On The Art of NDB DXing
by Sheldon Remington
©
1987-2000 All Rights Reserved
|
CHAPTER 3: MAN MADE NOISE
PART 1: - Offensive Strategy If we were to poll longwave listeners asking what their worst obstacles to DX reception was, we would probably get a near-unanimous response that man-made noise headed the list. Most DXers unfortunately reside in urban or suburban locations, and virtually all of these places, plus many rural places, are plagued by man-made noise. Sometimes the round-the-clock noise is so strong that it completely obscures all signals on longwave, which are generally weak anyway. Possessing a deluxe receiver does little to alleviate the problem, which can be particularly disconcerting to newcomers to the spectrum below 540 kHz. I know of several DXers who have basically given up on longwave in despair over their "permanent" noise levels. Those who persist often find that their DX success varies in proportion to their success in eliminating man-made noise. This is made more challenging as the noise levels in most areas steadily rise with the progress of time and technology. Ace DXer Mike Mideke has commented that he finds himself in a treadmill situation in which each improvement he made in his listening setup over the years was countered by an equivalent rise in the mid-California noise floor. This is especially noteworthy since he resides some seven miles from the nearest powerline or other noise source. He also has observed ionospherically-propagated man-made noise! If this trend continues, it won't be long before people whose tooth fillings act as radio receivers will be complaining of bees in their head... There is a whole menagerie of different man-made noises being heard on radios these days. Luckily, we on longwave don't have to contend with the Woodpecker, ionosondes, jammers, or WA4DOG. Our main concern is with just two general types of noise sources: broadband harmonic energy from AC-line-operated devices and TV receiver horizontal oscillator harmonics. Both of these sources produce noise that could be described as variations on a buzz. This sound is so horrible that it's often chosen as the programming for jammer transmitters on the broadcast bands. For some reason, there has not been a wide awareness among DXers that these noise problems can be completely eliminated or at least greatly diminished with the right techniques, and perhaps, a simple homebrew black box of one kind or another. Imagine getting your home noise floor down to the same level as you would find on a rural DXpedition--the NDB band occupied only by signals and maybe some natural noise--so that the limits of your reception are controlled by nature, not by your neighbor. Now, let's proceed to make that dream become reality. As with the overload/spur problem in Chapter Two, noise control may require a combination of offense and defense. This chapter will discuss the offensive strategies of locating and controlling noise sources. Chapter Four will venture into defensive strategies, i.e., those that are needed if/when one finds they can't tame the noise at its source(s)--and that is the area where the most amazing things can be achieved! TV Oscillator Interference TV receivers have a powerful oscillator which operates at the horizontal sweep frequency, and since the waveform is not a sinewave, it also contains a rich spectrum of harmonics of that frequency. Due to the minimal shielding of consumer-grade TV receivers, they often radiate this spectrum of harmonics which are then heard by nearby DXers. The surest way to determine the presence of this problem is to note whether you hear a rough, unstable buzzing concentrated around known frequencies (See Table 1 for a listing of sweep frequency harmonics that affect the longwave band). In North America, the basic oscillator runs at 15.734264 kHz, although this can vary slightly if the TV set is receiving a non-standard signal such as that from a VCR or computer. If you have this kind of noise, sometimes called ITV (a reversal of TVI), you'll need to locate and suppress the offending TV receiver(s). This interference can be D/F'ed with a portable radio which has a ferrite rod (loopstick) antenna. By the way, you may not need a longwave-capable radio-the sweep harmonics are usually easy to hear on the low end of the AM BCB. It's helpful to confirm your quarry by simply turning the TV on and off, or in the case of a neighbor's TV, you can watch for the telltale glow to go off at night, listening for the interference to go away too.
Table 1. TV Horizontal Sweep Frequency Harmonics (System M) Studies have shown that the radiation takes place principally in three ways. In order of magnitude of importance: 1) Via stray coupling from sweep circuits into the AC line. The cure is simply to install bypass capacitors and/or an L-C (inductance-capacitance) line filter in the TV's line cord, preferably where it enters the chassis. Appropriate filters can be purchased or constructed as described later. Another tip (W6SAI, Ham Radio Dec. 1981) is to be certain, if the TV has a 3-prong AC plug (few do), that the ground wire is indeed grounded. 2) Via stray coupling from the sweep circuits to the TV antenna system ("rabbit ears," external antenna, or CATV coax lines). The remedy is a high-pass filter inserted at the antenna terminals of the TV; one such unit is the Radio Shack #15-581. 3) Directly from the picture tube and sweep-circuit wiring. Sometimes this is exacerbated by, to quote Glenn Hauser, "the problem of so-called repairmen who fail to replace tube shields properly and jack up the high-voltage to compensate for a weak picture--thus not only increasing (RF) pollution but heightening the danger of shock and X radiation." The strategy here is to shield high-potential leads, and perhaps also add extra bypassing in the sweep circuits; in severe cases it may be necessary to line the cabinet with foil or mesh screening. These are drastic measures and obviously should not be attempted without experience due to the lethal nature of sweep circuits! Incidental radiation of this type is covered by Part 15 of the FCC Rules, and the user of the TV is obligated to take steps to eliminate it. The owner should be advised to contact the source from which the TV was purchased for appropriate modification of the set. The dealer in turn can obtain the necessary information from the set manufacturer. ARRL (225 Main St., Newington, CT 06111) keeps a list of "contact people" for each manufacturer as does the FCC. Broadband Powerline-Related Interference It is an unfortunate fact of life in an electrical society that almost every electrical device is capable of emitting broad-spectrum noise. Variously known as hash, line noise, or spark discharge, this problem is partly the result of our use of AC rather than DC for power distribution. Generally, it doesn't matter much what exact devices are causing the noise; the result is the same: a continuous spectrum from 60 Hertz on up to VHF. Sometimes the noise spectrum will have broad peaks or dips in the frequency domain, but usually the noise is strongest on its lower harmonics which is just where longwave DXers are listening. The list of possible polluting AC devices could go on endlessly, but some of the more notorious ones include light dimmers, motors with brushes, fluorescent lamps, and devices with thermostats, such as fish tank heaters. In all cases, the devices are, or should be if properly designed, noise-free until they become defective. The offensive is often complicated by the simultaneous presence of multiple sources. But this should not be insurmountable it you proceed step-by-step, starting with your own home. Again, a portable receiver is very helpful, in part because its battery permits operation without any powerline connections (which might further complicate the search by receiving conducted noise in addition to radiated noise). The simplest strategy is to listen for a reduction in noise as you remove power from possible sources. At first, you can try shutting off your house circuits, and if the noise drops, go unplug each device that's connected to that circuit. You then filter and/or repair the device and move on. Once you're absolutely satisfied that your home contains no noise sources, you are ready to expand the search. At this point, you should consider involving the local electric power company. For one thing, it is often the powerlines themselves which produce the interference-due to improperly installed or maintained pole hardware mostly, although worn-out mercury and sodium street lights that fail to stay ionized for more than a couple of minutes at a time (causing the starter to function repeatedly) will also produce strong intermittent noise if they are less a block or two away. For another, well-managed power companies either have a staff expert or an outside consultant who has the equipment and experience to track down RF noise sources. In fact, you will be doing the company a favor by notifying them of such problems since they can save headaches by fixing problems before they worsen. If you are successful in tracking an interference source to a particular power pole, look for and note the pole number which is usually nailed or stenciled on the pole. Then report it to the power company by street location and number. The professional RFI sleuths make particular use of a characteristic of most noise sources, namely, that the low-frequency radiation carries the farthest, and higher frequencies will be confined to the vicinity of the source. You can try this same technique if you have a multi-band portable receiver. The trick is to keep the radio tuned to the highest frequency on which the noise is audible; you will then be in the smallest possible zone of interference. Unfortunately, the search is complicated by the conduction of noise along powerlines. Often you'll be drawn to a series of seemingly conflicting indications, and you'll have to draw a rough map to figure out what's going on. Directional (ferrite) antennas can just add to the confusion, and they usually don't operate above 1700 kHz. The best method might by to get someone to drive along slowly while you operate a portable receiver from the passenger seat. Also, you might have to silence several noise sources before you can uncover the real culprit which most interferes with your home receiver. For more information on this kind of search, you should get a copy of the excellent Interference Handbook by William R. Nelson (Radio Publications, Inc.). Mr. Nelson has over thirty years' experience as an RFI troubleshooter for Southern California Edison, and he gives many helpful case histories of tough interference situations he's solved. Nearly all devices causing powerline interference are regulated by Part 15.25 of the FCC Rules. Usually, (for an ominous exception see QST Feb. 1987, pp. 52-53) the FCC will be on your side as a radio listener and can be called upon if all else fails to get the cooperation of the owner of the interfering device in suppressing the noise. However, diplomacy is always the best policy; in fact, sometimes it will be worth your while to make and install the filters yourself, though, legally you don't have to (and if the device goes bad, you'll be blamed). Good tips on diplomacy can be found in Nelson's book and in the ARRL booklet, RF Interference. Now down to the nuts and bolts of filtration. First, both above-mentioned books have directions for building and applying filters for a variety of applications. Secondly, commercial AC line filter are available, such as the Radio Shack Cat. # 15-1111. One simple homebrew filter is shown in figure 1.
The parts values are completely non-critical; the larger the values, the lower the cutoff frequency. The values shown give a cutoff well below 100 kHz. The capacitors must be non-polar (ceramic disc, etc.) and rated at least 1000 volts. The inductors should be as large as conveniently possible; a good size might be 50 turns of #12 (to handle a 1000-watt load) on a 1.5-inch diameter air core form, although ferrite cores can be used. Build the filter in a metal box, installed as close as possible to the device's chassis, and grounded. Two or more such filters can be cascaded in series for extreme cases. With a wall-mounted light dimmer, there is unlikely to be enough room for a filter, so the simplest solution is to replace the dimmer with one that is noise-free. The Lutron brand is reputedly quiet, or you may find some brands which clearly mention RFI suppression. Also, dimmers that have only three (off, dim, bright) settings are noise free. Though this is actually a defensive ploy, you might also want to install a filter on the incoming AC line feeding your receiver. If your radio exhibits less noise when switched over to batteries, then its AC supply is inadequately filtered. The Politics of RF Pollution Virtually all broadband radio noise is the result of cost-cutting in the design and maintenance of electrical devices and powerline systems. It does add slightly to the cost of things when filters are added or extra breakdown margins are allowed. The designers are usually aware of the problem, but they perceive insufficient demand to justify the extra cost. This also holds true, as most hams know, of RF susceptibility. In my case, I pick up the phone, and KHLO-AM pours out of the earpiece! Several years ago (the late 1970's and early 1980's), the CB boom drove these issues to the forefront, as the FCC was deluged with complaints of RF susceptibility. The FCC came close to requiring improved mandatory standards, but then the CB fad went bust, and the pressure dropped off. In the broadcasting bands, the traditional approach has been for governments to authorize very high transmitter power levels (like 50 kw up to the megawatt range), thus overcoming urban noise levels by brute force. So, if anyone needs to make their voices heard by their government on
this issue, it is we the DXers. Especially on LW, both NDB and LowFER
DXing is limited to very low transmitter levels. But, DXers on all bands
are working at the margin of detectability relative to natural noise, so
it in all our interest to see some improvement in the RFI situation. We do
have an organization which can and should accept some responsibility for
lobbying on this issue: ANARC (and Europe has EDXC, The DU's have SPARC,
etc.). LWCA is an ANARC member club, and it would be appropriate for our
club representatives to commence agitation on this matter.
| |||||||||
