The bane of your existence
If you use, operate, design, install or maintain a radio-frequency communications system, you at some point will have to deal with a phenomenon known as radio-frequency interference, or RFI. In this article, we will define the specific kinds and types of RFI, how each type manifests itself, and some of the protection devices available.
RFI comes in three basic types:
- Natural noise
- Manmade noise
- Other radio systems
We will discuss each type of RFI in this article.
Natural RFI does exist, and by using good engineering practices, the effects can be reduced. In most cases, a grounded antenna lessens the effects of natural RFI. The following are the causes of such interference.
Cosmic radiation: Radiation and signals do arrive from outer space. In addition, the radiation that does come from space is ionizing, which means that it also is harmful to human tissue, such as skin and eyes. Fortunately, the amount of cosmic radiation is very low, and for most radio systems, the effects are negligible.
Thunderstorms: The lightning discharges from thunderstorms create static “crashes” in radio receivers, and the lower the frequency, the further away that these crashes can be heard. On a VHF low-band system, this can be hundreds of miles away. On a VHF high-band system, this can be dozens of miles away. On UHF and above, only stations in a radius of 50 miles or less will be affected by the lightning discharge.
Besides the interference issue, a lightning hit on a radio system will cause damage to the system. A direct hit of lightning to an antenna usually will result in serious damage.
Wind: When the humidity is very low, and the wind is high (above 30 mph), this wind will cause a potential difference of hundreds of volts to appear on ungrounded antennas. When the voltage reaches a point where it can arc across the coax terminals and antenna or radio connections, it will discharge with a loud crash similar to lightning discharging in close proximity to the station receiver. In some cases, it can damage the pre-amplifiers in the stations, so good engineering requires you to use only grounded antennas in locations that have very low humidity and a history of high winds, such as mountaintop sites.
Static electricity: In some instances, you will hear a static electricity discharge on a channel that is in close proximity to a thunderstorm, but there is no lightning present at the time. Again, a grounded antenna keeps this from occurring.
When an electrical circuit is energized or de-energized, it will emit a small amount of RF energy that will be of very short duration. If you have radios on top of a building that have hundreds of circuits that are turning on and off all of the time — such as air handlers, air conditioner compressors and elevator motors — the sum of these very short random pulses will become a constant roar to the radio systems on the building and the radio receiver noise floor will be high.
Besides the transient pulses described above, electrical motors put out RFI when the brushes on the armature are worn. New brushes and cleaning the inside of the motors usually fix these problems.
Power transmission lines also insert energy into the radio spectrum, but this normally is not very problematic if all of the connecting hardware is bolted together properly. However, when the connections are old and rusty, or when carbon exists in the dust on the insulators, then the power lines will radiate an extremely high level of RFI. This kind of RFI will shrink substantially the range of the radio system; ergo, the source of the power-line RFI must be located, especially for systems that are using narrowband or digital modes of operation. The power company will gladly help to fix these problems, as they have three good reasons for doing so. These are:
- Electricity is going though their network and nobody is paying for it.
- They are providing poor service to a customer who has this kind of problem on their lines.
- In the U.S., the FCC will fine the power company $50,000 to $100,000 for failing to fix this problem — if it is reported to the commission.
The power company will cooperate with you, but you have the responsibility to find the specific area from where the RFI is emanating.
The last kind of manmade noise that you need to be vigilant about concerns the use of compact fluorescent light bulbs, or CFLs, on towers and building rooftops. These kinds of light bulbs may save electricity, but they play havoc with the radio signals of the radio receivers at the site, as they radiate an extreme amount of RFI. You cannot have these kinds of bulbs on a tower or building rooftop.
Radios can and do cause interference with each other. The types of interference usually fall within one of nine RFI classifications, and the remedy is associated mainly with the classification of the problem or root cause. The classifications are as follows:
- Spurious emission
- Spurious response
- Desense and blocking
- Co-channel interference
- Adjacent channel interference
- Transmitter noise
- Image frequency
We will discuss a few of these issues individually.
Intermodulation, or IM, occurs when two or more radio frequencies combine to form other frequencies. Intermodulation comes in three forms. These are: transmitter, receiver and external. Transmitter intermodulation is the most common type, and it is the responsibility of the offending transmitter operator to correct this problem. The other two are a nuisance, and it usually takes a combination of fixes to correct these issues.
If there is one good thing about intermodulation, it is that the frequencies involved are related mathematically and can be predicted and corrected relatively easily. We will use some examples to demonstrate how the math works.
(2xA) — (1xB)=C
(2×150) — (151)=149 MHz
(2 X A) — (1 X B)=C
(302) — (150)=152 MHz
These examples are called third-order IM products. This is derived by adding the numeral 2 in the equation (which is the coefficient of A) to the numeral 1 (which is the coefficient of B). If the mix was caused by (3xA) — (2xB)=C, then this would be a fifth-order IM product. Similarly, if the mix was caused by (4xA) — (3x B)=C, then this would be a seventh-order product. The coefficient is directly proportional to the number of harmonics that are present in the signal produced by the radio's transceiver. We will cover harmonics in greater depth in the next installment. Note that if no number is present, a 1 is always assumed as the coefficient, as it represents the fundamental frequency.
There can be multiple channels involved, and it is not uncommon for a mix such as the following to occur:
1A+1B+1C — 1D — 1E=F
This example would be a fifth-order product.
In the past, almost every IM product was an odd-order product. This is because odd-order (3, 5, 7, 9, 11, etc.) products fall within the same general frequency band as the individual channels that are involved.
However, as more radio signals are popping up in more places, even-order (2, 4, 6, 8, 10, etc.) products are being introduced. An example would be the mixing of a 929 MHz paging system and an FM broadcast station.
1B — 1A=C
929.1 — 90.1=839.1 MHz (which falls within the input side of a cellular radio system)
As you can see, the second-order IM product falls outside the band where the contributing frequencies are located. This is true of all even-order products.
With the cellular band occupying 825 MHz through 850 MHz and 870 MHz through 895 MHz, and 800 MHz trunking occupying 805 MHz through 823 MHz and 850 MHz through 868 MHz, the entire VHF band is vulnerable to even-order IM products. This occurs when such equipment is operating near equipment that is operating in any of the cellular or 800 MHz trunking bands.
Sometimes multi-channel trunking radio systems cause intermodulation products within their own systems. The next example will demonstrate this phenomenon.
B+C — A=D
455+451 — 450= 456 MHz
In this example, 456 MHz is the receiver frequency for the 451 TX channel. This third-order product happens on every combination of a multi-channel trunking system.
Intermodulation can and does occur in transmitters, in the front-end RF stages or mixer stages of receivers, and on any surface where non-similar metals or rust occur in close proximity to strong radio signals (over 0.2 VAC, or voltage in alternating current).The key to correcting IM is to eliminate one or more of the participating interfering channel frequencies.
Spurious emission occurs when a transmitter broadcasts a signal that is not mathematically related to the channel or the low-level stages in that transmitter. The most common cause of a spurious emission is a bad connection or oscillation within a low-level stage in a transmitter.
Spurious emissions are always FCC violations and must be corrected once it is brought to the attention of the radio system operator or licensee.
Meanwhile, just as a transmitter can have a spurious emission that puts out energy on an errant, non-assigned or related frequency, the local oscillator can do the same thing in a receiver, and that receiver will have good sensitivity to a channel to which it is not supposed to be listening. This is called spurious response. Normally, you can retune the receiver's local oscillator to correct this problem.
Part 1: Class is in session: Basic LMR and FCC definitions
Part 2: Start at the beginning: Understanding LMR user needs
Part 3: The devil's in the details: Conducting a user-needs survey
Part 4: Decisions, decisions: Understanding the LRM procurement process
Part 5: Let's get started: System engineering begins with RF planning
Part 6: The lynchpin: Receiver planning and noise interference
Part 7: Connecting the dots: How to connect LMR sites
Part 8: The next piece of the puzzle: Understanding dispatch communications
Part 9: Now the real work begins: How to select a suitable LMR site
Part 11: Winning the battle: What causes radio frequency interference
Ira Wiesenfeld, P.E., is a consulting engineer who has been involved in the radio communications business since 1966. He is a senior member of the IEEE and has been a licensed amateur radio operator since 1963. He can be reached at firstname.lastname@example.org.
Robert C. Shapiro, P.E, is the senior manager-systems engineering for PlantCML, an EADS Company. He serves on the TIA TR8 committee as the TSB-88.4-C task-group chair and is a senior member of the IEEE. He can be reached at email@example.com.