Major Improvements for Short-Wave Reception
July 1963 Radio-Electronics

July 1963 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

This 1963 Radio-Electronics magazine article discusses the importance of the incoming signal vertical angle, or delta (from the Greek letter, Δ), at which radio signals arrive for optimal transoceanic short-wave signal reception. It highlights that most listeners are unaware of this crucial factor and thus miss out on capturing signals from far-off stations. The piece provides insights into how to calculate delta and suggests various antenna setups, such as vertical wires with ground radials, to improve long-distance reception. It also explains how existing structures like TV towers can be repurposed for superior short-wave reception. The author emphasizes that understanding and utilizing the concept of delta can significantly enhance the quality of long-distance radio reception.

Major Improvements for Short-Wave Reception

By Harold B. Churchill, W2ZC, ex-W3XM

A record total of listeners are now enjoying intercontinental radio reception. The short-wave audience has more than doubled since the end of World War II. Listeners are using just about every type of receiver from the latest innovation to pre-war models.

Few of these listeners realize that a careful change in their receiving systems can improve their results 5, 10 or 15 times. Once that change is made, rare-catch stations on the far side of the globe will start appearing and those normally logged bang in with the wallop of continental US broadcasters.

The single most important element in really good transoceanic reception is a value called delta (from the Greek letter, Δ). It is the vertical angle at which a signal arrives. This angle varies with distance, conditions and frequency, in a specific way. Unaware that delta is at work, the listener puts up an antenna he feels will do the job and hopes for the best.

Very long distance radio signals travel by reflection from the ionosphere. A radio signal shot from one continent to another may make the circuit via one bounce, two or even three. What happens en route fixes its angle of arrival at a receiver. If you think about it a moment, it's all very logical. You can see in the head illustration how this works. Note the signal from station A, say in the US, is coming in to the listener's antenna at a far higher angle than really distant station B. Like the catcher's mitt, the listener's antenna may miss one, and secure the other. Or it may partly muff both, and the net result is mediocre to poor reception of both A and B. The listener may explain his results with: "Oh, conditions just aren't good." The truth is, the signals may be whizzing by his wire as if it were nonexistent, yet laying a sizable signal into a competitor's aerial.

Angles of Arrival

Three factors decide the incoming angle of a short-wave signal-distance between transmitter and receiver, frequency used, and ionospheric behavior between the two points. This is true for domestic reception and rare-catch dx.

If you tried to compute all these factors at once, these variables and the math would be staggering. Fortunately, the answers have been so important to international communications and foreign trade that they've already been calculated. For instance, a communications concern may need to know: What is the vertical fire for Japan? Saigon? London? Indo-China? As of 1963, we know rather exactly.

Published for the first time in this form is the chart of Fig. 1. It allows a rapid estimate of incoming signals (or values of delta) on the trans-Atlantic circuit between Europe and the US East Coast. These are the median values to use when designing top-performance receiving antennas.

As time and conditions change, exact arrival angles vary up and down, but measurements show they again return to these specific values and coincide with them more often than not. We even know the exact percentage of time they will vary from these values.

Fig. 1 shows us a lot about the typical trans-Atlantic circuit. Take New York to London, and it isn't too different from New York to Paris, Rome, Berlin or Madrid. The characteristic delta for 15 mc is about 9°. At the 7-mc end of the short-wave broadcast spectrum, 23°¹.

Let's see how the typical listener's dipole performs on a European band, say 7 mc. First, its vertical responsiveness. This is the balloonlike curve in Fig. 2. Right off, you see its major response is directly up. In other words, delta equals 90°. Strictly skyward. However, the arrow at the right shows that the 7-mc European band usually arrives at 22°. So it muffs this overseas band by a good 68°. Not too good by any standard. Works a bit. perhaps. But it's really marginal. Now let's try this typical setup at 18 mc. We cut it precisely to length to give it every advantage t ere is. Without using space for another polar diagram, we find it is busily receiving at a delta of some 30°. Unhappily, Europe is arriving at 70 on this frequency. So it misses the ball again, this time by 23°. Conclusion: it isn't doing much of a job at this frequency, either.

At this point, the reader will say, "Well, my own dipole does pretty well with London and Moscow." But here the credit goes to superpower and top-flight engineering at the London and Moscow ends. Actually, the typical dipole abstracts only some 15% of the available European signal on a vertical-angle basis, and does it about 20% of the time. Small wonder far-side-of-the world stations are rarities on the dial.

Dollars vs Decibels

If a big percentage of any transoceanic signal goes whizzing by this average installation, how can more of it be captured? Going back to Fig. 1 again, you'll notice that the high end of the spectrum, where the superdistance usually lies, is the region of very low incoming angles. For example, 7° for 18 mc, 11° for 14 mc, and some 13° for 13 mc.

What's the best way to receive signals that come in at low, sizzling angles? There are several. One is genuinely costly: quadrupling the height of the dipole you are using and stacking and phasing counterparts under and beside it. Another approach uses knowledge instead of money, and can turn out first-rate results: switch the whole antenna concept to vertical polarization, and use a vertical wire working against ground radials.

Practical applications quickly Perhaps the most interesting quarter-wave vertical developed to date is the one in Fig. 3. It is compared in Fig. 4 with the conventional doublet. It has a further advantage over the simple wire in that its performance broadbands across any cluster of overseas stations, and receives them all about as well. A third advantage is that it can be fed by a standard 300-ohm TV line. Its length (height) in feet can be computed by L_ft = 234/F_mc.

In supporting a vertical, the approach is the same as in stringing a horizontal wire, but your horizontal run now becomes fine rope or cord. It must not be wire, since the overhead metal disturbs the vertical's performance. Excellent support points are between sizable trees or adjacent buildings. The shorter the run, the tighter the line can be, and the more erect the antenna. By far and away the best supporting cord is No.4, 1/8-inch 100% Nylon rope, breaking strength 450 lbs, and thin enough for a presentable appearance. A pulley-and-weight support system is best, but the Nylon itself has elasticity, so the two add.

For installations where no horizontal rope line can be erected (for instance, in a city or treeless development) excellent self-supporting verticals are available. Those manufactured by Hy-Gain, for example, include trap-loading coils and function on several bands. They have roof-top supports, also usable on the ground.

Space the three wires of the vertical 12 to 15 inches apart for SWL frequencies. Use hard-drawn copper or preferably brass tubing for spreaders. Solder the wires to the spreaders for good electrical connections.

One most important adjunct to the superior long-distance performance of the vertically-oriented antenna is the ground directly under it. Quite unlike the simple horizontal dipole, the vertical uses the ground as an electrical return path for signal currents. This interrelationship is responsible for its unique distance capability. For genuinely top-flight performance, the listener should lay a number of ground radial wires under the antenna, extending from it like the spokes of a wheel.² At the center and directly under the antenna, drive a ground rod. Bond the radials to it. Then dig a circular trench around the rod and fill it with rock salt. Radials should be at least as long as the antenna, and preferably longer. For an apartment house installation, the radials can be laid on the roof, their center preferably bonded to the apartment house metal roof (some morning before the superintendent wakes up).

Still More Gain

The next progression in long-distance reception appears in Fig. 5. This is the half-wave vertical dipole, and exceeds the quarter-wave in Fig. 3 because it has a still lower value of delta. It also has a degree of broadbanded response, and will successfully bridge a cluster of overseas stations. It can be directly fed by standard 300-ohm TV line and the system very carefully balanced. A single-wire, half-wave dipole conventionally fed by coaxial line is not shown in this series. Though widely used, it attempts to marry a balanced antenna to an unbalanced line, with the result the coaxial's outer sheath or braid starts receiving what is usually noise. Like the three-wire vertical, this antenna should have radial wires underneath it for maximum performance.

Still one more step upward in long-distance gain is the antenna shown in Fig. 6. Never before appearing in published form, it was invented by a leading antenna engineer³ and is available in  limited numbers to those interested in far-side-of-the-world reception. Its complex configuration of wires basically acts as two half-waves-in-phase vertically oriented, with receptivity at an exceptionally low delta value. It is the first known configuration of two-half-waves-in-phase matched directly by a standard 300-ohm TV line, allowing very simple installation. A series of ground radials greatly aids its very-long-distance performance, as it does for the preceding antennas.

Fig. 6 - An added step in distance gain - two half-waves in phase. The upper and lower halves fire together.

Your TV Tower

It may come as a distinct surprise to many long-distance listeners, but a conventional TV tower is often tailor-made for very good short-wave reception. It can often be used with little modification, and turn out superior results. The electrical equivalent of the towers erected by AM broadcasters, it has a striking resemblance to the antenna in Fig. 3. As one engineer (also a short-wave fan) remarked the other day: "My TV tower has been wasted on television!" But your tower may serve for both. Here's how.

Consider it as an exactly vertical metallic mass grounded at its base (or it should be for lightning protection). A close cousin of the antenna in Fig. 3, the frequency of its top performance is determined by its height. Its natural resonant frequency can be approximated with a tape measure (including the height added by the top-positioned TV antenna), and its quarter-wave response very nearly determined by the simple formula:

F_mc at resonance = 230/Height In feet

This departs from the normal quarter-wave resonance formula, since most TV antennas act like a top-loading "hat" on a broadcasting tower, causing a synthetic increase in height.

Variable Height Antennas

Of course the resonant frequency of the conventional tower is predetermined by its height, but there is a good chance it may lie close to an overseas broadcasting frequency. For example, an overall height of some 31 feet comes very close to optimum for the European 7.3-mc band. Then, too, the electrical length of a fixed tower may sometimes be increased to exactly the right value by adding an aluminum pole top-mast.

But the most valuable of all TV towers for good short-wave reception is the crank-down type. Its owner has a tunable vertical that can be peaked on nearly any overseas band simply by raising and lowering its height (and therefore resonant length). The settings can be marked, and the owner in effect has a calibrated mast which can be pre-set for any overseas band.

Your initial approach to the tower is made this way. Clip the TV lead-in at the tower base and insert a connect-disconnect plug. For short-wave use, wrap the lead-in around the tower. For TV reception, the lead-in is reconnected to the TV set. Guy wires, if any, should be insulated from the tower. Ground the tower base thoroughly via a ground-rod, driven in and rock-salted. For top results, lay radials around the tower base in spoke form, bonded at their hub to the tower base. Buried an inch deep, they are no hazard to walkers.

Next comes tentative calibration of the variable-height mast. Lower it to its minimum height, and if possible attach a tape measure to its highest point, or TV antenna. Then, with the tape measure secured, raise the tower to its greatest height. As you do so, attach tentative placed markers (indelible laundry ink on white adhesive tape) to the lifting cable. This is your first step in calibration. By the preceding formula, so many feet will be close to such-and-such a frequency. Use enough RG-58/U coaxial cable to allow a connection between the very bottom of the tower, and a point farther up, and the antenna terminals of the receiver.⁴ Solder about 3 feet of rather heavy, flexible insulated wire to the center wire of the coaxial line. At its end attach a battery clip large enough to fasten securely to a leg of the tower. Bond the outer braid of the coaxial line to the base of the tower, at the point where it is connected to the ground. Tentatively attach the battery clip to the tower, starting, say, 1 foot from the base and moving progressively upward, later in final adjustment (Fig. 7).

Fig. 7 - Hookup for using TV tower as antenna for distance reception.

The listener is now ready for more exact tower tuning and calibration. Raise the tower to the point where the formula shows it is nearly resonant to a desired short-wave broadcast band. Then tune your receiver to this frequency, log a station and check signal strength on the S-meter. Now raise and lower the tower a bit for a maximum S-meter reading. Once tower height is set, start varying position of the clip connection, again looking for a point giving maximum reading. For peak results, both should be maximized once more, this time by small amounts, since their positions are slightly interrelated. When optimum positioning has been determined for each band of interest, record the tower elevation and battery-clip settings so they can be repeated.

The sometimes surprising performance of a vertical is illustrated by the experience of one East Coast listener who put up a sizable vertical Yagi pointed at Tokyo. His compass bearing was a perfect shot via the North Canada-Alaska-Petropavlosk, USSR, circuit, a wild and uninhabited route to say the least. He snapped on his receiver as soon as the feedline was connected, and heard nothing except sporadic cracks of ignition noise.

Discouraged, he called a friend from a nearby research laboratory, who checked his general compass bearings, and then went to the receiver. The unmistakable snap of ignition again came in - soon identified as a farm tractor visible several fields away.

"Why, your antenna is as hot as a firecracker!" the engineer exclaimed. "Try it later, and be sure your receiver's on Tokyo's frequency." The vertical's constructor did as told, and suddenly there was a virtual thump as the Hammarlund's avc latched on to Radio Tokyo, swamping everything else.

So if you're aware of slight ignition noise in your first listening, don't be concerned. It's more likely to be the sign of an asset, not a liability. Many veteran vertical listeners size up its appearance during off hours as a forerunner of later good reception. More often than not, it's advance proof the installation is superreceptive to the low-grazing angles that are the bearers of genuinely good long-distance reception.

1 Note to European readers: 1n general, these angles hold when listening in the reverse direction. Say, to the Voice of America.

2 Note to CB users and amateur mobile operators: park your car over the ground screen of a broadcast station (required by FCC), and you'll have the most remarkable results yet.

3 Dr. Dean O. Morgan, 927 Highgate, Alexandria, Va.

4 Coaxial line is ideally suited here, since we have an unbalanced antenna feeding an unbalanced line.