Who needs all that receiver sensitivity on HF?

Having listened and spoken to many radio enthusiasts over the years, I have found that many myths and misconceptions about the theory of radio abound. One of them is the debate on which receivers are good and which are bad. There are far too many reasons to mention here why one receiver should be better than another, but in this article I will concentrate on sensitivity requirements for HF receivers.

Sensitivity is how good a receiver is at receiving weak signals. The factor which ultimately determines receiver sensitivity is temperature. All electronic components generate noise due to the random movement of electrons, and if the electron movement can be reduced by cooling the components, this random movement can be reduced, and hence the noise. Receiver front ends used in satellite systems and radio telescopes are cooled to temperatures not too far from absolute zero (-273ΓΈ Celsius) by immersing the components in a liquified gas, to reduce the noise to a minimum. These lengths are not required on HF as will be seen. Sensitivity also depends on the receiver bandwidth. The narrower the bandwidth, the more sensitive a receiver can be made. The wider the bandwidth, the more noise, whether it be self generated in the receiver or noise picked up on the receiving antenna will be passed through the receiver. The bandwidth required for communication is proportional to the rate of change (bandwidth) of the information to be transmitted, and for optimum communication, efficiency the receiver bandwidth must be tailored to the bandwidth of the transmission.

For broadcast television, 6MHz of bandwidth is required to carry all the picture information, and about 200kHz is required for broadcast FM. Bandwidths for HF communication are usually considerably less than this. Speech contains frequencies from about 100Hz to 10kHz, however, many of these frequency components are at a very low level and are redundant as far as intelligibility is concerned. For broadcast AM in the UK, an upper frequency limit of 4.5kHz is imposed, which still gives acceptable fidelity on speech, although music doesn’t sound quite so good. This requires a receiver bandwidth of 9kHz (because in AM two sidebands are transmitted, each occupying 4.5kHz of spectrum). These two sidebands carry identical information, so one of them (along with the carrier which carries no information) can be dispensed with, leaving a single sideband (SSB) signal.

It can be seen that because the bandwidth is reduced, in this case by a factor of two, only half the receiver bandwidth is required thus providing an effective sensitivity increase. (It doesn’t matter whether the upper or lower sideband is used, because the information they contain is the same, except that the lower sideband is transmitted frequency inverted, but this is sorted out by the receiver). For communication speech, it is generally regarded that 1.8KHz to 2.4kHz is required, and one or both of these bandwidths are commonly used in receivers designed for SSB operation. The rate of information on CW (at the speeds used by most HF stations) is very low, and bandwidths as low as 30Hz can be used. This gives a tremendous advantage as regards noise reduction, and this is probably the main reason why CW is still so popular in amateur circles, although it is rapidly becoming an outdated mode for commercial HF communications, being superseded by other forms of data transmission, most of which contain some form of error correction.

On frequencies to up to 30MHz, with a dipole or reasonable length of wire used as an antenna, in the absence of a wanted signal, a level of background noise above that generated by the receiver will be received. This will be quite high in an urban environment, coming mainly from man made sources, but even in a quiet country location a reasonable level of noise will still be received. This is mainly what is commonly called static or atmospheric noise, and is caused by the signals from hundreds of lightning flashes per second from all the storms on the earth being propagated around the globe. This level will vary with the time of day and time of year depending on propagation conditions. If the storm is very close, hundreds of volts will momentarily appear at the receiver antenna terminal from a close discharge. The level of this static tends to fall off at higher frequencies, where cosmic noise (noise generated away from the earth) predominates, mainly coming from our milky way and the sun. Receiver specifications for amateur receivers are normally given in uV (micro volts), with different figures being given for the different modes, because of the different bandwidths used. For SSB, figures of .25uV for 10dB signal to noise ratio in a 2.4kHz bandwidth are typical for a modern amateur HF communications receiver.

If narrower bandwidths are fitted for CW then the sensitivity could be considerably better than this easily by a factor of 10 or more (.025uV), moreover, figures for AM will be slightly worse than the SSB sensitivity. With all but the worst antenna, the atmospheric and cosmic noise will generate voltages higher than this figure. On the lower frequencies at night the ambient noise might easily rise to 40dB or more above this figure, this equating to a signal level of 25uV in a 2.4kHz bandwidth. It is obvious from this that wanted signals below this level just cannot be received, much of the receiver sensitivity being wasted.

As the frequency is increased the ambient noise level decreases, and it is not until about 30MHz that most of the receiver sensitivity can be utilised. Receiver sensitivity can also be expressed in terms of noise figure. This is way of expressing the receiver sensitivity which takes into account the different bandwidths, and it is a measurement in dB that the receiver falls short of a receiver having no internal generated noise, a figure of 7 to 10dB being typical for an HF receiver. On low frequencies, it is often better to use an amount of antenna attenuation. A range of attenuation is often provided on modern amateur receivers, and in some cases will prove useful in minimising receiver generated spurious signals due to deficiencies in the strong signal handling performance of the receiver. It can be seen that 40dB can be used on the lower frequencies with no degradation of the wanted signal, which will be riding on 25uV of ambient noise. Situations where good sensitivity is required, is where a very short antenna is used and the level of ambient noise picked up by the antenna is less than or equal to the receiver sensitivity.

It is in this case that a good ATU comes into its own by matching the antenna impedance to 50 ohms, and transferring as much of the signal in the antenna as possible into the receiver, thus overcoming it’s internally generated noise. Using an ATU with a large antenna, although it might increase the signal level on the S meter, will not improve the readability of the signal, because the level of ambient noise will increase along with the wanted signal. In some cases it might improve readability, such as the case where very strong out of band signals (now rejected) might cause receiver overload. Likewise, using a preamplifier with a large antenna will not improve the signal readability, and stands a good chance of degrading the readability due making the receiver’s high signal performance even worse. The only way to increase the readability of a signal is by using another type of antenna which favours the wanted signal over that of the ambient noise, hence the popularity of directional antennas.

Sensitivity once upon a time used to be the be all and end all of a receiver specification. Nowadays it is easy to make an HF receiver with internal noise significantly less than ambient noise from the antenna, and there are far more important criteria than sensitivity that makes a good receiver. If you are still using an old insensitive valve receiver on the lower frequencies you will be missing nothing as far as sensitivity is concerned, and because of the superior strong signal handling capabilities associated with valve receivers, at least under strong signal conditions the likelihood is that you will be listening to the right signal and not some spurious one! The next time you buy an HF receiver, don’t choose it because it is a few points of a uV more sensitive than another one – there are many other more important factors to be considered. I have tried to keep the explanations simple so that some of the basic principles can be understood by all, and I hope the more knowledgeable among you will forgive me for this. In future articles, I will be discussing some of the other characteristics that go to make a good HF receiver.

Tony Richardson.

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