interference-reducing antennas for shortwave broadcast listeners

IEEE TRANSACTIONS ON BROADCASTING, VOL. 34, NO. 2, JUNE 1988 159

INTERFERENCE-REDUCING ANTENNAS FOR SHORTWAVE BROADCAST LISTENERS

Dr. 0. G. Villard, Jr. SRI International

333 Ravenswood Avenue Menlo Park, CA 94025

Abstract

It is possible to realize small-compared-with-the- wavelength directional antennas optimized for the needs of shortwave listeners. Although these designs go well beyond the traditional loop, they are not difficult to build or to use, and further simplification is a reasonable expectation. improved rejection of both ground- and sky-wave interference can be obtained with minimum modification to the associated receiver, even in typical indoor environ- ments. Measurements suggest that single-source sky- wave signal reductions of 20 dB can be achieved on the average. This is sufficient in many instances to give satisfactory separation of two signals on the same frequency of comparable strength and having azimuth differences of 90 degrees or greater. It seems likely that widespread use of these antennas would significantly reduce the severity of co-channel interference in shortwave broadcasting.

It has been found that significantly

Introduction

Efforts have been made in recent years to provide shortwave listeners with the advantages of antenna directivity as a means for combating interference of either local or distant origh. antenna designs are too bulky to be used indoors and are intended to be used for both transmitting and receiving. A directional antenna for broadcast reception should be compact, portable, and capable of separating signals and interference in the electromagnetic clutter of a typical indoor environment.

The majority of HF

Since these are daunting goals, it is encouraging that there has been some success in achieving the above objectives both in Europe [1,2] and in the United States. [ 3 ] This paper outlines some technical prob- lems and gives examples of what has been achieved to date.

History

In 1983 Mr. S . Wysocki of the Polish Department, Radio Free Europe (WE) in Munich, West Germany, was sent by a Polish listener a recording of a jammed RFE broadcast received first on a standard wire antenna and then on a special loop design. Since the improvement was impressive, it was decided to make knowledge of the technique available to other listeners. loop design details could be obtained from Poland, Mr. Erwin Straub of the RFE/Radio Liberty (RFE/RL) Engineer- ing Department designed an easy-to-construct shielded loop made of readily available TV coax cable plus a variable capacitor from a medium-wave radio. [4] Con- struction and operation details were made available to listeners in printed form and were also transmitted in- to Poland in special broadcasts of an innovative kind devised by Mr. Wysocki in which listeners were asked in advance to make use of tape recorders in order not to miss or forget essential details. polish listeners and other evidence make it clear that a significant number of these loops were constructed and used to reduce the effects of jamming. Only about 2% of those mentioning the loops reported that they had been of little or no help. [ 5 ]

Because no

Letters from

At about the same time, Mr. P. Fraenckel of the European Services Department of the British Broadcast- ing Corporation (BBC) became interested in interfer-

ence mitigation because of a challenging--but imprac- tical--proposal put forth by a listener. Learning of the RFE/RL work, he encouraged Mr. G. W. Short and Mr. V. Smith of the Programming and Engineering Department, respectively, to collaborate on an ingenious vertical loop design particularly intended for listeners bothered by ground-wave interference of very great strength. [2,6] To prevent receiver overloading, this loop is provided with a shielded enclosure within which the receiver is located. The design is also noteworthy for its unusual construction materials, which consist of a cardboard carton, aluminum foil, and plastic film. No electronic components are required. Ground-wave rejection in excess of 40 dB was measured at a BBC experimental site. is obtainable at no cost from the External Services Department of the BBC. [7]

A pamphlet describing a similar antenna

Loops intended to be operated primarily in the vertical position, as in the case of the WE/RL and BBC designs, are very effective in reducing the strength of one ground-wave interferer at a time. However, the need also exists for an antenna that can reject more than one such signal, or signal component, at the same frequency but coming from different directions. led SRI International to develop in 1983, with 1- funding, a horizontal loop having these characteristics. This and a prototype anti-sky-wave design were successfully tested in Austria in August 1984 against both types of jamming coming across the border from Czechoslovakia in the 15-MHz band. Further development work was supported by the U.S. Information Agency, Voice of America in 1986. [3] Since completion of the work, the latest anti-sky-wave design has been successfully tested--also in Austria--by RFE/RL engineers.

This

Design Goals

The following antenna design goals have served as a guide.

The antennas should be readily portable and comparable in size with a receiver with its whip ex tended.

The antennas should operate indoors, and be affected as little as possible by reradiation from building framework, wires, pipes, etc.

They should be rotatable (when appropriate) by use of a simple turntable or equivalent.

They should give deep, stable nulls when reject- ing either ground-wave or multipath sky-wave signals.

In most situations, a unidirectional (cardioidal) pattern is more useful than a bidirectional (figure of eight) pattern.

Ability to discriminate one signal from another is the desired goal; accuracy of direction finding is of little importance.

Antennas should be located near the receiver and be easily accessible for adjustment. than one easily performed tuning setting per received frequency should be necessary.

No more

0018-9316/88/0600-0159$01.~ 0 1988 IEEE

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( E ) Interaction between the antennas and the user's body should be as small as possible.

Sensitivity should be comparable with that of the receiver's built-in whip.

Construction should require only readily available passive materials and electronic components.

( 9 )

(10)

(11) In connecting the antennas to existing receivers (either line or battery operated), little or no modification should be needed.

(12) The devices should be tunable or switchable to cover several shortwave bands.

( 1 3 ) If possible, the antennas should protect the associated radio from direct pickup or overloading by extremely strong signals.

Although no one antenna design achieves all these goals, several come closer than might at first be ex- pected.

Fundamental Considerations

Ground-Wave Propagation

Signal energy reaching the listener over an essentially line-of-sight path will be called for simplicity "ground-wave'' propagation. Such propagation is charac- terized by (1) arrival angles close to the horizontal, (2) comparatively great path stability, and ( 3 ) largely vertical polarization. These characteristics make straightforward cancellation techniques attractive for ground-wave rejection.

Sky-Wave Propagation

Even at times of normal propagation, ionospheric transmission is highly variable, resulting in continual changes in received-signal amplitude and phase. most circumstances, there are a number of signal components distributed over a range of angles in the vertical plane, since the requirements for transmission can be satisfied, in general, by more than one such path. once.) In the majority of situations, all components travel over the short great-circle path between transmitter and listener. The magneto-ionic effect causes individual-path polarization to rotate as a consequence of ionospheric reflection. both by changes in individual path length and by polarization changes.

In

(For example, two or more hops may be present at

Signal fading is caused

Sky-wave signal components spread in vertical angle o f arrival are important in the present context, because to reject such signals, an antenna must have a pattern null which is narrow in azimuth but which simultaneously extends over the needed angular range in the vertical plane. i n this respect because their null is deep only at one angle perpendicular to the plane of the loop.

Indoor Reception

Conventional vertical loops fall short

Building construction falls into three categories: (1) steel-framed reinforced concrete; (2 ) brick or stone, perhaps with some steel framing; and ( 3 ) wood framing. Building types (2) and ( 3 ) are usually suitable for HF reception and crude direction-finding indoors. However, buildings in the first category are often effective shields; in that situation, indoor direction-of-arrival measurements usually give the direction to the nearest window, rather than the direction to the station. Nevertheless, if the receiver is placed within a few feet of a window, some directional

discrimination is usually possible.

RF energy is frequently guided indoors in all types of buildings via pipes or wiring. Strength of the signal thus conveyed is strongest when the conductor is resonant with a high Q, but the probability of this being the case is fortunately low. Because of interconnections between pipes and wires, and the presence of lossy dielectrics (such as plaster), resonances are normally well damped.

These and other considerations affect the choice of impedance levels for indoor directional antennas. impedance designs (for example, foreshortened conventional monopoles) respond most strongly to electric fields; relatively low-impedance antennas such as loops are sensitive to magnetic fields. Electric-field antennas are easily detuned by small capacitances, such as those between the antenna and the human body, and are sensitive to reflections from high impedance objects such as those generated by the human body or by brick or plasterboard walls. They also respond strongly to evanescent electric fields off the ends of wires act- ing as transmission lines to conduct energy from outside to inside.

High-

As it happens, typical houses tend to have more sources of high-impedance reflections (plaster board, etc.) than they do sources of low-impedance reflections (extended conducting surfaces, etc.). To the extent that this is true, the spatial discrimination of low- impedance receiving antennas will be less likely to be degraded in the average indoor situation.

Signal Polarization

Outdoors, in proximity to the earth, and indoors in most situations, the predominant signal polarization-- either sky wave or ground wave--will be found to be vertical. (This conclusion is relatively little affected by ground conductivity and radio frequency.) In general, any horizontally polarized component tends to be reduced in intensity by the conducting earth, or by its equivalent indoors. In these circumstances, a vertically polarized antenna receives--on the average--a stronger signal than its horizontal counterpart.

Of course, in sky-wave propagation there will be in- frequent times when the only signal energy present may be horizontally polarized, due to the rotation of polarization of signal components. Directional antennas should therefore be designed to avoid degradation of the depth of the all-important null at such times.

Direct Receiver Leakage

Since it is practical to reduce ground-wave inter-

The ference by large amounts, the possibility of direct leakage into a given receiver must be considered. severity of this effect, which in a typical case may be estimated by operating a given radio with its antenna disconnected or its whip retracted, is often appreci- able. Fortunately, many external anti-interference antenna arrangements are flexible enough to cancel not only signal incident on the antenna, but also signal which leaks directly into the set. is made more critical, of course, by the presence of a leakage component.

The nulling process

Signal leakage can often be reduced by placing small portable receivers with plastic cases on a piece of metal somewhat larger than the set itself. The metal acts as an extension of the set's internal ground. However, the performance of some receivers of this type can be adversely affected by close proximity to metal because of resonant-circuit detuning and increased in- terstage coupling. In those cases, some minimum spac-

161

ibg (perhaps half an inch) should be maintained.

Some Guidelines in Directional Antenna Design

General

on nulls for directional discrimination, since maxima are comparatively broad. For best results with sky waves, the nulls should be designed for good performance at all significant angles of elevation in the null direction. The exact nature of the response ih other directions is relatively unimportant so long as there

Small-compared-with-the-wavelength antennas depend

a response of adequate strength.

Sources of Directivity

At the most fundamental level, directivity depends upon phase differences between signal energy induced in different parts of a given physical structure. plicity, in the case of small-compared-with-the-wavelength antennas, no more than two structures (or two parts of the same structure) are normally chosen to be independent signal sources. In the case of end-fire directivity, the antenna must have extension in the direction of wave travel. With broadside directivity, the extension is in a direction perpendicular to that of wave travel. End-fire directivity is often used to obtain unidirectional patterns, but it is difficult to make the vertical-plane null extend over a wide range of vertical angles. Broadside directivity, on the other hand, can give a bidirectional null whose depth is independent of elevation.

Need for Sensitivity

For sim-

Response in the maximum-response direction should be sufficient for adequate signal-to-noise ratio. though most broadcasts have comparatively great strength, inexpensive receivers tend to be insensitive, and field strengths are, in most cases, weaker indoors. Sensitivity can be improved by an increased aperture, use of resonance, or (where permissible) preamplifica- tion.

Polarization Response

Al-

Directional discrimination can normally be improved

Most configur- by choosing a structure which in the desired direction is responsive to one polarization only. ations which have a desirable directional pattern for one polarization, but at the same time respond to the opposite polarization, have a less desirable response to that polarization.

When a loop is vertical and the distant station, when nulled, lies in the plane of the loop, response to horizontal polarization is very low, assuming that the feed to the loop is well isolated. This is a generally desirable situation. In off-null directions, there may be response to horizontal polarization, but it normally adds to the total received signal and so is of little consequence.

A possible disadvantage of vertical linearly polarized antennas is that at times (usually at the opening and closing of transmission on a given waveband) a small number of modes is propagating (usually at a low angle) and the net signal polarization is linear and rotating. At such times, there will be deep nulls in the output of any linearly polarized antenna, and antennas having a mixed polarization response may show less fading. However, the superior sensitivity and directional discrimination of linearly polarized vertical elements outweighs this consideration.

Element Interconnections

It is desirable to avoid direct antenna element interconnections to transmission lines when possible. For example, in vertical broadside loop arrays with elements interconnected by a horizontal transmission line, precautions are needed to attenuate current flow on the outside of the line, because the near fields created by these currents couple readily to the pattern-determin- ing vertical portions of the structure. Although fer- rite beads are effective in breaking up currents on the outside of coax, they are ineffective for reducing coupling when the impedance level is high, which is often the case. use of inductive coupling plus (if necessary) Faraday shielding or the equivalent.

Low-Impedance Elements and Shielding

Direct interconnections can be avoided by

Elements made of flat metal strips three or four inches in width have an impedance level low enough not to require shielding against local electric fields. axial line can be used, if desired, but the unshielded wide-strip conductor performs at least as well and may be easier to obtain and construct. makes a satisfactory conductor.)

Co-

(Aluminum foil

Since low-impedance antennas are less detuned by the capacitance of adjacent objects and are less influ- enced by reflections from partial reflectors such as the human body, they are especially suitable in situations where adjustment by an operator is necessary.

Neutralization

In compact antennas, coupling resulting from the close spacing between elements can lead to severe tuning interaction. Coupling may be reduced by neutralization, for which a number of techniques have been described. [ 81 Neutralization is typical19 effective over a given shortwave band but, in switching bands, it may be necessary to switch neutralization as well.

Ability to Reject Sky-Wave Multipath Components Over a Wide Range of Vertical Angles

The ability to attenuate high-vertical-angle signal components determines the extent to which a deep, stable null can be obtained with fading sky-wave signals. loop plus vertical dipole sense antenna gives a cardioidal pattern whose ability to null at least low-vertical-angle components of fading sky-wave signals is significantly better than that of the loop alone. difference in action is pointed up by the fact that a loop by itself has a Comparatively narrow null in a plane perpendicular to that of the loop, whereas the loop-dipole combination has a broader null in the plane of the loop.

It is interesting that the familiar vertical

The

When the signal source lies in the plane of a loop, loop response is independent of vertical angle, to a first approximation. On the other hand, the response of a vertical dipole falls o f f with increasing vertical angle and becomes zero for a downcoming angle of 90 degrees. that although null depth may be excellent for low-angle vertical-plane components, it falls off steadily as the angle o f arrival of these components increases. sky-wave signal-discriminating ability of the antenna is proportionately decreased.

The net effect in a loop-dipole combination is

The

One way to ensure better vertical-plane response is to use the broadside directivity principle as employed in the Adcock antenna. Such antennas have a null ex- tending continuously from the horizon on one side through the zenith to the horizon on the other side.

162

(However, if vertical dipole elements are used, there will be a region of low sensitivity directly overhead.)

Discrimination of the whip-loop can be improved by making the response of the whip change with vertical angle in approximately the same manner as the loop, or vice versa. One approach is to rearrange the whip dipole so that it resembles a bent dipole, which happens to be the case in the coplanar twin loop (CTL) described below.

Representative Designs

RFE/RL Loop

This is a shielded loop made of readily available television cable and one standard MW tuning capacitor (Figure 1). Connection to the receiver is by coaxial transmission line, which in some cases requires receiver modification. able wooden frame, permitting the loop to be tilted and rotated for best interference rejection.

An important feature is an adjust-

I I TORECEIVER

BOTTOM PART FILED ROUND TO ROTATE IN HOLE

Figure 1 Shielded loop developed by RFE/RL, Inc. Rejects one ground-wave signal at a time, plus sky-wave signals having low vertical angles of arrival.

This loop is intended to take advantage of the stability of ground-wave signals. It can also be used to attenuate sky-wave signals when the incoming vertical- plane signal components arrive at a low vertical angle. This occurs in the case of multihop transmission when ionospheric electron density is barely sufficient to support propagation (for example, just after transmission begins in the morning or just prior to failure in the evening). In these circumstances, simple loops

give deep and relatively stable bidirectional nulls. However, as increasing electron density causes more vertical components to appear, the nulls fill in and become too variable to be useful.

BBC Loop

This vertical loop is similar to the above in application, but incorporates provisions to shield the associated receiver, thus greatly reducing the likelihood of direct-signal breakthrough (Figure 2 ) . [ 6 ] It can be constructed of aluminum foil, sheet plastic, and a wooden or cardboard box. (No electronic components required.) Foil and plastic form a high-Q tuning capacitor. Loop sensitivity is high. Interfering-signal at- tenuations in excess of 40 dB have been measured under controlled conditions. Negligible modification of the receiver is required.

GAP BETWEEN / ENDS OF LOOP

CARDBOARD BOX

CLEAR PLASTIC SHEET

Figure 2 Loop antenna developed for British Broad- casting Corp. Application is similar to RFE/RL loop. receiver.

Note technique for shielding

Horizontal Loop Antenna (HLA)

Unlike vertical loops, this antenna attenuates ground-wave interference on the basis of polarization (Figures 3 through 5) . [ 9 ] There is a negligible effect on sensitivity to ordinary sky-wave signals, irrespective of their arrival direction except when all propagating modes have very low vertical angles. (Such signals then behave like time-varying ground waves.) Construction is similar to the BBC loop. stable surface is required for support. Low-impedance, wide-strip construction minimizes hand capacity and re- duces losses when operating close to the ground where ground-wave signal polarization is most nearly vertical. By suitable tilting, as shown in Figure 5, total signal received from incoherent signal sources in different azimuths can be minimized. the HLA is appreciably higher than that of the conventional whip. It may be tuned by means of a homemade capacitor as is shown in Figure 2. The HIA has been used successfully to uncover broadcasts rendered un- intelligible by ground-wave jamming in Europe.

A flat,

The sensitivity of

163

FLAT METAL OR FOIL (THICKNESS UNIMPORTANT1 MOUNTED ON WOODEN BOARD FOR STIFFNESS

10-cm WIDTH 200 TO 300-pF CAPACITOR f E (MEDIUM-WAVE-TYPE O K )

L- -5 0.3 TO 1.3 rn 4 CONNECTION FOR PORTABLE RECEIVER:

I '

RECEIVER

Figure 3 Circuit diagram, Horizontal Loop Antenna ( H W

Figure 5 HLA rejects ground-wave signals coming from more than one direction

FIAT STRIP

SIGNAL TO BE NULLED

Figure 6 Circuit of one embodiment of the Coplanar Twin Loop (CTL)

Figure 4 Adjustment of HLA tuning for best signal reception

Coplanar Twin Loop (CTL)

The CTL is an improvement on the Twin Loop Antenna described in Reference 3 . When this device is oriented vertically, it rejects ground-wave interference well, particularly when path splitting in the vertical plane is present. However, it is intended primarily for at- tenuating sky-wave-propagated signals (Figures 6 and 7). [3,9,10,11] The unidirectional action is provided by the outer loop, which is sensitive to both electric and magnetic fields. frequency of the station being received, and should be

This loop must be tuned to the Figure 7 Portable CTL with built-in receiver. Useful for locating and avoiding regions of severe field distortion.

164

retuned whenever the operating frequency is changed by 10 or 20 kHz. Although there are two loops, the over- all adjustment is straightforward, because the inner loop is tuned only once per waveband. (A small untuned loop inside the inner loop can be used for output coupling and provides desirable transmission line isola- tion. [12]) Except for bandchanging, the CTL can be said to be a one-control device.

Figure 8 shows a typical polar pattern obtained with a multipath sky-wave signal. The points represent one-minute signal averages. Figure 9 is a pattern made under similar circumstances, but with the outer loop detuned. inner loop is barely discernable. lated vertical-plane polar pattern.

The free-space figure-of-eight pattern of the Figure 10 is a simu-

180 AFRTS BETHANY FREOUENCY: 15.330 MHz DATE: 20 JUNE 1986 TIME: 1530 TO 17W GMT

(0830 TO 1 OOOLT) LOOPS ON GROUND

Figure 8 Cardioidal CTL polar pattern. One-minute signal averages taken to smooth out effects of fading.

The method of operation of the CTL is unusual. When the outer loop is tuned to resonance, the current that flows in it has the correct amplitude and phase to create a null, or "shadow", in the ambient magnetic field at the position of the inner loop. Since for all practical purposes, the inner loop responds only to magnetic field, because of its low impedance, the electric field simultaneously generated by the outer loop has little effect. a relatively high impedance, its current is derived from both E and H fields of the incoming signal. components of current are in phase quadrature. signal direction changes by 180 degrees, the phase of current flow in the outer loop changes because the phase of the H-field component changes 180 degrees. Conditions will then no longer be correct for the ambient H field at the location of the inner loop to be balanced out. loop and thus in the receiver. This action has Been verified both by tests and by computer modeling (see Reference 3, pages 78-83).

Because the outer loop is designed for

These When

This causes signal to reappear in that

The adjustment procedure for the CTL is as follows:

0 de9 I

180 AFRTSBETHANY FREQUENCY: 15.330 MHz DATE: POJUNE 1986 TIME: 1530TO 1700GMT

(0830 TO 1oooLT) LOOPS ON 4C-FI TOWER

Figure 9 Same conditions as Figure 8; response of inner loop of CTL alone. of normal loop response.

Note the vestige

Figure 10 Simulated vertical-plane response pattern of CTL (made with horizontally polarized test signal and CTL on its side)

With the outer loop detuned, the inner loop is tuned to resonance,

While simultaneously adjusting the loops' direction for the best rejection of an unwanted signal, the outer loop is tuned for deepest rejection of that signal,

The loop may then be rotated 180 degrees for maximum reception of the nulled signal, or to some other angle which gives the best separation between a wanted and an unwanted signal.

Since both loops have impedance levels lower than that of a conventional inductance-loaded dipole, CTL ad- justments are simplified by absence of significant hand capacity effects. Front-to-back ratios comparable with that of Figure 8 can normally be obtained. The antenna has been tested both in Europe and the United States.

165

Good vertical-plane response of the CTL results in part from minimum physical separation between the loop and the "whip," which is in reality a part of the outer loop, and in part because the halves of the "whip" are semicircular in shape rather than straight.

Antennas Generating Nulls in the Ambient Electric Field

Antennas based on an electric-field analogue of the CTL's magnetic-field shadow are possible (see Figures 14, 15, and 16 of Reference 3 ) . Designs based on this principle have attractive features; for example, with battery-operated receivers using whips, no connection between antenna and receiver is required. However, in view of their sensitivity both to hand capacity and to weak reflections (for example, from the body), they have not been developed further.

An Example of Broadside Directivity

An experimental bidirectional antenna, called the Decoupled-Element Broadside Array (DEBA) (Figures 11 and 12), consists of two parallel wide-strip resonant loops whose output is combined in phase opposition. can be thought of as an Adcock antenna using tuned loops for sensitivity and for good response overhead, plus decoupling for compactness. cally, a null is generated which extends from the horizon on one side through the zenith to the horizon on the other side. This gives the best possible rejection of multiple vertical-plane modes and is particularly useful when short-range sky-wave transmissions must be attenuated by antenna directivity.

It

When operated verti-

Figure 11 An example of broadside directivity: circuit diagram, decoupled-element broadside array (DEBA)

When the antenna is operated with the loops horizontal, a null extends in all directions at the horizon. However, the antenna is sensitive to downcoming sky- wave signal components irrespective of direction of arrival, provided that they have a reasonable angle of elevation in the vertical plane. Thus it is useful for general-purpose reception in exceptionally noisy locations. Interference traveling at angles near the horizontal is attenuated both by the phase relationship between the loops, and because the plane of the loops is essentially orthogonal to the electric field lines of vertically polarized ground waves.

The antenna has fair sensitivity due to the use of resonance. neutralization, (2) choice of a low impedance which keeps losses low in spite of a metallic shield halfway between the loops, and (3) the lack of direct connec-

Its compact size is made possible by (1)

Figure 12 Laboratory model of the DEBA

tions between the loops, made possible by inductive coupling. Tuning is surprisingly straightforward. Neu- tralization is such that when one loop is resonated to a given station, and its response is measured separate- ly, tuning the other loop through the same resonant frequency has very little effect. This greatly simplifies tuning the second loop to provide a deep null when its output is combined with that of the first loop. though the antenna would appear to be a significant improvement on the Adcock, it is probably too complex for the casual broadcast listener. Therefore further development has not been pursued in the present context, although in other applications it might be quite useful.

Additional Designs

Al-

One proprietary loop design uses resistive loading to achieve unidirectional response and a wide response frequency range without tuning or bandswitching. Pre- amplification and relatively large size overcome intrin- sic low sensitivity. not to be as great as might be desired. [13]

The front-to-back ratio appears

Another proprietary directional antenna of ingenious design achieves great bandwidth and a unidirectional pattern; it is both compact and passive. [14] With suitable circuitry, the pattern can be electronically rotated. Sensitivity to electric fields is balanced against magnetic-field sensitivity to achieve a unidirectional response. tivity when a passive structure of practical size is used, and decreasing effectiveness of sky-wave nulling as vertical-plane angles increase.

Disadvantages include poor sensi-

Additional Aspects

Signal Discrimination Using Vertical Angle of Arrival

The CTL can be operated horizontally and rotated so that the pattern null is swept in the vertical plane. This will null signal components on the basis of their vertical angle of arrival. tic of multimode ionospheric propagation that at any given time, one vertical-plane mode exceeds the others in amplitude. through that mode can produce a change in receiver output that is sufficient to identify the existence of the mode and its approximate angle. The resulting vertical- angle-measuring ability is useful for intercomparing receiving locations, estimating station locations (i.e., single-station radiolocation), and discriminating signals in elevation as well as in azimuth.

It seems to be characteris-

When this occurs a CTL null swept

166

Built-in Diversity Reception

An unexpected property of the CTL is that it can provide outputs suitable for diversity reception. on-course or reference output can be derived from the outer loop; a nulled output is available from the inner loop. put, the null output in most situations is still well above the noise level. When an output from terminals having a 20 dB null in the transmitter direction is compared with that of the reference, the fading is found to have a low degree of correlation, presumably because the propagating paths are so different. path must have significant amounts of sidescatter.) The correlation is much lower than when one loop is horizontal and the other vertical in a configuration suitable for polarization diversity reception. The CTL diversity action, in fact, appears to be comparable with spaced-antenna diversity. It is unusual to find such low correlation in so compact a structure.

Wide Bandwidth Unidirectional Response Without Tuning Adjustment

The

Although 20-dB or more below the reference out-

(The nulled

Two methods for achieving this have already been mentioned. [13,14] A different scheme using active electronics has been found to operate over at least a four-to-one frequency range, but due to its high impedance level and indifferent vertical-plane response, development has not been carried further.

Conclusions

Principles useful in the design of directional antennas capable of reducing interference in shortwave broadcast reception have been identified. have been given of designs which go beyond the time- honored vertical loop. (Nevertheless, such loops re- main a useful option.) Perhaps the most important single characteristic of any design is the ability to perform well indoors. This calls for the lowest possible impedance level combined with reasonable sensitivity in relation to size. Other useful properties include simple construction, the ability to attenuate ground-wave interference from more than one direction simultaneously, plus the ability to obtain a deep stable null in the case of one multihop sky-wave signal at a time. Many of these goals have been achieved in compact structures that are not too difficult to adjust. Although experimental antennas capable of very wideband (i.e., order of a decade) response without any adjustment have been built, they are handicapped by low sensitivity in relation to size, high impedance level, and nonuniform response over the desired range of vertical angles. There is reason to believe that further improvements are possible, particularly in designs in- corporating active elements.

Examples

Acknowledgment

The author acknowledges the substantial contribu- tions of C. A. Hagn and G. H. Hagn to the HLA.

References

[l] The Board for International Broadcasting 1985 Annual Report, Board for International Broadcast- ing, 1201 Connecticut Avenue, N.W., Washington, D.C., 20036, September 1984.

[2] S . Mukherjee, "Indoor loop aerial for short- waves," Wireless World, p. 38, April 1985; see also G. Wareham, "Shortwave Loop Aerial," Elec- tronics and Wireless World, p. 41, February 1986.

( 3 1 0. G. Villard, Jr., K. 3. Harker, G. H. Hagn, "Interference-Reducing Receiving Antennas for Shortwave Broadcasts," VOA Final Report, Contract IA 22082-23, SRI International, Menlo Park, CA, January 1987.

[4] E. Straub, "KW-Rahmenantenne," Radiowelt, pp. 28-30, October 1985.

S . Wysocki, Radio Free Europe/Radio Liberty, Inc., Oettingenstrasse 67, Am Englischen Garten, 8000 Munich 22, West Germany, private correspon- dence, 1987.

G. W. Short and V. Smith, "The Box Loop," Elec- tronics and Wireless World, vol. 93, no. 1616, June 1987.

[5]

[ 6 ]

[7] George Short, "How to Make A Box Aerial," BBC Ex- ternal Services, Science Industry and Exports Unit, Bush House, London, England, WC2B4PH, 1987.

( 8 1 L. A. Moxon, HF Antennas for All Locations, London: Radio Society of Great Britain, 1982.

[9] G. H. Hagn, 0. G. Villard, Jr., C. A. Hagn, and M. J. Toia, "The Wide-Strip Horizontal Loop Antenna (HLA): An Effective Solution for Ground- Wave Interference to Shortwave Reception," - ceedings of the Fourth International Conference on HF Radio Systems and Techniques, April 11-14 1988, pp. 145-150, Institution of Electrical Engineers, London, England.

[ l o ] 0. G. Villard, Jr., "Portable Unidirectional HF Aerial for Reducing CO-Channel Multihop Sky-Wave Interference, 'I Proceedings of the Fourih Inter- national Conference on HF Radio Systems and Tech- niques, April 11-14 1988, pp. 141-144, Institu- tion of Electrical Engineers, London, England.

[11] 0. G. Villard, Jr., "The Spatial Notch Filter--A Compact Unidirectional Receiving Antenna for Sky- Wave Interference Rejection and Direction Find- ing," =, American Radio Relay League, Newing- ton, CT (to be published in September 1988).

[12] D. E. Barrick, "Miniloop Antenna Operation and Equivalent Circuit," IEEE Trans. Antennas and Propagation, vol. AP-34, pp. 111-114, January 1986.

[13] J. A. Lambert, "A Directional Active Loop Receiv- ing System," Radio Communication, vol. 58, no. 11, pp. 944-949, November 1982.

[14] A. Luedtke and W. F. Bentley, "Directional Annular Slot Antenna," U.S. Patent 4,229,744, October 1980.

interference-reducing antennas for shortwave broadcast listeners

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