By Joseph Buch
NASWA Journal, December 1992, and January 1993

Most antenna handbooks optimize antenna design for maximum effectiveness for transmitting applications. Design engineers go to great lengths to maximize efficiency, minimize mismatch to the feedline, and maximize radiation in the direction of the horizon with as little upward radiation as possible for maximum distance per hop.
We will look at the applicability of these concepts for the SWL and explore why you may want to adopt totally different approaches for your DX receiving antenna.
We will show why antennas close to the ground work well for receiving.
We will show why small, active antennas can actually outperform full size dipoles and random wires.
And we will examine a new adaptation of the wave antenna which was successfully used in the sands of Iraq.

Antennas that couple maximum energy out of a transmitter will also couple maximum energy into a receiver. The sensitivity of modern receivers is so good, however, that the ability to hear a weak station is determined more by the atmospheric noise and interference on the listening frequency than by the signal level delivered to the receiver. This noise is especially limiting on the tropical band frequencies below 5.9 MHz. To test your receiver, disable the receiver AGC, remove your antenna from the input and substitute a 50 ohm resistor. If the speaker noise is less on the resistor, your receiving sensitivity is limited by outside noise. Try this on several frequencies. Your results should look something like the curves of Figure 1. Thus, the performance of a receiving antenna depends upon its ability to discriminate between the desired signal and the external noise environment, not the efficiency of the antenna.

What kinds of antennas can discriminate against external noise?

There are several designs which have proven useful. There are three general classes of receiving antenna which can improve the signal to noise ratio of signals compared to a high, horizontal half-wave dipole. The first type is the loop. It offers the advantages of small physical size, portability, and the ability to be rotated to null out interfering signals The next class of antennas is the long wire mounted close to or on the ground. Such antennas can exhibit good directional characteristics but their large physical size prohibits rotation. Finally, active antennas have proven to be useful. Active antennas combine a short, inefficient antenna with a low noise amplifier to make up the signal loss resulting from the short antenna. Active antennas can often be located in places away from locally generated interference. The better ones provide good overload characteristics. The major disadvantages are that most active antennas are expensive and non-directional.

Why might I choose to use a loop antenna?

Loop antennas can be easily constructed by the SWL or purchased ready to go. The loop antenna pattern has deep nulls at 90 degrees to the plane of the loop. The pattern of a balanced loop antenna is shown in Figure 2.

Figure 2. Loop antenna pattern shows omnidirectional response to high elevation angle waves.

These nulls are most pronounced on signals arriving at vertical angles near the horizon when the plane of the loop is vertical. The nulls are sharp and deep but the maxima are broad allowing the nulls to be oriented in the direction of the noise without sacrificing much of the desired signal. The loop won't help much if the interfering signal is arriving from a high angle or from the general direction of {or 180 degrees from) the desired signal.

Most of the ham radio antenna manuals mention loops only in passing. A good design appeared in the 1989 edition of Proceedings. Written by Joe Farley, the article provides clear plans for the construction of a remotely-tuned, active loop. Proceedings is published by Fine Tuning. For those who do not care to roll their own, there are also several brands of amplified loops available from the usual distributors that advertise in the commercial SWL magazines. Commercial models are available in the $100 to $200 range The commercial loops come with active amplifiers to compensate for the loss of signal resulting from small physical size. Being inherently frugal (real cheap), I would prefer to roll my own loop taking advantage of the fact that the night-time atmospheric noise will usually allow a 30 db loss in efficiency before the signal to noise ratio is degraded.

I live in an area with outside antenna restrictions. What about buried or on-the-ground antennas?

Another class of antenna uses wires on, near or slightly below ground level. These antennas are called "wave" antennas because they extract energy from the wave as it travels down the length of the wire. The wave antenna achieves improved signal to noise because it is directional and because its low height minimizes static induced by charged particles blowing in the air Anyone who has tried to listen to a normal dipole antenna during a dry desert windstorm will appreciate this characteristic.

Figure 3. A simple Beverage antenna responds best to signals arriving from the East if the restistance is adjusted properly.

One of the earliest designs of this type is the "Beverage" antenna. No it is not a bunch of beer cans soldered end to end. The Beverage antenna was invented by a team led by Harold H. Beverage and first described in 1922. The historic article was re-published in the January 1982 issue of QST magazine and is recommended reading. The Beverage antenna, shown schematically in Figure 3, is simply a wire near the ground running in the direction of the desired station and terminated at the far end in a resistance to ground equal to the characteristic impedance of the line. This resistance is about 500 ohms and can be made variable for exact adjustment to minimize response off the back. A typical Beverage pattern is shown in Figure 4.

How long should a Beverage antenna be?

The length of a Beverage antenna can range up to thousands of feet for long wave reception. Mr. Beverage stated in a letter publish in QST in December 1981 that Beverage antennas should not be longer than one wavelength at the frequency of interest. A wavelength at 3 MHz is 100 meters and scales inversely with frequency. So one wavelength at 6 MHz is 54 meters and at 1.5 MHz is 200 meters.

Figure 4. Beverage antenna directional pattern after optimizing the terminating restistance.

Because the wave in free space travels faster than in the antenna, the longer the antenna, the greater the phase difference between the current in the antenna wire and the approaching wavefront. Once this difference becomes greater than 90 degrees, the wave actually starts to oppose the current in the wire.

Because SWLs rarely read QST, they have been merrily using much longer Beverage antennas for decades with considerable success. John Bryant concluded in an article in the 1989 Proceedings that the optimum length is about 5 wavelengths. Victor Misek says in his Beverage Antenna Handbook that the optimum length is about two wavelengths. In the mid 1970s the Canadian Communications Research Centre in Ottawa sponsored an exclusive study of the use of Beverage antennas for HF reception That work was briefly described by Belrose in QST for September 1981. Their work concludes that the longer the wire, the stronger the signal.

Thus, the experts cannot agree on what length is best which proves the best length is not terribly critical and probably depends on local ground conditions. The bottom line is that one or two wavelengths is probably sufficient for good results.

I live in an apartment. Can an active antenna help overcome locally generated interference?

Yes, active antennas can be used successfully. The balanced design of this particular antenna is claimed to cancel man-made noise. Because of their small physical size, these antennas can often be located in spots chosen to minimize pick up of locally generated noise. Such interference is often radiated directly from the source and indirectly by power lines, cable TV shields, telephone lines, wire reinforcement in stucco walls, metal structure in apartment buildings, TV antenna lead ins, etc. Every conductor is a potential re radiator of interference.

The direct signal can add in phase with the indirect signal or the two can add out of phase resulting in cancellation of the interference. If you explore your receiving location with a portable SW radio, you will find places where the cancellation effect can the noise to diminish or disappear. The null spots are usually quite small physically. The null locations will tend to change with frequency because the signals which are canceling each other travel different distances. As the frequency (and consequently the wavelength) of the signal changes, the relative phase of the direct and indirect signals will change. The physical place where they cancel will, therefore, move. An antenna which takes advantage of this effect must be physically small enough to stay within the null spot. Full size antennas occupy a lot of area making it difficult to keep them within a null region.

Recently, I was plagued by a touch control lamp in my neighbor's house which radiated strong, wideband buzzing harmonics across the HF bands. Not knowing immediately what was causing the problem, I explored my house with a portable SW radio in search of the offending signal. 1 discovered a location in the attic above my garage where the interference could not be heard. The direct signal from the lamp and the signal radiated from the power wiring combined at this one spot and canceled each other out An active antenna at this spot would be immune to the interference.

Do active antennas have a downside?

Active antennas have at least two problems. One is susceptibility to overload and inrermodulalion. The World Radio and TV Handbook did a review of active antennas a couple of years ago where they evaluated the various brands for overload performance. I no longer have the book but remember that the more expensive units gave the best performance (surprise!). Alan Johnson reviewed a representative unit in the December 92 NASWA Journal. The other disadvantage, which is common to all physically small antennas, is that small antennas are more susceptible to fading than large antennas. HF waves propagated via the ionosphere can simultaneously arrive at the receiving antenna after having traveled different distances. When this happens, the waves can add to improve reception if the waves are in phase. Conversely, if the waves are out of phase, deep fades of 20 dB or more can result. Long wire antennas do not experience this effect to the same degree, because their greater physical size means that somewhere on the wire, as any instant, the waves do not cancel.

The small antenna can thus experience a momentary gain of 6 dB as the waves coherently add in phase and momentary fades in excess of 20 dB when they cancel. The DXer who is patient can wait for the peaks to occur at the time the ID is given. Murphy's law, however, dictates that IDs will always occur coincident with fades.

Some engineers combine the active antenna amplifier with the loop and active vertical whip to provide a compact, efficient, directional antenna. Techniques also exist to phase active antenna to achieve multiple simultaneous beams and to generate multiple nulls On interfering signals. These techniques usually require many antenna elements, computer controlled phase shifters and specially designed software. Such antennas have folded application in military and intelligence circles but the complexity is beyond the capability of SWLs without PhDs in electrical engineering to implement.

I have a large yard but outside antennas are forbidden. The copper police keep demanding that t take my antennas down. What can I do?

We have discussed the Beverage antenna which can be mounted close to the ground. Another design that has gained some popularity is called the "snake". The snake has been around for about 15 years but knowledge about it has been confined to the radio underground (pun intended). The snake is shown schematically in Figure 5.

The coax cable center conductor is connected to the receiver antenna terminal. The shield is left floating at the receiver. The shield and center conductor are shorted together at the far end. Aficionados of this antenna say it has very low noise and is broadbanded. Personally, I find little theoretical reason to believe this antenna would be any better than an equivalent length of insulated wire laid on the ground. With cheap coax selling for $.20 to $.50 per foot' to extra expense of using coax seems unwarranted.
I have used a 100 foot wire buried a few inches below ground level for about a year now. The gain is down about 30 dB referenced to a half wave dipole on most signals around 6 MHz. When conditions are good the signal to noise at this frequency approximates the dipole. When signals are disturbed by ionosphere absorption effects, the additional loss of the buried antenna reduces the already weakened signals so the dipole has a signal to noise advantage. In the medium wave spectrum the buried wire outperforms the dipole. I use this antenna to listen to New York City broadcast stations during the day from any location some 290 miles to the south. The buried wire is susceptible to noise pickup where the wire enters the house. I can hear any computer monitor and printer doing their things on this antenna but not on the dipole.

One solution to the local noise problem is to combine the single buried wire with a coax feedline to shield the line from noise pickup in the radio room. A design which I intend to try is shown in Figure 6.
The coax cable should only be long enough to keep the wire away from the noise source. The ohm resistor serves as a load for the coax cable An improperly matched cable could cause the antenna to appear to be a near short circuit across the receiver antenna terminals at frequencies where the coax length is an electrical quarter wavelength. The 50 ohm resistor provides a reasonable impedance match which avoids such resonance effects.

The answer depends on what you want to do. SW broadcasters targeting international audiences aim for the lowest possible radiation angle to minimize the number of hops the signal takes bouncing between the ionosphere and earth. Each bounce increases the loss. The objective is to provide consistently strong signals on the target area every day. The SW DXer on the other hand often waits months for the unusual propagation condition that brings in that weak Bolivian or Indonesian station.

These anomalous propagation modes are often associated with ionosphere ducting, grayline, or "whispering gallery" effects with high arrival angles.

Figure 7 shoves the elevation angle response of a horizontal dipole over perfectly conducting ground.
Over normal dirt the pattern has the same shape except that the nulls are less deep and the peaks are not as strong. As the antenna height is reduced to a quarter wavelength, the pattern shifts to direct the peak response straight up. At lower heights, the pattern shape remains nearly constant, only the efficiency decreases. Antennas which are less than 40 feet high and operating at 6 MHz and below will have this kind of vertical response pattern.

A high antenna which concentrates reception at low angles will be the best day to day performer. A low antenna which maximizes reception from high angles takes best advantage of the unusual conditions that result in the really rare DX catches. Unusual propagation is most frequently observed on the tropical bands because the transmitting stations usually use law dipoles to ensure good signals at ranges of a few hundred miles. When conditions are right, the high angle radiation is propagated to the other side of the world instead of being reflected to the target areas. The signals arriving from high angles are most efficiently captured by receiving antennas with high angle patterns.

Murphy must have been asleep when the laws of physics were drafted because he allowed the easiest antenna to erect to also be the best for DXing under anomalous propagation conditions.