in existence is possible to engineer.

How tough would that be?

Since you had to ask what kind of device you needed, and it's (and that's OK) not clear you fully understand what you're measuring there: In all honesty, very tough.

An electrical engineering degree with a focus on RF electronics, plus signal processing and estimation knowledge would be a plus.

It takes decades of experience and large engineering teams to produce sensible spectrum analyzers.

You could try to implement one based on an SDR, but the problem will be making it reliable enough so that a calibration would still mean something the next time you turn it on. Also, there's analog reasons that SDRs are available cheaper than spectrum analyzers, and it's that the filters and dynamic ranges necessary for a measurement device are simply way more expensive to buy or build than what SDRs come with.

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    $\begingroup$ A spectrum analyzer to track down something generating RFI seems like an excessively expensive solution. $\endgroup$ Jul 9, 2020 at 12:49
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    $\begingroup$ Read @PhilFrost-W8II 's answer, Marcus. Thousands of RFI sources have been successfully tracked down with very simple equipment. What would be the advantage of a spectrum analyzer over a portable AM radio? $\endgroup$ Jul 9, 2020 at 15:54
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    $\begingroup$ @MarcusMüller You can read my answer to see what I propose. What I want to know is why you need to accurately quantify the field intensity to find and unplug a noisy switching power supply. $\endgroup$ Jul 9, 2020 at 17:06
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    $\begingroup$ @PhilFrost-W8II and I like your approach, as it targets the source of observed interference: If you can't notice its presence with your radio, it's not a problem (to you)! Comparing mine and your answer, I have taken OP's question "what is the device used to detect noisy appliances" more in a literal, maybe. The appliance is noisy doesn't mean its interfering with your electronics so far. $\endgroup$ Jul 9, 2020 at 17:09
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    $\begingroup$ @MarcusMüller I think that's the problem. This is the amateur radio stackexchange, so it's a safe assumption the question is about reducing interference to an amateur radio receiver, and not conducting rigorous EMC testing on every appliance in in the apartment. And you've answered, "sorry, it's basically impossible, give up" when really this is a problem that's easily solved for as little as $0 incremental cost. $\endgroup$ Jul 9, 2020 at 17:23

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