By Joe Buch, N2JB
(NASWA Journal, March 1993)

Fading is the most important cause of distortion that detracts from the enjoyment program material transmitted on shortwave. Fading occurs on strong signals and weak signals. Increasing the transmitter power does little to improve the distortion caused by fading. An analysis of the different causes of fading is presented this month along with some ideas on measures broadcasters and listeners can take to reduce the effects of fading.

What causes fading?

There are two primary causes of signal fading on shortwave multipath cancellation and polarization rotation. Each type of fading results from different mechanisms and each has its own remedies. These effects can be minimized by appropriate design of transmitting antennas, receiving antennas, receiving techniques, and redundancy in the receiver configuration.

Figure 1. Multipath propagation causes fading when waves arrives out of phase.

Multipath propagation results in the signal being received by the listener over two or more paths. A typical circuit is shown in Figure 1. One path consists of a single, low angle hop from the transmitter to the receiver. The other path consists of two hops at a higher angle. The two hop path is physically longer than the single hop path. When the two waves combine at the receiver' they can be in phase. In this case the waves act to reinforce one another making the received signal stronger. Because the delay difference is a function of the path length difference, the waves can just as easily arrive out of phase causing cancellation or what we normally call fading.

Wavelength is inversely proportional to frequency. The bandwidth of an AM transmission is normally about 10 kHz. Sidebands extend above and below the carrier frequency by 5 kHz or more. Waves arriving via multiple paths can cancel at one frequency but not at another. This effect gives rise to what is called ''selective fading". Selective fading can result in the carrier fading while the sidebands remain strong. The result is severe distortion similar to overmodulation. Selective fading can also result in one sideband fading while the other remains strong. Distortion also results from this condition.

The polarization of the signal after it passes through the ionosphere is rotated by a phenomenon known as "Faraday Rotation". Normally, the signal transmitted by the broadcaster is horizontally polarized. (WWV is an exception to the rule; they use vertical polarization.) This means that the electric field is parallel to the surface of the Earth. When the wave transits the ionosphere, the presence of free charged electrons and the Earth's magnetic field combine to cause the electric field vector to rotate. Thus, the wave returns to Earth with the electric field oriented randomly. When the receive antenna wire is aligned parallel to the electric field vector, maximum energy is captured from the passing wave. When the wire is oriented perpendicular to the electric field vector, no energy transfer occurs. As the orientation of the electric field vector shifts, the signal fades up and down The amount of rotation of the electric field vector at any instant is a function of frequency so selective fading can occur at different frequencies within the signal bandwidth.

How can the situation be improved?

International broadcasters already attempt to minimize multipath propagation by concentrating radiated energy at the horizon. Curtain antenna arrays and rhombic antenna designs used by the big guns attempt to minimize high angle minor lobes to reduce the amount of energy reaching the receiver via more than one path.

FM broadcasters have found that automobile reception fading is reduced by transmitting circularly polarized waves. Circular polarization can be thought of as simply horizontal and vertical radiation with a 90 phase shift between the two polarizations. Although such transmissions would seem to be beneficial in reducing polarization-induced fading on HF paths, no international broadcasters are known to be using this technique.

The effects of selective fading can be minimized by the use of single sideband (SSB) and synchronous detectors. Single sideband reception of a double-sideband, AM signal allows the operator to select either the sidebands above the carrier frequency (USB) or below the carrier frequency (LSB). Ignoring the other sideband allows that sideband to fade without contributing to distortion. The SSB receiver also reconstructs the carrier. Selective fading of the carrier does not contribute to SSB distortion because the carrier is filtered out before the detector.

High fidelity reception of SW signals using the SSB technique is not possible for two reasons:
1. The frequency of the reconstructed carrier is not exactly the same as the frequency of the transmitted carrier;
2. The frequency of the signal as received will wander due to path length variations which result in Doppler shift.
Listen on 5 MHz some night when YVTO and WWV are both competing. Beats of three or four Hz are clearly evident even though the frequencies of both stations are controlled to better than one thousandth of a Hz by atomic standards. The Doppler shift varies erratically with time in an unpredictable manner. Three or four Hz error does not bother voice intelligibility but would be noticeable to a good ear on music.

Doppler shift can be eliminated by allowing the reconstructed carrier to track the incoming signal. This is where the synchronous detector shines. Signals demodulated by a sync detector are not subject to selective fading of the carrier as long as there is enough carrier present to keep the detector locked. Long time constants on the carrier tracking, phase locked loop allow the synchronous detector to "fly wheel" through short carrier fades without losing lock.

These distortion causing effects can also be reduced by the use of diversity reception. Harold Beverage of Beverage antenna fame died on January 27, 1993 at the age of 99. He had over 90 patents to his credit but the two he was most proud of were the wave antenna, as he called it originally, and diversity reception. There are three types of diversity reception techniques which the SWL can use to minimize fading and distortion:
1. Frequency diversity;
2. Spatial diversity;
3. Polarization diversity.

In all three cases the same information is receive via redundant paths and combined at the receiver site into a high quality signal.

Define what is meant by frequent diversity

In frequency diversity reception the same program material is received simultaneously on two or more frequencies. If the frequencies are separated by more than 100 kHz, the paths will likely fade at different times. Let's say the BBC can be heard on 12.095 and 15.070 MHz. You will need one receiver on each frequency. Combine the outputs and you have frequency diversity reception. One caution must be observed. The transmissions should originate from the same site. Relay stations are typically fed by satellite circuits. Each satellite hop to geosynchronous altitude results in a round trip time delay of i/4 second. An annoying echo effect is all that will be heard if the two frequencies do not originate at the same site.

How are the signals combined?

There are two general methods, pre-detection and post-detection, for combining diversity signals. Pre-detection combining can have some signal-to-noise ratio advantages but requires complex circuits and phase-stable paths. Because ionosphere-reflected signals continuously shift in phase as the path length changes, post detection combining is the most practical technique for the SWL. One approach could use the AGC voltage from each of two identical receivers to control the switching logic. The logic would select the stronger signal at any instant.

A simpler approach would be to feed each receiver into the separate inputs of a stereo audio system. The combining is then done in the listener's brain. As fading occurs the aural image of the sound will appear to wander between the two stereo speakers.

Figure 2. Two receivers, two antennas and a stereo amplifier makes a poor-mans diversity reeiver.

This wandering effect might at first be annoying but with a little "getting used to'' will provide a dynamic demonstration of the variability of ionospheric paths, If the listener finds this effect annoying, the stereo amplifier can be configured for monaural operation. In any case the gain of each receiver should be limited by use of the RF gain control to avoid noisy, distorted audio during fades.
Such a system is depicted in block diagram form in Figure 2.

How does spatial diversity work?

Spatial diversity is based on the fact that if the signal is received on two antennas separated from each other by more than a wavelength or so, the fades will not be correlated. Fades on one antenna will not be accompanied by simultaneous fades on the other antenna. In this case both receivers shown in Figure 2 could be tuned to the same frequency.

Another form of spatial diversity can be obtained with a single receiver by simply using a longwire antenna of more than one wavelength, The theory here is that the large physical size of the antenna means that somewhere on the wire the multipath waves are not canceling so there will always be an output, Longwire antennas, mounted high and in the clear, will exhibit gain and directivity at low elevation angles, Because high angle multipath signals are discriminated against by such an antenna, fading effects are reduced compared to a half wave dipole or smaller antenna.

What is polarization diversity?

Polarization diversity allows the receiving system to extract energy from the wave regardless of the orientation of the electric field vector. One approach to polarization diversity is to orient the two antennas in Figure 2 so that one antenna responds to horizontal fields and the other responds to vertical fields.