What is a DX cluster?

A DX cluster is a network of linked servers, called nodes, that share spots: short reports saying which station is on the air, on what frequency, and when. You connect to one node, and any spot entered anywhere on the network reaches you within seconds, so you can tune straight to the DX.

Each node serves its own users and passes traffic to the nodes it is linked to. Besides DX spots, the network carries announcements, WWV propagation reports and talk messages, and most nodes offer utilities such as callsign lookups, beam headings and sunrise and sunset times.

The DX cluster began in the 1980s with PacketCluster, written by Dick Newell, AK1A, for DOS. Users reached their local node over VHF packet radio and nodes were linked by radio. As the internet spread, nodes added telnet access and internet links, and newer programs replaced the original software.

Newell stopped developing PacketCluster around 1992, but its command set and spot format became the standard. In 1998 Dirk Koopman, G1TLH, began DX Spider in Perl, designed to be compatible with the AK1A program; Wikipedia describes it as the most widely used of the internet-based cluster programs.

AR-Cluster, by Terry Gerdes, AB5K, ran on 32-bit Windows with built-in internet links; Gerdes was among the few hams who worked to bring the cluster network onto the internet. Lee Sawkins, VE7CC, first wrote a client for AR-Cluster, then his own server, CC Cluster. Gerdes died in 2018 and VE7CC no longer maintains CC Cluster, yet many nodes still run both programs.

A spot is entered by one operator, the spotter, on the node they are connected to. It carries the spotter's callsign, the frequency in kHz, the DX callsign, a short comment and the UTC time. The node passes it to its linked nodes, and each of them passes it on to its own users.

On DX Spider the command is DX followed by frequency, callsign and remarks, for example DX 14025.0 XX9ABC CW up 2. Nothing checks that the station is really there: a spot is only as good as the operator who sent it, which is why spots for the same station from several spotters are worth more than a single one.

There are three common ways in. A telnet client connects to a node by host name and port and logs you in with your callsign. Logging and contest programs open the same telnet session for you. Web sites and mobile apps read the network and show spots without a terminal.

Telnet gives you the full command set of that node, including your personal filters, which most nodes remember from one session to the next. Logging software adds band maps and click-to-tune. Web pages and apps are the easiest to start with; they usually trade the raw commands for their own filters and alerts.

A skimmer is software that decodes every signal in a slice of spectrum at once and turns the callsigns it hears into spots. The Reverse Beacon Network collects skimmer spots from receiving stations around the world, so you can see who is calling CQ, and where your own signal is heard.

CW Skimmer, by Alex Shovkoplyas, VE3NEA, was released in early 2008 and exports its decodes through a built-in telnet server. The RBN grew out of an e-mail exchange in March 2008 between Felipe Ceglia, PY1NB, and Pete Smith, N4ZR. Today it carries CW and RTTY spots on one telnet feed and FT8 and FT4 on another.

RBN spots look like normal spots, but the spotter callsign ends in -# and the comment holds the mode, the signal-to-noise ratio in dB, the CW speed and the type, for example: DX de LZ4AE-#: 7018.3 RW1M CW 28 dB 18 WPM CQ 2259Z. On DX Spider and CC Cluster you turn them on with SET/SKIMMER.

Read a spot from left to right: who heard it, where, whom, what they said and when. A typical line is DX de IK2ABC: 14025.0 XX9ABC CW up 2 1432Z. IK2ABC spotted XX9ABC on 14025.0 kHz at 14:32 UTC, working CW and listening 2 kHz up.

The callsigns in this example are invented. Some nodes add the spotter's locator, country or state after the time; AR-Cluster can switch that on and off.

A handful covers daily use: SH/DX to list recent spots, filters to cut what you do not want, SH/WWV for propagation reports, SH/SUN for sunrise and sunset, ANNOUNCE for messages and BYE to leave. Syntax differs between DX Spider, AR-Cluster and CC Cluster, so check HELP on your node.

The examples below are for DX Spider unless noted. The trap to know: on DX Spider SH/DX 20 means the last 20 spots, while on AR-Cluster show/dx 20 means spots on 20 metres and show/dx/20 means the last 20.

POTA and SOTA spots come from their own sites, pota.app and SOTAwatch, not from the DX cluster. They identify an activator in a park or on a summit by reference, such as US-1105 or I/LO-001, and self-spotting is normal and accepted, because activators need hunters to find them.

A POTA reference is a country prefix and a number: US-1105 is Mount Nebo State Park in Arkansas, IT-0001 a park in Sardinia. A SOTA reference is association, region and summit: I/LO-001 is Monte Cevedale in Lombardy. POTA spots expire about 30 minutes after the last one, and the POTA rules let hunters re-spot an activator they hear.

An alert is a saved search that runs against every incoming spot. When a spot matches your rule, such as a callsign, a DXCC entity, a band or a park, you get a notification instead of watching the list. The quality of an alert depends on the spots behind it.

Cluster filters decide what reaches your screen; alerts decide what is worth interrupting you for. Keep alerts narrow: a rare entity on the bands you can work, not every spot on 20 metres. A busy station will be spotted many times, so a good alert system groups repeats rather than notifying you for each one.

Spot only stations you have heard or worked, with the correct frequency, and only when the spot is useful to others. Do not spot yourself on the DX cluster, do not use spot comments to chat or thank the DX, and check that nobody has spotted the same station a few minutes earlier.

Self-spotting on the DX cluster is widely considered bad form, and many contest rules, such as those of CQ WW and the IARU HF Championship, prohibit it. POTA and SOTA are the exception: there, activators are expected to spot themselves on the programme's own site. Questions and messages belong in ANNOUNCE, not in a DX spot, and even there sparingly.

HamQSO puts DX cluster, POTA and SOTA spots in one live list on iOS and Android. You can filter by band and mode, set alerts for a callsign, country or park, and see the beam heading and local time at the other end. It is free and needs no account.

Live DX spots come from the DXFun Cluster when you enter your callsign, or from HamQTH without one. The first alert is free; Pro adds unlimited alerts and advanced filters.