On The Art of NDB DXing
by Sheldon Remington
©
1987-2000 All Rights Reserved
|
Chapter Eleven: Visiting Beacon Sites
The previous chapter concluded with a discussion of tracking down unidentified beacons using loop bearings. Of course, we're not limited to just visiting unidentified beacons. Finding and examining any beacon, even a local one, is a challenge and a pleasure, especially if we've heard or tried to hear it at the home QTH. We can try to discover why some beacons are pests and others rarities. We can determine with certainty whether an officially-listed NDB that nobody can hear is just getting out poorly or is not even on the air at all. To find a particular beacon, we can use some combination of the loop-bearing method and digging into the official data to get the geographical latitude and longitude of the site. These data are essential to the navigator, so they're much more accessible than for transmitters on other radio bands. Ken Stryker shows coordinates to one minute (about a mile) in his Beacon Guide and Updater. In some cases, more detailed (but sometimes erroneous) coords can be found in Jurgen Trochimczyk's Radio Beacon Handbook. Publications used by navigators, such as the Jeppeson Guide and the US Department of Commerce's Airport/Facility Directory for aviators and the US Defense Mapping Agency's Radio Navigational Aids, Publications 117A and 117B for mariners, naturally give detailed coords, but again, the errors creep in. Some (but by no means all) NDBs are shown on Sectional Aeronautical Charts, and these are also worthwhile to have for plotting coords and bearings, as they have detailed latitude/longitude grids. These charts and other aero publications can be purchased at general aviation airports. For 1750-meter beacons, their locations can be gotten directly from their operator, who would probably welcome a visit from a DXpeditioneer. But consider asking not for the complete directions to the Lowfer beacon, rather perhaps just a clue or two to help home in on the site, or just the coordinates. After all, homing on an unknown location is half the fun. When we arrive within a few miles of the beacon, we should begin a process of stopping for quick bearings, which at this point we no longer need to write down, but just use them for dead-reckoning which way to proceed. At intervals that seem appropriate based on how rapidly the strength is increasing or how near we're getting to a mapped position, we repeat the D/Fing. This is best done with a portable receiver's loopstick, held away from the vehicle body, powerlines, fences, etc., as these will distort the directional pattern. Usually the 180° ambiguity of ordinary loops won't present much problem, if we keep track of the trend of successive bearings. In the case of the Sony 2002, and probably some other portables, both the whip and the loopstick are active on LW. So when the whip is extended vertically, it offsets the loopstick's nulls, making one of them deeper than the other, thus eliminating the ambiguity. If we're using a vehicle-mounted receiver, we can rig up a hand-held loop connecting to the receiver. For close-in D/Fing like this, the loop can be small and simple, and a preamp is unnecessary since the signal will be so strong. Homing can also be done with a car-top active whip system, but it will be less straight-forward. We have to guess which direction to proceed, then see if the signal strength rises or falls, as in the "hotter/colder" children's game. To reduce wasted time with this method, it'd be a good idea to plot the strength changes on a detailed road map. When we get really close, whether using a loop or a whip, we may need to switch in an attenuator or otherwise reduce the gain to avoid meter-pinning and overload. At this point the approach will be constrained by the availability of roads, since we're in an unfamiliar area and rarely will there be a road going in the same direction as our bearing. It's necessary to proceed slowly, keeping our eyes peeled lest we drive right past the beacon site. Having a passenger can speed up this process and allow the driver to devote full attention to the road. Beacon Antenna Configurations Now we're almost upon the beacon and need to know what to look for. Firstly, 1750-meter antennas can take almost any imaginable configuration, including semiinvisible wire arrays. So it might be good to ask the operator in advance what it looks like. As for NDBs, this writer has visited over fifty of them, and found that there are certain most commonly seen antenna types; refer to the diagrams below. In the case of marine NDBs in the US, they generally use a 30-to-50-foot high tubular vertical, usually painted white and self-supporting; a minority of them have a tiny top hat, probably as much for corona reduction as for capacitive loading. Naturally. they are always situated close to some major shoreline feature, often in a strikingly photogenic setting. Many are perched atop the same pedestal as light or sound nav-aids. A few marine beacons are affixed to lighthouses or even atop buildings.
Aeronautical NDB's, by contrast, can take many forms, although these are all just different mechanical realizations of the loaded Marconi vertical antenna (as are the 1750-meter and marine beacons). Those expecting broadcast-type lattice towers will find a few among aero NDB's. Sometimes these are mounted atop a mesh screen perhaps 25 feet square, and elevated several feet above the ground; this is known as a counterpoise, and it helps reduce ground losses in poor soil.
The most common aero NDB antenna, at least in the western US, is the flattop wire array, also known as a "tee." The flattop consists of one, two, or three parallel horizontal wires a few feet apart, with a central downlead to the transmitter enclosure. These wires are interconnected at varying points, either the ends or center. Two-wire flattops usually have a separate downlead for each wire, but the three-wire arrays have only one feeder. A couple of cases are known where a flattop is end-fed, although this is poor engineering. Flattops range in length from perhaps 60 to 300 feet, and in height from 15 to 60 feet. They are suspended via lengthy insulators between two utility poles or occasionally from steel towers. So when we're looking for an aero NDB, we should especially watch for what looks like a detached electric powerline that's so short it only needs two poles to contain its entire length.
The second common type of aero NDB antenna is the umbrella. This consists of a (usually) self-supporting tubular mast 30 to 40 feet high surmounted, by a top hat consisting of radial spokes. These are 5 to 10 feet in radius and typically 6 to 8 in number, with or without a wire skirt connecting the radial ends (which aids mechanically but not electrically). Often they have what looks like an upside-down bucket part way up, which contains a loading coil. The remaining aero NDBs take some odd forms. One is the "inverted coathanger" loop which, despite the appearance, is fed with a single wire into the transmitter, rather than the balanced pair we'd expect in a receiving loop or an HF quad. They are constructed, like the flattops, from heavy wire suspended between a pair of wood poles. Another unusual form is the vertical cage antenna, consisting of several vertical wires hung around a wood pole and spaced outward with insulators. No doubt there are numerous other peculiar NDB antennas waiting out there to be discovered by the DXpeditioneer.
Transmitter enclosures are generally not very large. Some are in small white prefab huts, windowless with just a locked door and a blower vent or two. Sometimes a standby generator can be spotted nearby. Other transmitters are housed in one or two metal boxes attached to a post near the base of the antenna. Usually the transmitter manufacturer (mostly Nautel or Southern Avionics) and model number will be labelled on the outside of the box. If we're lucky, there may be a portal in the box showing an antenna current meter, calibrated in RF amps. Other things to note are the ground radial wires connected to the base of the system, and the design of the feedthrough insulator for the antenna downlead, with its associated lightning gap. Examining and photographing or sketching these details and the antenna configuration can teach us useful lessons about LF beacon installation which are applicable to our 1750-meter operations as well. One caveat: Observe discretion when visiting NDB sites--touch nothing, and secure permission before crossing private property. On a single tour, it's not difficult to visit a score or more beacon sites. Planning is made easier if we use a map and list of all officially-listed NDBs for our travels, such as contained in the NDB DXers Newsletter. Frequency-order listings are best for DXing, but geographical-order maps/lists are best for beacon touring. If you're planning an LF DXpedition, and agree to share your observations, send an SASE to this writer for a map and/or list of all NDBs in that region with info about where they've been logged. This can be supplied for any reasonably small region in the world, excluding Europe, Africa, and the Middle East. Try to allow at least a month in case the info has to be specially drawn up. Let's not forget to bring a camera or sketchbook so we can keep a
permanent record and share our discoveries with other DXers. In fact there
are already numerous such photos sitting in our collections and just
waiting for a volunteer to compile them and somehow print them up as a
Beacon Photo Album for a wider circulation. Such a project could even be
expanded to include photos of notorious longwavers and their
equipment--any takers?
|
