On The Art of NDB DXing
by Sheldon Remington
©
1987-2000 All Rights Reserved
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CHAPTER TWO: THE SPURIOUS RESPONSE PROBLEM
Do you remember the first time you ever listened to a receiver having a longwave band? If you were like most of us in North America, you were probably greeted with an awful-sounding mixture of manmade noise and the combined programs of several of your local BCB stations. Maybe a handful of NDB's were strong enough to be heard through this cacophony, or maybe not; certainly there was no hope of hearing real DX. At this point a lot of potential longwavers probably throw up their hands in despair and are not heard from again. Indeed, this writer existed in that state for more than a decade after first acquiring a receiver with LF (a Knight-kit Star Roamer). Finally, LWCA came on the scene, and listings began to appear of real longwave DX, heard by those who either had solved these problems or were located in one of the rare places not plagued by them. This DX motivated me to go ahead and apply the solutions learned through hamming and BCB experience; suddenly my log total, which had been stagnant at a few dozen NDBs, rose to several hundred and has risen steadily ever since. This chapter will discuss cures for the spurious response problem, and the following chapter will deal with manmade noise. Spurious responses are known variously as spurs, crossmod(ulation), intermod(ulation), mixing products, or (somewhat incorrectly) images. Their causes and cures are best divided into two general categories: those originating with receiver deficiencies and those which are external. Unless you have a very "crunch-proof" receiver, the first category will dominate and must be solved first. Then, once you're satisfied any remaining spurs are due to external sources, you can pursue those, if they are objectionable. Receiver Problems Most receiver-based spurs are due simply to the first-mixer stage being overdriven into its non-linear region by strong signals which sneak through the front-end selectivity circuits. This can happen even through the receiver is tuned to a frequency well separated from the strong signals. Occasionally, the overloading will be in an earlier (preamp) stage. Only recently has this problem received proper attention from receiver manufacturers (and homebrewers, too), as more and more DXers find themselves immersed in strong RF fields from local transmitters. The latest latest generation of pricey receivers are the first to contain high-level mixers or even to specify a figure for dynamic range. The early solid-state receivers were very poor-the designer would ask you to use the RF gain control or DX/local switch to reduce overload-of course this would also remove the weak LF DX along with it! But the newer equipment, such as the R70, R71 series, the R7A, the NRD 515 and 525, and the receiver sections of Kenwood amateur transceivers, will substantially outperform the classic hollow-state (tube) units like the military-surplus R390A, in this respect. In fact, at the present rate, receiver overload may soon become a thing of the past, except perhaps in portables where battery life must be considered (crunch-proof front-ends have relatively high power consumption). A dynamic range spec. of 90-100 dB should suffice for most users; 50 to 80 was typical of older receivers, while 120 dB is easily exceeded in state-of-the-art designs. You can find measurements of this parameter in QST product reviews, and computations have appeared in Ham Radio magazine and in the literature of Sherwood Engineering, Inc. (1268 South Ogden St., Denver, CO 80210). Cures for Receiver Overload It is possible to retrofit a high-level mixer and oscillator into an existing receiver, but it would be a major undertaking. Theory, design, and test methods can be found in profusion in recent QST articles, ARRL Handbooks, and the ARRL Solid State Design for the Radio Amateur. A much easier cure for a spur-ridden receiver is to use an outboard filtering device at the receiver input-this can be just as effective as a crunch-proof front end. These filters work by rejecting the frequencies of the interfering signals, while allowing the desired LF frequencies to pass unimpeded. They also can be either fixed-tuned or variable--many DXers like to have both, for extra protection. The fixed-tuned type can use either a sharp cut-off lowpass filter, set for about 500 kHz, or rejection notched tuned to individual interfering signals, or both. This type allows one to tune freely across the LF spectrum without operating any controls other than the receiver tuning itself. The variable type are known as pre-selectors; these pass only narrow slices of the spectrum, so they must be re-peaked every time you shift your listening frequency by more than a few kiloHertz. Commercial Filters If you desire to purchase a ready-made unit, there are at least five sources that I know of offering longwave filters: (Note: This list was valid as of early 1988.) LWCA Editor Ralph Burhans (Burhans Electronics, 161 Grosvenor St.,Athens, OH 45701) has some inexpensive preamps for active whip/loop antennas. These incorporate one or more of the following filters: fixed lowpass; fixed bandpass (for example, one unit covers the 160-190 "LowFER band" and rejects other bands); or rejection notches (set by Ralph to the frequencies specified by the user). A number of LWCA members' success stories with Ralph's designs have appeared in The LOWDOWN; see, for example, March 1986, page 22. Send an SASE to Ralph for his catalog. Another LWCA member, Sal DeFrancesco (LF Engineering Co., 17 Jeffrey St., East Haven, CA 06512) manufactures as active whip antenna, the L-400B, which incorporates a fixed lowpass filter. A preamp for high-impedance wire and loop antenna, the model L-201 also has a fixed lowpass filter. Sal's design also have received testimonials from LFers; see, for example, page 10 of the February 1987 Lowdown. Send an SASE to Sal for his new catalog. The large Japanese manufacturer Yaesu-Musen Co., Ltd. (17210 Edwards Rd., Cerritos, CA 90701) makes two devices intended as accessories for their FRG-series receivers, which are nevertheless perfectly useable with any brand. The FF-5 is a fixed lowpass filter which lists for $20. The FRT-7700 is a tunable passive preselector with coverage down to about 150 kHz, and it also contains a lowpass filter like the FF-5. It has antenna and bypass switching, a stepped attenuator, and a helpful 4-position impedance matching switch; even a skilled homebrewer would be hard-pressed to pack so many features into such a compact unit. The FRT-7700 has not been reviewed in The LOWDOWN, but IRCA members give it high marks, and all of my own DX has been filtered through one for over five years. At $65, I believe it's the preselector of choice for those not needing coverage below 150 kHz. Yaesu sells their equipment only through ham stores; one that stocks these units and offers quick mail-order service is Amateur Electric Supply, Inc. (5710 W. Good Hope Road, Milwaukee, WI 53223, Phone (414) 358-0333). Grove Enterprises (140 Dog Branch Rd., Brasstown, NC 28902) offers one or more passive preselectors which tune all the way down to about 100 kHz. Their model TUN-3 was reviewed by W.R. McIntosh in the September 1982 Lowdown, page 7. Grove has a catalog available for the asking. MFJ Enterprises, Inc. (P.O. Box 494, Miss. State, MS 39762, Phone (601) 323-5869) has the model 955, another passive preselector which tunes down to 10 KHz; a brief description can be found on page 304 of the 1986 WRTH, or write to/phone MFJ for their free catalog. MFJ products are available direct from the manufacturer, or via ham stores, some of which discount the 955 below its $80 list price. Homebrew Filters While all of the above devices can eliminate your spur problems, you also have the option of building your own. In fact, these filters are among the simplest devices in all of electronics, so they can even be constructed successfully by those with no experience at all. The LF literature abounds with design examples: in The LOWDOWN, in Ken Cornell's Scrapbook series, and in the series of articles written by Ralph Burhans for Radio-Electronics, available as a reprint for $4 postpaid from Ralph. If you wish to construct a fixed low-pass filter, below is a diagram of one that is simple, yet effective. Simply connect it between your antenna and the receiver input. In severe cases, you can stack multiple filters in series. It is always worthwhile to experiment with grounds; sometimes a particular ground configuration will be especially good or bad from the standpoint of spurious rejection, without having any noticeable effect on the desired LF signals.
External Rectification Once you've filtered the signals enough that any receiver overload is eliminated, there may still be a few weak-to-moderate spurs audible. This will likely be the product of a problem called external rectification. It occurs when two large pieces of metal are in a strong RF field. The inevitable corrosion at the point of contact will act as a rectifier like the old catwhisker-and-galena crystal set. This rectification produces sum-and-difference frequencies from the strong signals, which are then re-radiated by the pieces of metal. This may occur in your antenna/feedline/ground systems if they contain poor (non-soldered) connections. It may also occur in nearby fences (particularly long, rusted wire fences), railroad tracks, or any other large pieces of metal, sometimes even hidden inside the structure of buildings. Some older cities with lots of corroded metal and large numbers of powerful transmitters--Boston and Mexico City are both notorious for this--will have such a profusion of external spurs that the only solution is to go on DXpeditions to quiet environments outside the urban area. Frequently, external spurs will be observed to change abruptly in strength when the wind blows or precipitation occurs. A strategy for locating these pieces of metal might be to use a portable receiver with a directional loop antenna, tuned to the spur itself. The loop nulls and the signal strength should lead you to possible culprits. Then, try a kick or sledgehammer blow to see of the spur is affected. Cures require electrically isolating the two members, or electrically isolating a low-resistance path around the corroded joint. Grounding the objects may also help. The search then continues until you find another source of spurs, and so on until you're satisfied. Miscellaneous External Causes Another source of LF spurs is a peculiar mixing process that occurs in defective broadcast transmitters. When one station with nonlinearities in its output stages is in the strong RF field of another station, energy from the latter is picked up by the first station's antenna and travels the "wrong way" down the feedline and into the transmitter. There, it is mixed with the transmitter's own output, sent back up the feedline, and re-radiated as spurs from the antenna. This has lately proven to be a problem in San Jose, CA and in Honolulu, where multiple BCB transmitters are diplexed into a single tower. Generally, these cases are discovered when direction-finding with a portable receiver leads straight to that station's tower. Of course, the offending station is responsible for cleaning up this type of emission. Finally, an obscure and relatively low-level source of spurious mixing,
but one beyond our control, is the atmosphere itself. This was discovered
several decades ago in Europe, when skywave signals from a certain
longwave broadcast transmitter were found to be cross-modulating with the
signal from another in Luxembourg, which lay directly beneath the skywave
path. It turned out that the Luxembourg transmitter was so powerful that
it modulated the propagational qualities of the atmosphere in its
vicinity. Perhaps one of our European members could supply more details
about this peculiar process, which is known as the Luxembourg Effect.
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