Application Note
Radio Frequency
Interference in Hospitals
Hospitals are replete with sources of interference and many applications that require the ultimate in reliability to
assure patient safety. Patient telemetry systems, cellular telephones, bluetooth devices, security radio systems,
police and paramedic radios, Wi-Fi, microwave ovens, cordless telephone systems, light dimmers, fax machines,
floor buffers, elevators, elevator controls, nearby cellular base stations, broadcast transmitters, and ultrasound
systems all contribute to the harsh RF environment in hospitals. This may not be a lot worse than many
businesses, but lives are at stake so it is critical to be able to quickly solve interference problems.
Patient Telemetry
Patient telemetry systems typically operate in the 608 MHz to 614 MHz, 1395 MHz to 1400 MHz, and
1427 MHz to 1432 MHz WMTS bands, as well as other bands such as the 2.4 GHz ISM band which is also
used by Wi-Fi, microwave ovens, cordless telephones and other devices. There are also telemetry systems
in the 5 GHz 802.11(a) band. The 608 to 614 MHz range in the United States is reserved for radio astronomy and
patient telemetry systems, so it generally is free of in-band interference from other transmitters. However, that
does not mean it is interference free. There are many sorts of unintentional radiators which emanate broadband
noise that can easily disrupt communication.
Equipment that is operating in the WMTS bands (608 MHz to 614 MHz, 1395 MHz to 1400 MHz, and 1427 MHz
to 1432 MHz) has primary status and is protected from interference by other devices. These bands are the only
parts of the frequency spectrum that is designated exclusively for wireless medical telemetry systems. To be legal,
hospitals that intend to deploy telemetry systems in the WMTS bands must ensure that device registration has
been completed before the equipment is operational.
FCC Requirements for WMTS Equipment Registration
The FCC Report and Order, ET Docket 99-255 that created the WMTS bands requires that all WMTS operators
register their equipment before they put it in operation:
Section 95.1111 (Frequency coordination).
a.Prior to operation, authorized health care providers who desire to use wireless medical telemetry devices
must register all devices with a designated frequency coordinator.
Types of Interference
Interference can be broadly grouped into two categories: first, interference from devices that are intended to
transmit signals and second, unintended radiation from devices that aren’t supposed to transmit signals.
The basic interference hunting process is the same regardless of the source, however it may be more difficult to
find the unintended radiators since they may be very intermittent and they can look like anything, a floor buffer, a
FAX machine, light dimmers, a malfunctioning electric blanket, microwave ovens, elevators, hair dryers, etc. The
radiation from unintended emitters can be broadband and very dirty since the ultimate source is often a noisy
motor with dirty brushes that make poor contact to the rotor and spark as they do so. Light dimmers can be a
significant source of interference. Often they are installed in patient rooms and while none are particularly clean to
begin with because they work by chopping off part of the power line’s sine wave causing many harmonics to be
generated, overstressed dimmers tend to get significantly worse over time before they fail completely.
Intended emitters include such devices as Wi-Fi access points, cell phones, walkie-talkies, remote controls for
toys, two-way pagers, police and ambulance radios, broadcast transmitters, cell phone base stations, paging
transmitters, and land mobile repeaters (they are on the roofs of many hospitals), diathermy machines and more.
Conducted Interference
Ultrasound equipment tends to be susceptible to interference conducted into the equipment over the power line in
the 2 MHz to 18 MHz range. Harmonics of the oscillators in many switching regulator power supplies can be in the
frequency range used by ultrasound equipment. You can recognize such interference by repetitive lines on the
instrument’s display. Most small power supplies employ switching regulators so if such interference is noted, a
hunt for such power supplies is in order.
Cell Phones
Cell phones operate on many different frequency bands. The ITU-R approved the following bands for cell phone
operation: 806 MHz to 960 MHz, 1710 MHz to 2025 MHz, 2110 MHz to 2200 MHz and 2500 to 2690 MHz.
Not all countries follow this exact plan. In the United States, for example, these bands are in use.
Experienced interference hunters have noted that very seldom are cell phones the cause of interference to
telemetry systems in hospitals and usually when there is interference it is because a doctor’s cell phone went
off inches from the patient’s telemetry equipment.
Current / Planned Technologies
Band
Frequency
SMR iDEN
800 MHz
806 MHz to 824 MHz and 851 MHz to 869 MHz
GSM, IS-95 (CDMA), IS-136 (D-AMPS), 3G
Cellular
824 MHz to 849 MHz and 869 MHz to 894 MHz
GSM, IS-95 (CDMA), IS-136 (D-AMPS), 3G
PCS
1850 MHz to 1910 MHz and 1930 MHz to 1990 MHz
3G, 4G, DVB-H
700 MHz
698 MHz to 806 MHz
3G, 4G
AWS
1710 MHz to 1755 MHz and 2110 MHz to 2155 MHz
4G
BRS/EBS
2496 MHz to 2690 MHz
Wi-Fi and other sources in the 2.4 GHz band
The 2.4 GHz IMS band is unlicensed, and hence difficult, if not impossible, to manage. Telemetry equipment that
operates in this band generally has sophisticated interference avoidance algorithms, allowing them to change
frequencies to move away from occupied channels. If the use of other 2.4 GHz equipment is carefully managed in
or near a hospital, such deployment can be done successfully. Bear in mind that microwave ovens operate in the
2.4 GHz band and, while reasonably well shielded, they do radiate enough energy to disrupt telemetry operation if
the oven is too near a telemetry antenna.
Outside Signals
There is beginning to be smart grid deployment in the 1392 MHz to 1395 MHz and 1432 MHz to 1435 MHz bands in
the United States. It is worth noting in that these assigned frequencies are adjacent to the 1395 MHz to 1400 MHz,
and 1427 MHz to 1432 MHz WMTS bands. Since it appears that smart grid deployment will eventually be ubiquitous
in the United States, being aware of the potential for interference is worthwhile. In other places in the world 1.4 GHz
is used for cell phones.
The 608 MHz to 614 MHz band is in the middle of the UHF TV band - between channels 36 and 38. If you have
those channels or other strong TV channels in your area, they can readily deliver a very strong signal inside a
building and potentially cause harmful interference. The transition of analog to digital TV in the United States
caused many channels that previously operated in the VHF portion of the TV band to move to available UHF
channels, raising the potential for interference that hadn’t previously existed.
On the roofs of many hospitals is a forest of antennas for various radio systems, including paging transmitters, land
mobile of various sorts, microwave links, cellular base stations, etc. Most tall buildings lease space on their roofs for
such uses. It would be a good idea to get on the roof so you can discover and measure what is there. That way you
will be knowledgeable about the potential for interference. A good survey would cover as wide a frequency range as
your equipment can do. Keep records of the frequencies being used and take screen shots of the various signals so
if they show up as interference you will already know what they are. Ambulance companies usually have UHF radios
in their vehicles and walkie-talkies for the paramedics. Usual frequency assignments for such services are in the
470 MHz area; while not particularly close in frequency to the IMS and WMTS bands, a walky-talky be used close to
a telemetry antenna can cause fundamental overload of the telemetry receiver. When that happens, the telemetry
receiver’s sensitivity is severely regarded and may not be able to receive telemetry signals.
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Broadcast stations, AM, FM and television, can cause interference to hospital telemetry systems if they are
nearby. Fortunately broadcast stations are fixed systems with known operating parameters. If you are experiencing
problems in this area, careful shielding of the equipment, filtering of power lines and bypassing of wiring going into
equipment can usually remedy problems. One thing to note is that at night AM broadcast stations frequently run
different power levels and different radiation patterns from their antennas. If you are having problems that change
at night, this may be just the clue you need to resolve the problem and the solution to the problem will generally
get down to shielding, filtering and bypassing.
Hunting Interference to Patient Telemetry Systems
Patient telemetry systems are designed to allow patients to be ambulatory while maintaining constant connection
to the monitoring system. The way this is commonly accomplished is to have antennas along the hallways and in
other areas into which patients may go. The signals from the antennas are fed into a wiring room in which there is
typically one receiver. The signals from the various antennas are combined to feed all signals into that receiver’s
input. Having one receiver reduces the complexity of the system but makes it more difficult to locate sources of
interference since it could be coming from one or more antennas.
If interference is continuous, simply isolate the source to a single antenna by disconnecting antennas one at a
time until you disconnect the one with the interfering signal. Make sure that the nurses at the monitoring station
know what you are doing so they won’t be alarmed if a patient’s signal disappears abruptly. Once you know
which antenna is contributing the interference, take your spectrum analyzer and an antenna appropriate for the
frequency range of the telemetry system and go to the area served by that antenna. Since you will be searching in
a relatively small area, an omni-directional antenna is generally the most convenient for this sort of hunt. You will
be simply following the signal looking for the strongest point. Since you are looking for something that is on all the
time, many potential emitters can be eliminated from the hunt because they are used only intermittently. Probably
when you get to a point where the signal is strong, the emitter will be obvious. If it isn’t, keep the inverse square
law in mind - the strength of the signal doubles as the distance to the source is cut in half. Recall that an increase
of 3 dB is a doubling of the power. If the signal strength is climbing very rapidly you know you are very close to the
source. Pay special attention to devices installed near telemetry antennas.
If you are searching for an intermittent interferer, Anritsu spectrum analyzers have the capability to help you find
the emitter without having to sit in the wiring room staring at the screen for hours on end. To get an idea of the
nature of the interferer, use max hold on trace A to capture all signals that are seen. Let the instrument capture
traces until interference occurs. You will then have an idea of the frequency, amplitude and bandwidth of the
interferer. Figure 1 shows the signal from a microwave oven. This could cause significant interference if the
microwave oven is located close to a source antenna.
Figure 1: Microwave Oven signal captured using Max Hold
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Once you have a good sample of the interference then build a limit line to let the instrument capture future
occurrence of interference while not saving lots of traces in which there is no interference. To do this, create a limit
line that is higher than the expected signals but lower than the interfering signal. Using Anritsu handheld spectrum
analyzers you can create a complex limit line that has 40 segments. See Figure 2a for an example.
Figure 2a: complex limit line to capture an intermittent interferer
Figure 2b: Menu of save on event.
This limit line was built automatically by using limit envelope. Once you have the limit line, use it to capture enough
occurrences of the interference to get an idea of the time of day and the rate at which the interference occurs.
You do this by using the save-on-event capability to capture only traces in which a signal exceeds the limit line. It may
take a couple of tries with the limit line to capture the interfering signal when it occurs, but not too many other traces.
After you have captured a group of useful traces, copy them into your PC in a separate directory. You will be
creating a spectrogram from the traces by using the “folder spectrogram” creation tool in Master Software Tools,
software shipped with every Anritsu handheld spectrum analyzer.
In addition to creating a spectrogram, the folder spectrogram can show you the total power versus time, average
power versus time, peak frequency versus time, a time chart that shows the number of sweeps captured versus
time. All of these are useful when searching for an intermittent interferer.
Spectrogram
This is a great tool for noticing time and frequency patterns of intermittent signals. A spectrogram provides the
frequency, time and power level of all signals in the measured bandwidth.
Figure 3: Folder Spectrogram
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Average Power versus Time
If you have interference that becomes worse at certain times of day, this is a great view to see what time of day
the interference is happening. If you spot a pattern, you will be able to be on-site to look for interference at the
times when interference has occurred.
Figure 4: Average Power Vs. Time
Peak Power versus Time
This display gives more clues that may be able to help you determine when you need to be in the hall with your
spectrum analyzer to find an interferer. Time is on the horizontal axis. The times of the first and last saved files is
shown in the legend.
Figure 5: Peak Power vs. Time
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Total Power versus Time
This display can help you to discover relationships between bouts of interference and the total power in the
bandwidth being swept.
Figure 6: total Power vs. Time
Peak Frequency versus Time
This display shows you the frequency at which the highest power level in the sweep occurred. This display can
help you to discover what frequency range is actually causing harmful interference by correlating occurrences of
interference to the frequency at the highest power level at the time that interference occurred.
Figure 7: Peak Frequency vs. Time
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Time Chart
For intermittent interferers, this is a powerful tool for discovering patterns in the timing of the interferer. If, for
example, interference peaks at the start and end of work shifts, think about activities that peak at those times,
such as heavy elevator use. It is possible that the elevator or elevator controller is generating interference.
Figure 8: Time Chart
Depending on the nature of the interference, you may not get useful information out of all the displays available to
you in folder spectrogram. However, one of them may give you just the insight you need to solve your interference
problem. In this case the spectrogram and the peak frequency vs. time chart give useful information. The Time
Chart, which shows the number of measurements saved per minute, shows that the vast majority of the signals
occurred in the last afternoon and morning and basically went to zero at night.
Keep notes of the measurements you make, where interfering signals are strong and you can generally find the
problem and devise a fix.
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