Playing music with computer-emitted interference

RFI-Sony-7600GSWLing Post reader, Chris Smolinski, writes:

Here’s my Mac Book Pro (2010 model) playing Mary Had A Little Lamb over the radio, by modulating the RFI (Radio Frequency Interference) produced by it and other computers. As picked up on a Sony 7600G receiver. I found the best reception was on the long wave band, although I could continue to hear it well above the AM (medium wave) band, past 1700 kHz. The signal was pretty much everywhere, no matter where I tuned, in 1 kHz steps.

Picking up radio emissions from computers is one method that can be used to spy on them.

I used the source code available here:

If you want to see some radio related software I’ve written, please visit

Click here to view on YouTube.

That is fascinating, Chris. While I was well aware that computers and mobile devices (of all stripes) produce RFI, I had no idea that it could be used for spying. I love how you’ve manipulated this interference to play a tune! What a creative hack!

Guest Post: London Shortwave’s guide to mitigating urban radio interference

London-Urban-CityMany thanks to SWLing Post contributor, London Shortwave, who is kindly sharing this guest post–a brilliant article he recently posted on his own website.

I’m very grateful: one of the most common questions I’m asked by readers is how to cope with the radio interference so many listeners and amateur radio operators experience in high-density, urban areas. If this is you, you’re in for a treat–just keep reading:

Dealing with Urban Radio Interference on Shortwave

by London Shortwave

Shortwave radio listening is an exciting hobby, but for many of us city dwellers who either got back into it recently or tried it out for the first time not long ago, the first experience was a disappointing one: we could barely hear anything! Station signals, even the supposedly stronger ones, were buried in many different types of static and humming sounds. Why does this happen? The levels of urban radio frequency interference, or RFI, have increased dramatically in the last two decades and the proliferation of poorly engineered electronic gadgets is largely to blame. Plasma televisions, WiFi routers, badly designed switching power adapters and Ethernet Over Powerlines (also known as powerline network technology, or PLT) all severely pollute the shortwave part of the radio spectrum.

Does this mean we should give up trying to enjoy this fascinating medium and revert to using the TuneIn app on our smartphones? Certainly not! There are many angles from which we can attack this problem, and I shall outline a few of them below.

Get a good radio

The old adage “you get what you pay for” certainly holds true even when it comes to such “vintage” technologies as shortwave radio. Believe it or not, a poorly designed receiver can itself be the biggest source of noise on the bands. That is because many modern radios use embedded microprocessors and microcontrollers, which, if poorly installed, can generate interference. If the receiver comes with a badly designed power supply, that too can generate a lot of noise.

So how does one go about choosing a good radio? and have fantastic radio review sections, which will help you choose a robust receiver that has withstood the test of time. My personal favourites in the portable category are Tecsun PL310-ET and Tecsun PL680. If you want a desktop radio, investigate the type of power supply it needs and find out whether you can get one that generates a minimal amount of noise.

It is also worth noting that indoor shortwave reception is usually best near windows with at least a partial view of the sky.

Tecsun PL310-ET and Tecsun PL680, my two favourite portable shortwave radios.

Tecsun PL310-ET and Tecsun PL680, my two favourite portable shortwave radios.

Identify and switch off noisy appliances

Many indoor electrical appliances generate significant RFI on the shortwave bands. Examples include:

  • Plasma televisions
  • Laptop, and other switching-type power supplies
  • Mobile phone chargers
  • Dimmer switches
  • Washing machines / dishwashers
  • Amplified television antennas
  • Halogen lighting
  • LED lighting
  • Badly constructed electrical heaters
  • Mains extension leads with LED lights

Identify as many of these as you can and switch them all off. Then turn them back on one by one and monitor the noise situation with your shortwave radio. You will most likely find at least a few offending devices within your home.

Install an outdoor antenna

If you have searched your home for everything you can possibly turn off to make reception less noisy but aren’t satisfied with the results, you might want to look into installing and outdoor antenna. That will be particularly effective if you live in a detached or a semi-detached property and have a garden of some sort. Of course, you will need a radio that has an external antenna input, but as for the antenna itself, a simple copper wire of several metres will do. An important trick is making sure that the noise from inside your home doesn’t travel along your antenna, thus negating the advantage of having the latter installed outside. There are many ways of achieving this, but I will suggest a configuration that has worked well for me in the past.

Fig.1 Schematic for an outdoor dipole antenna.

Fig.1 Schematic for an outdoor dipole antenna.

I have used a three-terminal balun (positioned outdoors), and connected two 6 metre copper wires to its antenna terminals to create a dipole. I then connected the balun to the radio indoors through the feed line terminal using a 50? coaxial cable. In the most general terms, the current that is generated in the antenna wires by the radio waves flows from one end of the dipole into the other, and a portion of this current flows down the feed line into your radio. The balun I have used (Wellbrook UMB130) is engineered in a way that prevents the radio noise current from inside your house flowing into the receiving part of the antenna.

Wellbrook UMB130 balun with the feed line terminal disconnected

Wellbrook UMB130 balun with the feed line terminal disconnected

Antenna preselectors

There is a catch with using an outdoor antenna described above — the signals coming into your radio will be a lot stronger than what would be picked up by the radio’s built-in “whip” antenna. This can overload the receiver and you will then hear many signals from different parts of the shortwave spectrum “mixing in” with the station you are trying to listen to. An antenna preselector solves this problem by allowing signals from a small yet adjustable part of the spectrum to reach your radio, while blocking the others. You can think of it as an additional tuner that helps your radio reject unwanted frequencies.

Fig.2 Schematic of a preselector inserted between the outdoor antenna and the receiver

Fig.2 Schematic of a preselector inserted between the outdoor antenna and the receiver

There are many antenna preselectors available on the market but I can particularly recommend Global AT-2000. Although no longer manufactured, many used units can be found on eBay.

Global AT-2000 antenna coupler and preselector

Global AT-2000 antenna coupler and preselector

Risk of lightning


Any outdoor antenna presents the risk of a lightning strike reaching inside your home with devastating and potentially lethal consequences. Always disconnect the antenna from the receiver and leave the feed line cable outside when not listening to the radio or when there is a chance of a thunderstorm in your area.

Get a magnetic loop antenna

A broadband loop antenna (image courtesy of

A broadband loop antenna (image courtesy of

The outdoor long wire antenna worked well for me when I stayed at a suburban property with access to the garden, but when I moved into an apartment well above the ground floor and without a balcony, I realised that I needed a different solution. Having googled around I found several amateur radio websites talking about the indoor use of magnetic loop receive-only active antennas (in this case, “active” means that the antenna requires an input voltage to work). The claim was that such antennas respond “primarily to the magnetic field and reject locally radiated electric field noise”[*] resulting in lower noise reception than other compact antenna designs suitable for indoor use.

Interlude: signal to noise ratio

In radio reception, the important thing is not the signal strength by itself but the signal to noise ratio, or SNR. A larger antenna (such as a longer copper wire) will pick up more of the desired signal but, if close to RFI sources, will also pick up disproportionately more of the local noise. This will reduce the SNR and make the overall signal reading poorer, which is why it is not advisable to use large antennas indoors.

The other advantage of a loop antenna is that it is directional. By rotating the loop about its vertical axis one can maximise the reception strength of one particular signal over the others, once the antenna is aligned with the direction from which the signal is coming (this is termed “peaking” the signal). Similarly, it is possible to reduce the strength of a particular local noise source, since the loop is minimally sensitive to a given signal once it is perpendicular the latter’s direction (also known as “nulling” the signal).

It is further possible to lower the effect of local noise sources by moving the antenna around. Because of the antenna’s design, the effect of radio signals is mostly confined to the loop itself as opposed to its feed line. Most local noise sources have irregular radiation patterns indoors, meaning that it is possible find a spot inside your property where their effects are minimised.

Many compact shortwave loop antennas require an additional tuning unit to be attached to the loop base (much like the preselector described above) but broadband loops do not. Wellbrook ALA1530S+ is one such antenna that is only 1m in diameter, and it was the one I chose for my current apartment. I was rather impressed with its performance, although I found that I need to use a preselector with it as the loop occasionally overloads some of my receivers when used on its own. Below is a demo video comparing using my Tecsun PL680’s built-in antenna to using the radio with the Wellbrook loop.

As you can hear, there is a significant improvement in the signal’s readability when the loop is used.

Experiment with a phaser

Although the loop antenna dramatically reduces the levels of ambient RFI getting into the radio,  I also have one particular local noise source which is way too strong for the loop’s nulling capability. Ethernet Over Powerlines (PLT) transmits data across domestic electrical circuits using wall socket adapters, as an alternative to wireless networking. It uses the same frequencies as shortwave, which turns the circuits into powerful transmitting antennas, causing massive interference. One of my neighbours has PLT adapters installed at his property, which intermittently become active and transmit data. When this happens,  it is not merely noise that is generated, but a very intense data signal that spreads across the entire shortwave spectrum, obliterating everything but the strongest stations underneath. Fortunately, a mature piece of radio technology called antenna phasing is available to deal with this problem.

Fig.3 The principle of antenna phaser operation (adapted from an original illustration in Timewave ANC-4's manual)

Fig.3 The principle of antenna phaser operation (adapted from an original illustration in Timewave ANC-4’s manual)

Signal cancellation using phase difference

A phaser unit has two separate antenna inputs and provides one output to be connected to the radio’s external antenna input. The theory of phase-based signal cancellation goes roughly as follows:

  • The same radio signal will arrive at two different, locally separated antennas at essentially the same time.
  • The phase of the signal received at the first antenna will be different to the phase of the same signal received at the second antenna.
  • This phase difference depends on the direction from which the signal is coming, relative to the two antennas.
  • The phaser unit can shift the phases of all signals received at one antenna by the same variable amount.
  • To get rid of a particular (noise) signal using the phaser unit:
    • the signal’s phase at the first antenna has to be shifted by 180° relative to the signal’s phase at the second antenna (thus producing a “mirror image” of the signal received at the second antenna)
    • its amplitude at the first antenna has to be adjusted so that it is the same as the signal’s amplitude at the second antenna
    • the currents from the two antennas are then combined by the unit, and the signal and its mirror image cancel each other out at the unit’s output, while the other signals are preserved.

Noise sampling antenna considerations

To prevent the possibility of the desired signal being cancelled out together with the noise signal — which can happen if they both come from the same direction relative to the antennas — one can use the set-up illustrated in Figure 3, where one antenna is dedicated to picking up the specific noise signal, while the other is geared towards receiving the desired broadcast. That way, even if the phases of both the noise and the desired signals are offset by the same amount, their relative amplitude differences will not be the same, and thus removing the noise signal will not completely cancel out the desired signal (though it will reduce the latter’s strength to some extent).

It is possible to use any antenna combination for phase-based noise signal cancellation. However, one has to be careful that, in the pursuit of removing a specific noise source, one does not introduce more ambient RFI into the radio system by using a poorly designed noise-sampling antenna. After all, the phaser can only cancel out one signal at a time and will pass through everything else picked up by both antennas. This is particularly relevant in urban settings.

For this reason, I chose my noise sampling antenna to also be a Wellbrook ALA1530S+. The additional advantages of this set-up are:

  • It is possible to move both loops around to minimise the amount of ambient RFI.
  • By utilising the loops’ directionality property, one can rotate the noise sampling loop to maximise the strength of the noise signal relative to the desired signal picked up by the main antenna loop.
Two Wellbrook ALA1530S+ antennas combined through a phaser

Two Wellbrook ALA1530S+ antennas combined through a phaser

And now onto the phaser units themselves.

Phaser units


DX Engineering NCC-1 (image courtesy of

I have experimented at length with two phaser units: the MFJ 1026 (manual) and DX Engineering NCC-1 (manual). Both solve the problem of the PLT noise very well, but the NCC-1 offers amplitude and phase tuning controls that are much more precise, making it a lot easier to identify the right parameter settings. Unfortunately this comes at a price, as the NCC-1 is a lot more expensive than the MFJ unit. As before, a preselector is needed between the phaser and the radio to prevent overloading.

Below is a demo of DX Engineering NCC-1 at work on my neighbour’s PLT noise. I have chosen to use my SDR’s waterfall display to illustrate the nefarious effect of this type of radio interference and to show how well the NCC-1 copes with the challenge.

Cost considerations

Fig.4 Final urban noise mitigation schematic

Fig.4 Final urban noise mitigation schematic

It would be fair to say that my final urban noise mitigation set-up, shown in Figure 4, is quite expensive: the total cost of two Wellbrook antennas ($288.38 each), a DX Engineering phaser ($599.95) and a Global AT2000 preselector ($80) comes to $1257. That seems like an astronomical price to pay for enjoying shortwave radio in the inner city! However, at this point another old saying comes to mind, “your radio is only as good as your antenna”. There are many high-end shortwave receivers that cost at least this much (e.g. AOR AR7030), but on their own they won’t be of any use in such a noisy environment. Meanwhile, technological progress has brought about many much cheaper radios that rival the older benchmark rigs in terms of performance, with Software Defined Radios (SDRs) being a particularly good example. It seems fair, then, to invest these cost savings into what makes shortwave listening possible. You may also find that your RFI situation is not as dire as mine and you only need some of the above equipment to solve your noise problems.

Filter audio with DSP

If you have implemented the above noise reduction steps but would still like a less noisy listening experience, consider using a Digital Signal Processing (DSP) solution. There are a number of different approaches and products available on the market, and I shall be reviewing some of them in my next post. Meanwhile, below are two demo videos of using DSP while listening to shortwave. The first clip shows the BHI Compact In-Line Noise Elimination Module at work together with a vintage shortwave receiver (Lowe HF-150). The second video compares using a Tecsun PL-660 portable radio indoors on its own and using the entire RFI mitigation set-up shown in Figure 4 together with a DSP noise reduction feature available in the SDR# software package, while using it with a FunCube Dongle Pro+ SDR. As a side note, it is worth remembering that while DSP approaches can make your listening experience more pleasant, they can’t recover what has been lost due to interfering signals or inadequate antenna design.

Set up a wireless audio relay from your radio shack

The above RFI mitigation techniques can result in a rather clunky set-up that is not particularly portable, confining the listener to a specific location within their home. One way to get around this is by creating a wireless audio relay from your radio shack to the other parts of your house. I did this by combining the Nikkai AV sender/receiver pair and the TaoTronics BA01 portable Bluetooth transmitter:

Head for the outdoors!

So you have tried all of the above and none of it helps? As a last resort (for some, but personally I prefer it!), you can go outside to your nearest park with your portable radio. After all, if shortwave listening is causing you more frustration than joy it’s hardly worth it. On the other hand, you might be surprised by what you’ll be able to hear with a good receiver in a noise-free zone.


Many of the above tricks and techniques were taught to me by my Twitter contacts. I am particularly grateful to @marcabbiss@SWLingDotCom, @K7al_L3afta and@sdrsharp for their advice and assistance over the years.

Thank you–!

What I love about my buddy, London Shortwave, is that he didn’t give up SWLing just because his home is inundated with radio interference–rather, he saw it as a challenge. As you can see, over the years, he has designed a system that effectively defeats radio interference.

I also love the fact that he uses an even more simple approach to defeating RFI: he takes his radio outdoors. A kindred spirit, indeed.

I encourage all SWLing Post readers to bookmark and search London Shortwave’s website. It’s a treasure trove for the urban SWL. We thank him for allow us to post this article in its entirety.

RFI: The Sound Kitchen now on shortwave radio

RFI-Logo-World-LargeMany thanks to SWLing Post reader, Mike who notes that Radio France International’s program, The Sound Kitchen, is now available over shortwave radio.

The Sound Kitchen staff reported this news in the show notes of their latest episode, Pieces of Eight:

“Fabulous news! We have a shortwave frequency again! It’s 13725 kHz, on the 22m band; you can hear us between 6.00 and 7.00 UT every day! We’ve had reception reports from New Zealand, Japan, Bangladesh and the UK, and although the frequency is “aimed” (or however that works) towards the African continent, give it a try! You never know!”

Fabulous news, indeed! Though–as they state–North America is not the target of this broadcast frequency and time, I will certainly be listening as night time openings on the 22M band are certainly possible. I imagine those of you in Africa, Asia and the Pacific can receive The Sound Kitchen with relative ease.

Richard’s search to find the best SWLing spot on campus


SWLing Post contributor, Richard Langley, has been seeking the perfect spot on the campus of the University of New Brunswick (UNB) to listen to shortwave. He recently shared the following:

Here’s a link to a brief video of my recording of last weekend’s VOA Radiogram “in the field” (a UNB parking lot):

Richard goes on to say that he’s found an even better location:

receiver_locations_smaller (1)

“That location on campus (green pin on attached image) turned out to not be noise-free on all bands. Found an even better location (red pin). Negligible power-line interference although still within Wi-Fi range of UNB’s system but no significant effects from that discovered yet. Got excellent reception of VOA’s Radiogram this past Saturday afternoon. Extremely clean waterfall in Fldigi. And virtually noise-free images [below]”

voa_radiogram (1)

Richard’s decoded message. (Click to enlarge)


VOA Radiogram decoded image

Many thanks for sharing this with us, Richard!

Those of you who live or work in areas with significant radio noise should consider scouting out a listening spot like Richard has. Also, you might be inspired by LondonShortwave who takes his radios to public parks. Regardless, moving your receiver as far away from sources of radio interference as possible will always yield better listening results.

Dave says not all Jameco power supplies are linear and regulated

Note that not all power supplies are listed as "regulated linear"

Note that not all power supplies are listed as “regulated linear”

Many thanks to SWLing Post contributor, Dave Zantow (N9EWO), who replies to a post published yesterday regarding some of Jameco linear power supplies. Dave writes:

“A bit of a caveat on Jameco’s these so called Linear power supplies. This is based from my own experiences so is not fiction.

Bottom of this page :

Over the years some of (but not all) these Jameco linear regulated power supplies are no longer clean for radio use.

Without changing the model number or description of the product, they have made changes with some (or much of ??) this “Linear Regulated” adapter line. Indeed they are still using a good old power transformer, but when it comes to the regulator part of the adapter, they have gone to switching type regulator device. So it produces a nice strong whine on a radio receiver just as a full fledged switching supply.

I had purchased a number of these so called linear supplies (sorry I no longer have the exact model number noted that I ordered) and experienced awful interference with any radio receiver. So I cracked open one of these to see what was up here and sure enough it was using a MC34063A inverting switching regulator .

Called Jameco and they flat out denied that they were using any switching devices in this Regulated LINEAR Jameco ReliaPro adapter. So I then sent a nasty gram email to the CEO of Jameco. I received an email back (was from the CEO too) and after some research they FINALLY did admit a change was made in some of the product line to use of a switching regulator . But he strongly made the point they would continue to still market these adapters as totally linear (yeah right ….nice guys).

I must add here that it does (or did not) NOT affect the entire line of these linear regulated adapters. About a year ago I ordered more (already had a few before) of the 12 volt 1 AMP model 170245 , and these are (or were anyway) totally clean and are excellent.

Also note that Jameco purchases up surplus “linear regulated” adapters from time to time. This 6 volt 500 ma one here is an example and is (or was anyway) nice clean one and uses no switching regulators. Our 2 tested samples of this adapter from about 5 years ago used a nice 7806 analog regulator. Perfect for use with many SW portables, (including the Sony ICF-SW7600GR with a plug change). But a warning again from experience , they are all subject to changes without any warning (and this one may have changed too for all we know ??)

They appear to stick the ReliaPro name as the manufacture on all adapters (if it was made by Jameco or not)

So Caveat Emptor.”

Duly noted, Dave! I’ve also noted that not all of the power supplies on their linear power supply page are listed as being a linear supply (see screen grab at top of page).

I may contact Jameco about this too and see if they can adjust their search results to properly reflect a selection of regulated linear supplies.

How to replace a noisy wall power supply

JamecoWallPowerSupplyRegarding noisy switching power supplies, SWLing Post reader, Dan Lewis, comments:

“Google the following: “Jameco linear wall transformer”, and you’ll find a suitable non-switching replacement.

Jameco still has a number of linear transformers in their catalog at reasonable prices. I haven’t bought anything from them in many years but when I dealt with them frequently a number of years back they were always reputable.”

Many thanks for your suggestion, Dan! Jameco is a reliable company and I’ve also been a long-time customer. If you know how to pick the proper power supply for your radio (or any other electronic device) click here to view a list of regulated linear supplies on Jameco’s website. [Also note this follow-up post.]Otherwise, keep reading…

How to find a replacement AC adapter/power supply

When you purchase a replacement power supply, you must make sure that several properties match that of the device it will power, else you could cause damage.

There are four properties you need to match: voltage, rated current, polarity and tip size.


Most consumer electronics are powered by and rated for 4.5, 5, 9, 12, or 13.8 volts DC. Of course, there are exceptions. It is important that you match the required voltage exactly. Most radios and electronic devices display their required voltage and voltage tolerance on the unit itself, on the supplied switching power supply, and/or in the owner’s manual.

Rated Current

Like voltage, rated current is usually displayed somewhere on the device, existing power supply or in the owner’s manual. Current is usually indicated in amps (A) or milliamps (mA). Unlike voltage, rated current on your power supply does not have to match the device exactly. You simply need to make sure the power supply meets or exceeds your radio’s required current.

For example, if your radio requires 800 mA (or .8 A) and you find a power supply rated for 500 mA, you should not use it. If you find a power supply rated for 2 amps (or 2000 mA), it exceeds the 800 mA rating, so you’re good to go!

Unlike voltage, your electronic device or radio will only draw the amount of current it needs from the power supply.


Click here to read more about tip polarity. (Source: WikiPedia)

Click here to read more about tip polarity. (Source: WikiPedia)

You’ll need to determine if your radio requires a plug with a positive or negative tip (a.k.a. center conductor).

Fortunately, manufacturers have long used standard symbols to make polarity obvious (see image).

You’ll typically find a polarity symbol printed on the back of your radio, near the plug-in point, in the owner’s manual or on the back of the existing wall adapter.

Note: Be very careful matching polarity! Some radios and electronic devices are not properly protected against reverse polarity; damaged can occur immediately after supplying voltage with incorrect polarity.

TJamecoWallPowerSupplyip/plug size

You need to make sure that the inner diameter and outer diameter of a replacement wall adapter will match that of your existing adapter.

This can be the most difficult property to match.

Occasionally, radio manufacturers will actually specify the tip size in their owner’s manual, spec sheets, or on the product page of their website. I’ve even had luck calling manufacturers and asking a technician for the plug size.


Specification sheets will typically indicate plug dimensions with an illustration.

Otherwise, you can always measure the existing power supply tip (both inner and outer dimensions) using calipers.

Once you have those dimensions, finding the appropriate replacement power supply is quite easy. Indeed, companies like Jameco provide specification sheets (click here for an example) that indicate dimensions for each power supply they sell.

Once you’ve matched the voltage, rated current, polarity and tip size, you can purchase a regulated linear power supply with confidence!

Keep in mind: there are most likely other devices in your home with noisy switching power supplies that could be causing radio interference. Check out my noise trouble shooting section of this article to help identify local sources of radio noise.

Update: check out this follow-up post regarding Jameco power supplies–not all are truly linear regulated.

Sangean blames AM interference on power supply and government regulation

Sangean-AMFM-RadioAfter the Sangean WR-15 received low marks for AM reception in an Amazon review, Bob of Sangean America replied that poor reception is due to the radio’s switching power supply–a design that is federally mandated.

Many thanks to Jeff over at the Herculodge for posting this (click here to read the full response).

It’s a shame the WR-15 can’t accommodate internal batteries as battery operation this would solve the problem.

If I owned the WR-15, I would simply replace the switching type power supply with a regulated power supply.

Looking at the back of the WR-15 (below), it appears it requires 12 volts DC, 1.2 amps and an adapter with a positive center tip. Though I’m judging this only from the image, the plug looks to be a common size.WR-15-back I bet I have a power supply that would fit the bill in my junk drawer.

Bob, at Sangean America, claims moving the radio at least one foot from the power supply should help. In truth, I believe much of the noise may be conveyed by the power cord itself, though I may be wrong.

It’s a shame Sangean engineers couldn’t compensate somehow for the noisy power supply as it seems this radio was actually marketed to AM radio enthusiasts.