We often hear reference to "noise" in antenna work, but often we are not sure what kind of noise is being talked about. So let's talk about noise and antennas. "Noise" comes in a wide variety of styles, and here is one way to divide the group into usefully smaller chunks.
Noise generally refers to any signal components at the point of demodulation except the intelligence of the desired signal. Noise may come from outside the receiver or be generated within it. In fact, all active devices in a receiver generate some noise, creating a so-called noise floor that determines the absolute minimum noise level for the receiver. Receiver designers constantly seek to improve the noise floor, but at the lower HF bands, other noise sources generally make the noise floor a hypothetical idea. Only at the upper HF regions can the receiveržs noise floor be perceived with an antenna connected.
1. Man-made noise: This category includes the usual machinery sparking, faulty signs, auto engine sparking, etc. As you can see from thinking about the sources, it largely derives from spark generation and hence produces useless RF over a wide frequency range. Most human-made noise is vertically polarized and of ground wave propagation. Hence, ground-mounted verticals are most susceptible to this category of noise. A horizontal antenna generally shows an immediate 3 dB reduction. Additionally, antenna elevation also helps reduce the noise level. Finally, a narrow-band antenna also reduces the total amount of noise energy in this category from reaching the receiver. A parallel feedline-ATU arrangement sometimes shows improvement over the same antenna fed with coax by filtration action, i.e., narrowing the bandwidth of the energy allowed to reach the receiver.
One technique that has been the subject of recent articles is the use of a short vertical noise sensing antenna (long enough to pick up local noise but too short for effective reception of propagated signals), inverting its signal, and combining the result with the regular antenna signal. With proper adjustment, local human-made noise can be canceled quite effectively, with only slight reductions in received signal strength. The benefit lies in the large improvement in signal-to-noise ratio, the truer mark of effective reception.
Except for very near by sources, such as an arcing pole pig, man-made noises create the most problems on the lower HF bands. Near by sources include any number of household devices, such as touch control lamps, dimmer switches, and similar devices. The noise from these devices often enters the receiver not only though the antenna terminals, but as well through the power lines and the grounded case and chassis. Remember that the receiver does not care if a signal varies around some preset bias level on the main signal line or if it alters the level of the common buss while the bias remains constant. The cure for these noise sources lies at the sources themselves.
2. Atmospherics: Atmospheric noise refers to radio frequency energy generated by natural phenomena and carrying no man-made communications intelligence. There are two main sources of "atmospheric" noise and energy coupling to antennas:
a. Sparks: Nature also generates wide-band sparks in the form of lightning. There are other atmospheric noise sources, but especially on the lower HF bands, QRN is largely propagated lightning signals. Lightning energy is generally AM modulated, and the difference between lightning modulation and some forms of rock music played by AM stations has been disputed. As with all spark energy, the energy decreases as the frequency increases, hence, the quieter high bands. There is little difference in the reception of propagated spark energy between vertical and horizontal energy, since the polarization is lost in the skip refraction.
Narrow-banding the pre-receiver reception system can reduce the total energy from such signals that reaches the receiver front end. One way to achieve a narrower bandwidth for signals entering the receiver is to place a narrow band, tunable filter at the antenna terminals. Except for pure receiving systems, this technique is not in wide-spread use by amateurs because it tends to attenuate outgoing signal power as well. The alternative is an equally undesired complex switching system to remove the filter during transmit cycles.
An alternative is to use high impedance antennas that require sharp adjustment, usually of an ATU, as the frequency is changed. Even though the bandwidth of filters and sharply tuned antennas is wider than the IF passband of the receiver, the effects of noise reception are reduced. Receivers respond to signals and noise over a wide range, and this response can activate the AGC system, create thumps and other unwanted outputs, and otherwise disrupt reception of the desired signal. The narrower the bandwidth of the receiving system at the antenna terminals, the less noise is a problem.
Since the perceived level of spark energy is the sum of energy received from all angles accepted by the antenna, narrowing the overall beamwidth of the antenna is one way to reduce noise levels. One might think that Yagis and other unidirectional antennas might achieve this, since they reduce radiation to and reception from the rear. However, their higher gains often offset this quieting for an overall tie with a dipole at the same height.
On the lower HF bands, one can feed certain vertically oriented loop antennas so that the horizontal radiation pattern is largely self-canceled. The horizontal portion of the antennas pattern is largely responsible for higher angle radiation in a broad vertical and horizontal beamwidth. By eliminating this portion of the pattern, as shown in Figure 1, the overall gain and receptivity to noise and signals is lowered. However, the remaining portion of the pattern is at low elevation angles that favor dx. Although the signals are usually weaker, their signal-to-noise ratio is much higher, resulting in higher quality reception. This technique, championed by ON4UN and other low-band DXers, proves that antenna gain is not everything, especially if that gain increases noise levels faster than it increases desired signal strengths.
b. Charges: The more that air molecules strike each other, the more they lose electrons and become charged. The thinner the atmosphere, as at high altitudes, the longer molecules can stay charged before recombining with lost electrons. It is from phenomena such as these that we get the static charge build-up on antennas. For most home antenna systems, charge build- up was no real problem with tube grids, but a real problem with solid-state front ends. The longer the antenna wire, the windier the location, and the drier the air, the more likely that static charge can build to damaging proportions. At the very least, static charge collection on an antenna is an additional noise source and problem.
For some antennas mounted very high, the energies involved could not be drained effectively before damage occurred to antenna elements. At the extreme, antennas can display St. Elmožs fire. The development of the quad loop was to solve HCJB's end coupling problem with its Yagis: at the high altitude of Quito, Ecuador, the energy coupling did not produce a mere glow; it was burning the ends off the antenna elements.
Loop antennas have no ends: hence, for a portion of the incoming energy, there is a reduction in the amount of energy coupled to the antenna from wire-end capacitance. Where the high voltage region is distributed across a wire length, whether vertical or horizontal, capacitive coupling is minimized. For this reason, some operators find quads and other loop antennas quieter than Yagis and dipoles.
Regardless of antenna type, static charge is simple to drain away. One technique is to have the antenna at DC ground. Some antenna designs are naturally at DC ground. Loops go from the coax center to coax braid, and if the braid is well grounded, the charge does not build up. Placing an RF choke across the antenna terminals or from the hot terminal to a ground line can continuously drain charge build-up. In some multi-band antenna systems, parallel feed lines can carelessly omit this protection, but a pair of RF chokes, one from each line to ground where the feedline enters the house, can protect equipment. However, remember that the impedance level at that point can be high, requiring a very high value of RF choke to ensure that significant signal energy does not go through the choke.
Modern receivers usually employ noise blankers to eliminate as much noise as possible. Older receivers employed noise limiters. A noise limiter simply clips a signal as it exceeds a preset level. This level was set somewhat above the level of the strongest anticipated intelligent signal so that what was clipped had to be noise energy. Unfortunately, clipping often introduced distortion and mixing products (see next entry) into the remaining stages of the receiver chain.
Noise blankers involve sending part of the signal energy to a separate amplifier. This amplifier is designed to detect energies of durations shorter than most forms of intelligence. Each energy pulse so detected is used to trigger a circuit that turns off a designated amplifier, usually in the IF chain. Hence, the amplifier is žblankedž for a period too short for the human ear to tell. Gone is the noise pulse. Unfortunately, noise blanker circuits are imperfect devices: the sudden on-off square wave cycling can create distortion of the desired signal, as well as some mixing products.
3. Mixing products: Two signals, neither of which is on the frequency to which we are tuned, can be mixed and produce a third signal (or a bunch of signals) that may fall on a frequency we want to use. The cure for mixing products begins by locating where the mixing occurs. If the mixing occurs in the receiver, then filtration of the unwanted frequency (or frequency range) is the best solution. If the mixing occurs externally to anything one's receiving and antenna system can control, then there is no cure immediately at hand. However, such problems often involve violations of technical standards by one or both of the signal generators involved as the sources of the mix, and patient bureaucratic pressure can sometimes alleviate the problem.
If the mixing occurs within one's antenna system, then there is usually something wrong with the system--bad connections, unwanted couplings, less than optimal tuning set-ups: all of these are correctable and should be part of one's routine periodic maintenance on the antenna system. Some mixing problems occur from žfront-end overload;ž that is, signals are so strong that, regardless of frequency, they enter into the mixing and amplifying chain of the receiver. Very often for low-HF operators, these signals are AM broadcast band signals. The cure usually involves a trap or narrow bandpass filter at the antenna terminals.
Along the way, we have mentioned ATUs, bandpass or trap filters, and other circuits external to a receiveržs antenna terminals as possible aids in noise reduction. It is possible to end up with a string of small boxes, each containing a certain kind of filter, and each connected in series with cables and connectors. Although this situation is typical ham, it is better suited to the test bench than to a communication system. Cables and connectors are invitations to new ways of noise pickup. A better system would be to build all of the necessary external noise reduction tuning and filter devices into a single case with direct (or switched) connections between them.
We have not covered all the noise sources. Power company equipment problems, such as arcing pole pigs, require a simple procedure: locate the problem transformer, keep on reporting the situation until you get action, and hope there is a ham on the technical staff that handles such complaints. RFI from light dimmers and other home products that use AC waveform chopping to control a voltage level has been noted in many articles and requires that we locate the source and cure it individually. Likewise with noise from computer timing circuits. ARRL maintains a bibliography of articles on RFI and related matters for use by interested amateurs.
A series of articles on antennas is hardly the place for political comment, but it is a sad though hardly surprising fact of life that commercial interests constantly pressure legislatures and regulatory agencies for relaxation of standards for interference and noise production. Counter- pressure from amateurs and others adversely affected by eliminible noise and interference is necessary to at least hold the line on the ever-noisier RF environment. Although there are no guarantees of success, especially since amateurs can rarely marshal the monetary resources for high-pressure lobbying, constant goading and enlightenment of regulators can at least minimize the rate of noise increase.
Although every amateur operator has a vested interest in identifying, controlling, and eliminating man-made sources of noise, we should all remember that noise is a reciprocal issue. We should also go to great lengths to ensure that our stations and test benches are not noise sources relative to any other devices able to pick up RF energy. This includes such non-RF devices as modern telephones, along with their portable RF cousins. The less we tolerate noise and interference that might emerge from our own equipment, the more justified we are in insisting that other interests do likewise.
The final defense against noise is ultimately the same as the final defense against both willful and unwitting QRM. That defense is to perfect our abilities as operators. When our equipment is state-of-the-art relative to noise elimination and reduction, we have nothing left to use in the war on noise but our well-honed skills as operators.
Operators skills involve three areas of effort: equipment adjustment, reception, and transmission. We need to perfect our abilities to get the most out of the equipment at hand. This includes experiential knowledge of where to point pointable antennas for the best signal-to-noise ratio; how to integrate bandpass, bandwidth, and blanker adjustments for maximum effectiveness; and a myriad of other subtle receiver settings. In addition, we need to constantly practice receiving signals in order to tune our ears more finely than any electronic decoding device can match. Lastly, we need to transmit not only with the precision that makes code or voice reception Q5, but as well with the intuition that alters code or voice speed and patterns to fit the operator at the other end of the line.
In the end, however, some folks are condemned to live in areas where noise is beyond control and even beyond the ability of the best noise blanker (in the receiver or between the ears) to handle. The solution, short of illegally de-powering these sources, is to save money and move to a quiet location--or to concentrate on portable operation. Short of those drastic remedies, however, antenna choice, feed system choice, filtration, noise cancelers, noise blankers, and operator skills can go a long way toward reducing currently unlivable noise to a mere constant irritation.
Updated 7-11-99. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.
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