On The Art of NDB DXing
by Sheldon Remington
©
1987-2000 All Rights Reserved
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CHAPTER SIX: MAN-MADE NOISE
Part Four - Blankers and RF Wrapup Automatic Blankers Between quieting powerline noise at its source (Chapter Three) and nulling it with directional antenna systems (Chapters Four and Five), it's likely that we can tame most interference problems. To that arsenal we can add yet another general strategy: noise blanking. Many DXers already have an internal noise-blanker in the IF of their receivers. These are automatic devices which sense high-level wideband pulses, and then act to shut down part of the IF chain for a period long enough to remove the pulse, and short enough to be inaudible to the listener's ear. Although mainly intended for HF/VHF ignition noise, they often help reduce LF powerline noise as well. As pointed out by Mitch Lee (Page 21, Lowdown, December 1986), these circuits require a wideband input, so their effectiveness is hampered when used with sharp preselectors or tuned loops. Synchronous Blankers Another way blankers can be controlled is by a timer circuit set for the repetition rate of line noise, normally 120 Hertz. The duration or width of the IF shutdown periods can be adjusted to just cover the duration of the line-noise pulses. Then, another control labeled phase is adjusted to center the blanking shutdown atop the pulses. This principle is used in the commercially-available Woodpecker blankers for HF OTHR-b QRM (over-the-horizon backscatter radar), but none have appeared for use on line noise. Nevertheless, such a synchronous blanker can be built by the amateur without much difficulty. Vince Pinto describes such a project on Pages 8-10 of the June 1983 Lowdown, with the schematic appearing on page 16 of the following issue. John Seamons has re-designed the circuit for CMOS devices and a lower parts count-this is shown in Figure 1. The 4538 CMOS IC has an internal Schmitt trigger for the 120 Hertz input, unlike the Pinto circuit, which uses a separate 7413 IC. The 40107 IC has an open-collector output, so it can yank down on an appropriate gating device. Vcc comes from a separate 12-volt supply (not shown); 5 volts can be substituted if you increase the 470-ohm resistor to keep the peak-to-peak trigger voltage equal to Vcc. Alternatively, the 120 Hertz trigger can be generated independently by a crystal oscillator, allowing battery operation, although the oscillator would need to have high stability.
John notes that this circuit works great on dimmer noise and to a surprising degree on powerline frying. The effect can seen dramatically with a spectrum analyzer. However, a single timing section probably isn't enough-perhaps 4 to 6 sections are needed for bad situations. The circuit can simply be cascaded, using two 4538's for each 40107. The output can be connected directly to the blanker rail in receivers having a blanker; Figure 2 shows where this is located in a Kenwood TS-430 Amateur HF Transceiver/General Coverage receiver. Vince Pinto attached his synchro blanker to a Drake R71A. Note that the receiver's automatic blanker being switched off doesn't affect the use of the blanker rail; in fact, it's important that the synchro blanker be set up with the automatic blanker switched off.
For receivers without any blanker, Vince notes that any of several gating devices can be added to the receiver. This could range from the RF line between receiver and antenna, to audio circuitry, although earlier in the chain is probably best. Vince's article contains lots of advice on applications and operation of synchro blankers. Western Update #48 has a completely different circuit for a synchronous blanker. Incidentally, some useful information on proper wiring of your home and rig is found in Dave Childs' "AC Power and Grounding" on pages 18-19 of the March 1984 Lowdown. Mike Mideke has confirmed the presence of noisy grounds in some configurations of his QTH. This concludes the discussion of man-made noise. Some months ago, I promised several correspondents, perhaps rashly, that this series would remove everyone's excuse that they couldn't DX because of such noise. So I would like to take this opportunity to ask readers to share successes and failures on this front. Maybe we should also share our results with the American media, as shown by the following anecdote: In late August, the U.S. President broadcast a speech to the Contras via a clandestine transmitter in Central America. Later that day, I heard a network newscast alleging that the broadcast had been jammed by Nicaragua, and a tape was played to illustrate the jamming. My ears perked up: this was surely the real thing--line noise in the flesh, rather than a jammer! Later, a BBC correspondent confirmed that, indeed, there had been no jamming--just Managua's noisy powerlines. Natural Noise Having eliminated all of your man-made noise, you should now be hearing only the natural noise at all times. Of course, this is anything but a constant: Edward N. Skomal's book, Man Made Radio Noise, notes that atmospheric noise at 300 kHz varies by more than 75 dB depending on time of day, season, location, and other variables. Statistically, the quietest period is in the late morning hours; unfortunately there is rarely any usable ionospheric propagation at that time, although ground-wave DXing can be worthwhile. At night, sometimes the noise does get very low (particularly in Winter), and DXing can be a delight on those nights. Still, some good DX can be heard even on moderately noisy nights, either between loud crashes or right over a lower level of atmospherics. So don't abandon your hobby during the summertime. In fact, what I believe to be the world's all time record distance for NDB reception, 9195 miles, was set by Mike Mideke in mid-summer. Limiters The only thing we can do on the receiver end to combat atmospheric noise is to limit its loudest peaks to no more than the peak strength of the signal you're receiving. This will at least help protect your ears and headphones. A limiter can be simulated by running the receiver's audio gain up near the point where it goes into overload, then using an attenuator to bring the headphones down to the desired loudness. Better yet, would be an actual limiter circuit in the receiver's IF or AF stages; these can be found in the ARRL Handbook and elsewhere. Hildreth Engineering (P.O. Box 60003, Sunnyvale, CA 94088) sells a sophisticated audio limiter and filter for about $70, which was favorably reviewed by Mike Mideke in the October 1985 Lowdown, Pages 16-17. Circuit boards, parts, kits, etc. can be obtained from Hildreth, or it can be homebrewed from the original article on Pages 113-120 of September 1985 Ham Radio. Some of the engineering statements in that article should be taken with a grain of salt, but nevertheless the device does work well on atmospheric noise. Further Antenna Considerations Unfortunately, thunderstorms are rarely localized, so an antenna system with narrow nulls won't be of much help. In fact, there is generally little to choose from among the commonly-used longwave receiving antennas. Loops, whips, verticals, shortwires, and longwires all perform similarly for DXing if properly used, with the exception of their ability to control man-made noise. An excellent treatise on antenna placement for maximum signal pickup and related topics is Vince Pinto's "Notes on Receiving Antennas for 1750-meter Weak Signals" on Pages 10-14 of the December 1983 Lowdown. By the way, I cannot emphasize enough how helpful this and other referenced Lowdown articles are; virtually every question asked by newcomers and veterans alike can be answered from wisdom contained in the last few years of the Lowdown. Back issues can be secured from LWCA headquarters and are well worth the low price. Returning to antennas, there is one type that can partially reduce atmospheric noise, provided that the noise isn't coming from the exact direction you're trying to receive signals from. This, of course, is the Beverage antenna. I've already touched on an advance type of Beverage, the Steerable Wave Array, in Chapter Five. A simpler type, the single-wire Beverage, can be effective on wide-area atmospheric noise, because it suppresses directions outside of its main lobe (which is in the direction the straight, long wire is pointed toward). A minimal installation consists of simply a piece of (usually insulated) wire stretched out in a fairly straight line, either on the ground, or a few feet high (on top of or through bushes and other foliage). Length should be at least 500 feet, and can range upward to several thousand feet or even more. The longer it gets, the narrower the main lobe will become. For more details, see the same references given in the Chapter Five discussion. With a Beverage, a noisy night will still be noisy, but getting a few dB here and there is the name of the game in any type of DXing. Receiver Noise, Gain, and Preamps It is not widely understood, either among longwavers or receiver designers, just how quiet LF can be. If the natural noise floor drops below the receiver's internal noise floor (or the available receiver gain is inadequate), then it's likely that some juicy DX would be audible if the receiver could be made more sensitive. Using small antennas or null-steering systems can yield low output levels which the receiver is unable to amplify sufficiently. Sometimes it can be hard to judge whether the noise floor we're hearing is primarily external or internal to the receiver. The short test whereby the receiver input is shorted as a noise comparison can be deceptive, since front-end noise and/or gain will usually shift with source impedance. My own philosophy is to have excess RF sensitivity and gain available, which can be reduced as needed. This is achieved with preamplifiers; naturally the preamp itself must have a low noise floor. Some commercially-available preamps were mentioned in Chapter Two. Many preamp circuits have also appeared in the Lowdown, especially of the JFET variety. These are probably best for high-impedance antennas such as whips, verticals, and short-wires. For low-impedance inputs such as loops, a carefully-designed bipolar circuit is quieter; I recommend Mitch Lee's MPSH-05 circuit on Pages 21-24 of the May 1984 Lowdown, with an addendum on Page 20 of August 1986. Marc Connelly also recommends this circuit for use with a series L/C tank circuit, either as a stand-alone preselector or as part of a null-steering system; see Pages 15-19 of the September 1985 Lowdown for details. Also popular is the regenerative preamp technique described by Ray Cole on page 14 of the May 1984 Lowdown, and by Ken Cornell in his LF-MF Scrapbook. This wraps up RF techniques in general. By the way, all these
techniques from Chapters Two to Six are equally applicable to NDB DX and
1750 Meters, and beyond, even into the BCB. Next time, we will begin to
explore some topics of more specialized interest to NDB DXers, starting
with special considerations: selectivity, ident pitch, and frequency
measurements.
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