Winning the battle

Last month, we discussed radio frequency interference, starting with the different types of natural and man-made noise that have a detrimental effect on system performance. In this article we continue to examine the causes of RFI and then proceed to the devices designed to protect against it.

Desense and blocking. If a strong signal — on the order of 60 dB or greater than the signal that you are trying to receive — is offset by one or more channels away from the receiver channel, you can get into a situation where the mixer stage of your receiver no longer functions properly and the receiver no longer hears the unit to which the channel is tuned. This is called “desense” or ”blocking,” and it is quite common at crowded radio sites.

As narrowbanding allows for more channels to be added to the VHF and UHF spectrum, you will see a lot more of this phenomenon. In addition, the normal adjacent channel rejection of the IF filter will not be accurate in those situations where the desense has impaired the receiver to the degree that it no longer will hear the adjacent channel, because the mixer no longer is working properly.

Co-channel interference. Because there is no such thing as an exclusive channel in the world of radio, there always will be situations where you have somebody else on your channel. Hopefully, this co-channel user is hundreds of miles away from you, but that is not always the case. The best thing you can do is avoid applying for a channel that is being used by another licensee in close proximity to you. In most situations, however, that cannot be accomplished.

If the co-channel station is more than 12 dB below your RF-signal level, you should not experience much interference. If it is less than 12 dB below your RF-signal level, there are mitigation techniques that can be used. These include moving to the opposite side of the tower, lowering the antenna height, changing CTCSS or DCS codes, lowering power levels, reducing receiver sensitivity, adding attenuators, or a combination of these items.

Adjacent-channel interference. An adjacent channel also can cause interference under certain conditions. Such interference generally is a factor of how strong the signal being received by the adjacent channel is compared with the strength of the desired signal. Most receivers allow up to a 60 dB difference in strength between the desired signal and the adjacent-channel signal before there is degradation to the former. Fortunately, mitigation techniques exist — which are the same or similar to the co-channel fixes — that can be used to alleviate these problems.

Transmitter noise. All transmitters put out noise on the frequencies close to the operating frequency of the transmitter. If you have a receiver that is within 200 kHz of a transmitter frequency, and in close physical proximity, you probably do have transmitter noise affecting your receiver signal’s minimum threshold. Distance and special filters usually are used to correct this kind of problem. Sometimes, you will need to move the receiver away from the transmitter site.

Harmonics. All transmitters and some other devices put out harmonics when power is applied. The number of harmonics produced can range from two to 10; the greater the number, the greater the signal distortion. An example of a harmonic-caused problem would be a transmitter operating at 152.1 MHz. If a receiver at the same site was operating at 456.3 MHz, it would hear the VHF transmitter, no matter what you did to filter the VHF signal out of the UHF signal. Good low-pass or other filters normally fix these kinds of problems.

Radio-frequency interference can be avoided or minimized by using the correct protection device for the circumstance. This last section will examine the major protection devices that can be used to help with RFI situations.

Cavity filters are very useful in correcting some RFI situations. They do this by eliminating or reducing at least one or more of the component frequencies in the RFI mix. There are multiple parameters that determine how effective a cavity filter is operating. These include: 

• Bandwidth. The frequency spread where the power level insertion loss through the cavity is at the 3 dB intercept points. 
• The Q[uality] factor. This is the reactance divided by the resistance. The higher the Q factor, the sharper the slopes of the cavity. 
• Power rating. The maximum power level that the cavity filter safely can handle.
• Insertion loss. The amount of signal loss in dB as the signal goes through the cavity. Many times, a low insertion-loss setting does not correct an interference problem.

Bandpass cavities allow a very narrow frequency band (10 kHz to 50 kHz) to pass through the filter, and cause a deep attenuation of all other frequencies through the cavity. These filters usually have an insertion loss of 0.5 dB to more than 3 dB, but the caveat is that the narrower the bandpass setting, the higher the insertion loss. When these cavities are set for 0.5 dB insertion loss, their effectiveness is only felt on channels that are more than 2 MHz or further away from the center channel.

When the insertion loss is set for 3.0 dB, then the bandpass cavity is effective down to 200 kHz or greater. Many systems cannot tolerate the high insertion loss that results from attenuating half of the transmitter power or reducing the receiver sensitivity by the same 3 dB — so be careful where you use these filters.

Notch filters remove a narrow band of frequencies with an attenuation of 25 dB to 35 dB on the tuned channels; when a notch filter is used, the pass channels can be as close as 100 kHz and have less than 0.5 dB insertion loss on all other channels.

Bandpass/band-reject filters allow some frequencies to pass and other frequencies relatively close in frequency to be notched at the same time. This makes these filters the optimum kind to use as duplexers in close frequency spaced repeaters of 300 kHz or greater in channel separation. They also can be used for situations where a pass-and-notch filter would help the interference mitigation.

Cascading filters. Sometimes the addition of a second filter in series with the first one will correct problems that a single filter does not fix.

High-pass filters. In some cases, it is desirable to attenuate all frequencies below a certain point, and let all frequencies above that point pass.

Low-pass filters work in the opposite direction — they are used when it is desirable to pass frequencies below a certain point and attenuate those above that point. Since all transmitters and isolators produce harmonics, you will find low-pass filters on every transmitter at every site.

Pre-selectors are bandpass filters that are designed to be wider than a single channel, such as a receiver multicoupler where the single receiver antenna is used to feed receiver channels that could be up to 10 MHz apart from each other.

Isolators are one-way filters that allow a radio signal to pass in one direction with minimal insertion loss (0.5 dB attenuation), with high loss (25 dB attenuation) in the other direction. They work by keeping transmitters that are in close physical proximity from having power come back into the transmitter from coupling through the antennas, mixing with the frequency of the transmitter itself, and coming out of the transmitter at a higher power on the IM mix channel.

These devices are used to control intermodulation — which occurs when two or more radio frequencies combine to form other frequencies — at transmitter sites. Isolators do generate even-order harmonics, so a low-pass filter is required after an isolator is used. The isolator must be rated to handle the power that the transmitter is putting into it. In some cases, two or three isolators must be deployed in series (cascaded) to achieve the desired isolation needed to keep the transmitter intermodulation under control.

Radio-frequency interference can and does occur anytime there is more than one channel in a given geographic area. However, by making the design engineers and installation technicians aware of the types of RFI and the ways to avoid or correct these problems, the radio systems should be able to work together.

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 10: The bane of your existence: How to deal with RF 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 iwiesenfel@aol.com.

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 bob.shapiro@eads.com.