MD - 204kHz - Enola (Harrisburg), PA


There is a sort of 'secondary' class of NDBs, called 'COMLO' beacons, or 'COMpass LOcators' which are part of a major airport's ILS (Instrument Landing System). They are usually located at the ILS' 'Outer Marker' (OM), some dozen kilometers from the end of and in line with the runway. This leads to the beacons being placed where they need to be as opposed to where it is convenient, and as a consequence in some pretty odd and surprising places.

This is certainly the case with 'MD'. It is on a sliver of land stolen from the edge of a parking lot opposite a school on a side street in the middle of Camp Hill, on the 'western shore' of Harrisburg. The little green hut and the short (20 feet high) 'phone poles make this a very casual looking affair.

OM NDBs are generally low power - they are not intended to have much range. The installation at MD ensures this as a practical matter; the antenna is a low (20 feet) inverted 'L' a mere 50 feet long, and being surrounded by blacktop on all sides cannot have a ground worth mentioning. It does not radiate well.

Outer Markers don't always have an LF NDB, but what they do have is an upward-pointing low-VHF yagi, or array of same; this radiates a low-power 75MHz signal which is automatically sensed in the aircraft when it overflys; it then knows it is at the Outer Marker. The NDB allows planes to find the general area of the OM in the first place. This yagi is plain in some of the pictures; here it is a single yagi, more often it is a pair pointing up in a 'Y" formation. Also visible is an 'X'-band microwave link to/from the airport, presumably from where the beacons are controlled. COMLOs in the US typically have a two letter abbreviated version of the ILS host airports identifier - in this case MD is from 'MDT' (Middletown), or Harrisburg InterGalactiDrome.

Some other pictures:

  • Moody OM and NDB antennas.
  • Hut and OM, parking lot. Taken at risk from middle of road!
  • NDB antenna end detail.
  • NDB antenna feedline connection.
  • NDB feedline leaving hut.
  • Hut, OM antenna, NDB fed end pole, microwave link antenna.

    © Steve Dove, W3EEE, 2003,4