Cuba Jamming Ham Radio? Listen For Yourself

A public SDR network triangulates the island as the source of mystery signals

6 min read
A modern home amateur ham radio station.
A modern home amateur ham radio station. Equipment includes a solid-state radio transceiver and a desktop computer interfaced to work with the radio.

As anti-government protests spilled onto the streets in Cuba on July 11, something strange was happening on the airwaves. Amateur radio operators in the United States found that suddenly parts of the popular 40-meter band were being swamped with grating signals. Florida operators reported the signals were loudest there, enough to make communication with hams in Cuba impossible. Other operators in South America, Africa, and Europe also reported hearing the signal, and triangulation software that anyone with a web browser can try placed the source of the signals as emanating from Cuba.

Cuba has a long history of interfering with broadcast signals, with several commercial radio stations in Florida allowed to operate at higher than normal power levels to combat jamming. But these new mystery signals appeared to be intentionally targeting amateur radio transmissions. A few hours after the protest broke out on the 11th, ham Alex Valladares (W7HU) says he was speaking with a Cuban operator on 7.130 megahertz in the 40-meter band, when their conversation was suddenly overwhelmed with interference. “We moved to 7170, and they jam the frequency there,” he says. Valladares gave up for the night, but the following morning, he says, “I realize that they didn't turn off those jammers. [Then] we went to [7]140 the next day and they put jamming in there.”

Valladres explains he escaped from Cuba to the United States in a raft in 2005. Like many hams in the large Cuban-American community in Florida, he frequently talks with operators in Cuba, and now he says the government there is “jamming the signal to prevent the Cuban people who listen to us and to prevent them from talking between them[selves].” Valladres has also heard reports that VHF 2-meter band repeaters have been shut down in Cuba.

Two-meter band radios are typically low-power handheld walkie-talkies used for short-range communications. Their range is often extended by using fixed relay repeaters, which retransmit an incoming signal using a more powerful transmitter and a well-placed antenna. Because Florida and Cuba are so close—only about 175 kilometers separates them at their closest point—it’s possible for 2-meter communications to cross the distance. “It was possible to go between Miami and Havana … with an external antenna you can talk to Havana easy because it’s not that far, it’s like 230 miles away,” says Valladres.

The short distance between southern Florida (with its large Cuban American population) and Cuba may also be one reason why the 40-meter band is plagued with interference while other shortwave bands used for long range ham radio, such as the 20-meter band, have been left untouched. Shortwave signals travel long distances by bouncing between the Earth and layers in the ionosphere high above. Signals in different bands are reflected by different layers, and these layers change depending on whether it is day or night. (Dealing with the vagaries of ionospheric propagation is one of the fun challenges of ham radio as a hobby.) The height of these layers also effectively sets a minimum range for a band, known as the skip distance. For the 20-meter band, the skip distance varies between 700 and 1600 kilometers in length, making it unsuitable for communications between southern Florida and Cuba.

During daylight hours, however, the skip distance of the 40-meter band allows much shorter range communications. “The 20-meter band…doesn’t allow you to work with Havana,” says Valladres, “But the 40-meter band… it is possible to work and talk all day long.”

Another factor is that 40-meter transceivers are relatively simple to build and maintain. “In Cuba, there are a lot of people that have homemade radios. And those homemade radios are [for the] 40-meter band, which is the easiest band to make a homemade radio,” says Valladres.

Valladres alerted other hams to the interference, and soon operators were comparing notes on a forum on the website for hams. Hams across the southern United States and as far away as Minnesota and Rhode Island as well as Suriname in South America reported picking up the signals. The hams soon turned to nailing down the source of the signal. In previous decades, locating the source of a distant shortwave signal would have required special direction finding equipment, but modern hams have an ace up their sleeves in the form of the public KiwiSDR network.

A KiwiSDR is a software defined radio board that attaches to a BeagleBoard computer running Linux. It can receive signals from 10 kilohertz to 30 MHz. Network-enabled, anyone with a web browser can access a public KiwiSDR station and tune in to whatever it is receiving. Crucially, the KiwiSDR software allows users to sample multiple stations around the world simultaneously. If at least three stations can hear a given signal, a TDoA (time difference of arrival) algorithm can estimate the origin of the signal.

Josh Nass (KI6NAZ), who is based in California and hosts the Ham Radio Crash Course channel on YouTube, was one of the first to use the network to locate the source. The weekend when the Cuban protests started, he says, “I noticed the interference covering a lot of the [40-meter] band. Then I had individuals reach out to me, Cuban-Americans living around the country but a lot of them out of Miami, and they said ‘we think there’s a coordinated jamming effort going on…’” This made him turn to the KiwiSDR network and the TDoA algorithm. “Sure enough, in parallel a lot of my friends were also doing the same, and we largely had the same [result], that it looks like the signals are coming from the eastern side of Cuba.”

As of this writing, the interference can still be heard via the KiwiSDR network: you can easily use a map interface to pick a station in Florida, such as the one operated by W1NEJ in Boca Raton, and listen in and then try your hand at locating the source.

If you’re new to ham radio or KiwiSDR, here are a few pointers: Set the frequency to 7106.5 to start with in the control box, and make sure to click the button for “LSB” to make sure you’re getting the right kind of demodulation to best hear the signal, which sounds like the unfortunate offspring of a frog and a Dalek. You can then use the “extension” drop down menu to select the “TDoA” option, which will give you a map of other stations you can combine with Boca Raton to localize the source.

“What you try to do is get a good sampling of SDRs that can all receive the signal that you want to detect, and that’s the tricky part of it, because you have to log in all these SDRs, go to the signal you are looking to triangulate, and make sure they can all hear it” before using the TDoA algorithm, advises Nass.

Alain Arocha (K4KKC) is a Florida-based ham who noticed the interference early and who has also been active on the QRZ forum. He agrees that the KiwiSDR software can give misleading results if not used correctly, so he went the extra mile and verified the ability of the KiwiSDR stations that he was using for his location hunting by transmitting his own test signal on another frequency and making sure the SDRs could receive it. Arocha says he’s been disappointed by the lack of response from some hams, who view the interfering signals as curiosity as they can deal with it by simply shifting to other bands or using digital modes unavailable to Cuban operators with their more basic equipment.

“The response has been real strong from the people who talk to Cuba, but some people couldn’t care less [as long it doesn't affect them], but this is making that part of the spectrum unusable,” says Arocha, saying he’s seen operators from Germany and Spain report the interference. “It gives me a sense of frustration for people not to give a damn about this.” In particular, Arocha is annoyed that the American Radio Relay League (ARRL) is not being vocal about the issue.

“We are aware of reports from radio amateurs of non-amateur signals observed in the amateur bands, and mostly likely originating in the direction of Cuba,” says the ARRL’s Bob Inderblizten (NQ1R). However, because American hams have access to so much spectrum and so the overall impact is limited in the United States, and because the ARRL is a national rather than international organization, “There’s no real role for ARRL” in this situation, says Inderblizten. 

However, he adds, “there’s a mechanism for amateurs to report intruders on the amateur band through the network of amateur network societies, as organized within the International Amateur Radio Union.” The IARU relies on national governments to enforce regulations, so if the interference is indeed being generated by the Cuban government, any notification is likely to have little effect. More pointed complaints would have to come from the US government, “and such bold and publicly reported interference is most certainly known by FCC and other government agencies,” says Inderblizten.

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