Read the full article in the December 2017 issue of Radio User
Author: Robert Connolly

Robert Connolly expands on some of the NDB antenna matters raised in his last column and offers tips regarding maritime beacon reception and on a Ukrainian online SDR receiver for NDB monitoring. 

Antenna Matters and Online Beacon Reception 

My diagram of an NDB station in my last column (RadioUser, October 2017: 35) might have caused some slight confusion with some readers. The image was a simplified sketch of an NDB station setup and the antenna used is not a normal dipole. It is, in fact, a multi-wire T antenna, where the horizontal wires (three or more) are separated by insulators. Unlike a normal (centre-fed) dipole, where the radiating element is the horizontal wire, the T antenna’s vertical wire is the actual radiating element, and this acts as a monopole antenna with capacitive top loading. Normally the supporting masts are around sixty feet high.

There are several antenna types available for NDB transmitters with the vertical monopole probably the most common one. Some installations use a vertical antenna that has horizontal radials located on the top. This is sometimes referred to as a ‘top-hat’ antenna, and functions in a similar way to the T antenna.

The other antenna type is designed for use on maritime platforms, such as ships or oil rigs. It is a horizontal long wire that is mounted around the helideck and can be either end-fed or centre-fed. If the vessel or rig has insufficient available space for this type of antenna, a vertical whip antenna encased in fibreglass can be used. Southern Avionics, a company that produces NDB equipment, has an interesting section on their website that provides examples and technical information on the various types of NDB antennas used.

A Dying Breed? 
It has been quite a while since I looked at what is now a rare breed of NDB: Maritime beacons. Prior to the year 2000, numerous marine NDBs remained operational around the world. It was these NDBs that I cut my proverbial teeth on when I started DXing NDBs many years ago.

In those days, several beacons were on the same frequency, in beacon chains. Here, each NDB had a one-minute transmission slot within a six-minute rotation. They transmitted their identification several times at the start of their cycle, followed by a long tone and then two identification transmissions before the next beacon began transmission on its allocated slot. To stand a chance of receiving all the beacons in a beacon chain, you had to attentively listen to each frequency, for at least six minutes.

With the demise of marine NDBs, some became differential global positioning system correction stations. However, some marine NDBs do remain, mainly in the Black Sea and Arctic Russia regions. Occasionally it is possible – suitable propagation conditions allowing – to receive one or two of these marine NDBs in the UK.

Trying to keep my database of current operational marine NDBs is very difficult because very few official documents now carry their details. Even marine navigation charts are of little use. These are not updated as frequently as other forms of navigation charts, for example aeronautical, with the result that they might still show marine NDB locations that have since closed.

I have a feeling – although no definitive proof – that some of the remaining Russian, Black Sea and Caspian Sea maritime NDBs are used for military, naval and navigation training, rather than as navigation aids for commercial vessels. Marine NDBs in the Russia n Arctic region are still used for commercial shipping navigation, especially as Russia is heavily promoting the use of their northeast passage to Asia. Many of these Arctic region beacons are operational only during the navigation season that runs from July to September. However, this is currently extended by the provision of several new Russian ice-breaker vessels.

While most marine and aeronautical NDBs are powered by a standard mains power supply, some of the Russian Arctic region marine NDBs are powered by a radioactive isotope, Strontium 90. This eliminates the requirement to provide a mains power supply in difficult and remote locations.