NDB of the Week: PIS

This morning, I concluded listening to my captured spectrum from two nights ago, logging maybe a hundred or so NDBs from all over Europe. Now, before you laugh and say this is not DX, remember this was only a 10-minute recording made during the early evening in the UK. So DX was not the objective in this exercise.

Path to PIS.

The last NDB - and one of the most distant (at 1687km) I heard from this recording - was PIS, located in Pisarovina, Croatia, primarily for navigation around Zagreb airport. The CW is found at 425.2kHz or so and operates continuously. A good image of PIS is found via Google Earth Street View, where the Marconi 'T' antenna design is clearly evident, the capacity hat strung out between two admirably substantial lattice masts, standing in stark contrast to some of the rural houses of the area.

'T' antenna type of PIS.

It's difficult to know exactly how much power PIS puts out, but information online reveals it's low power and "less than 50W". This seems to be correct, because the airport information provides the expected range for this NDB as up to 50 nautical miles (about 58 miles, or 93km). 

Official information about PIS (not including power information). Source: here.

Now, remember that this is what we can call the 'rated' range - the range at which an NDB receiver within an aircraft is expected to give reliable indication of the NDB signal, regardless of time of day and weather. This nominal distance has to take into account the fact that there are plenty of aircraft flying around that have very old and thus not cutting-edge sensitivity NDB receivers in the cockpit panel. Climb into any club light aircraft and you'll often find an NDB receiver with orange LED digits or entirely mechanical digits, coupled to worn-to-illegibility control dials, dating from the 1980s or even earlier. 

Whilst you might be lucky and have an aircraft with a 'glass' (computerised) cockpit, this old NDB receiver (displaying 920(kHz)) remains very common in many light aircraft today. It's also common to find boxes that don't work! The lower box, displaying 1200, is the radar transponder.

Even then, NDB signals are prone to erroneous indications, notably under strongly convective (charge-separating) conditions that lead to towering cumulus or cumulonimbus clouds, even if they are not producing lightning. On my qualifying cross-country flight back in 2003, such conditions were around in the Irish Sea and NDB signals were unreliable, the direction indicator preferring to point at clouds, rather than the NDB, at least when it was further away.

Anyhow, this is what PIS looks like, during its strongest showing of the ten minute recording, on the waterfall with the mini-whip as the 'antenna'. At its weakest, the characters can be impossible to distinguish. The signal on the left, by the way, is MIQ, an NDB related to Ingolstadt, Germany.

PIS NDB (CW at 425.2kHz)

Though it's meaningless in terms of aviation use, it's still pretty amazing that a sub-50W medium-wave transmitter can be heard at over 18 times its rated, navigational use range, with an E-field probe that fits in your hand!