Wednesday, February 24, 2010 10:01:41 AM MST |
Frequency modulation (FM) is a means of transmitting audio using electromagnetic waves. This same technology is used by local FM radio stations to broadcast music. Listen Technologies’ Stationary FM products wirelessly transmit audio from a sound source to portable receivers with headphones; primarily for assistive listening and language interpretation applications. FM signals can travel through most barriers – walls, ceilings, and floors. The distance a signal travels has many different variables such as Radio Frequency (RF) output power, the type and placement of the antenna, and the frequency band.
We often field technical questions regarding our different frequency bands. Common inquiries include:
- “Which frequency is best for my particular application?”
- “Does one frequency offer better audio quality over the others?”
This blog will focus on the differences between frequency bands to help narrow down your choices. First, I will describe Listen Technologies’ (4) four frequency bands and their regions for use.
Listen delivers FM products for Europe using the 863 MHz carrier.
Listen delivers FM products for Australia using the 150 MHz carrier.
72 – 75 MHz vs. 216 MHz
Each frequency band has different advantages over the other. Consider the following questions to help determine the right frequency for your particular application:
- Do I have a very long range that needs to be covered?
- Is distance of transmission a top priority?
- Is there a need to use more than (3) three frequencies simultaneously?
- Is audio quality a main concern?
- If using an external antenna, what is the clearance space with the ceiling?
- Is the transmission going to be used primarily outdoors?
- What length of cable will be used from the transmitter to the antenna?
- Which frequency is less likely to have interference with other signals?
Answering these questions will give you a better idea of which frequency will best suit your application. Here are a few facts and details about the two frequencies:
The transmission range doubles when using the 216 MHz frequency band. Using a dipole antenna (more efficient) the 72 MHz frequency has a transmission range of 1,500 ft. (457 m). With the same scenario 216 MHz will broadcast up to 3,000 ft. (914 m) line of site. In portable applications, using the LT-700 Portable Display FM Transmitter, the range is the same with both frequencies – 150 ft (45.7 m).
Because the 72 MHz band is larger, up to (6) six channels can be used simultaneously, while only (3) three channels can be used concurrently with the 216 MHz band.
System Frequency Response
Though slight, there is a bit of audio difference. The frequency response of the 72 MHz band is 50 Hz – 15 kHz (+/-3 dB of variance) while 216 MHz has a response of 50 Hz – 10 kHz (+/-3 dB of variance). For those who are unfamiliar with this terminology, the difference is audible with higher frequencies such as the hitting of a cymbal or playing of the flute. For reference here are the vocal ranges
of an adult human all of which fall in the frequency response of our products:
Soprano (240 Hz – 1.17 kHz)
Mezzo-soprano (220 Hz - 900 Hz)
Contralto (130 Hz - 700 Hz)
Tenor (130 Hz - 440 Hz)
Baritone (110 Hz - 350 Hz)
Bass (80 Hz - 330 Hz)
Antenna Clearance Space
The 72 MHz frequency has is a longer wave length
which means for the system to be tuned properly, the antenna must be longer, even though Listen Technologies uses only ¼ wave length antennas. So in a practical application, using an LA-122 Universal Antenna Kit, you will need 39 inches of clearance to use 72 MHz (a total of 80 inches is needed for full extension of this antenna). Because 216 MHz is a shorter wave length, the antenna is shorter – 13 inches. Obviously much less clearance is needed with the 216 MHz frequency band.
Listen Technologies offers an outdoor antenna for 216 MHz only, the LA-107 Ground Plane Remote Antenna (216 MHz). This antenna can be used in extreme weather conditions. Why does Listen not manufacturer an outdoor antenna? The answer is above. The antenna would be very large and difficult to manage.
If you are using an external antenna, cable length may be a necessity. Naturally, coaxial cable has an attenuation factor which means RF power is lost with its use. Please review the chart below for the amount of loss using two different types of 50 Ohm cable – RG-8 (considered low loss cable) and RG-58.
Translation – there is more signal loss using the 216 MHz frequency at any given length. How will this affect your range? Generally at 6 dB, half of the linear transmission range is lost. As an example, if you are using an LT-800-216 stationary transmitter with 100 ft of RG-58 cable, you can expect your linear range to be cut from 3,000 to 1,500 ft. (457 - 914 m).
|Cable Loss- 72 MHz|
|50 Feet||100 feet||50 Feet||100 feet|
|17.0 dbm||14.9 dbm||12.8 dbm||16.1 dbm||15.2 dbm|
|16.9 dbm||14.7 dbm||13.0dbm||16.0 dbm||15.4 dbm|
|16.5 dbm||14.7 dbm||12.9 dbm||16.0 dbm||15.4 dbm|
|16.6 dbm||14.2 dbm||12.9 dbm||15.8 dbm||15.4 dbm|
|Loss:||2 dbm||4 dbm||1 dbm||2 dbm|
|Cable Loss- 216 MHz|
|50 Feet||100 feet||50 Feet||100 feet|
|19.0 dbm||16.5 dbm||12.0 dbm||17.7 dbm||16.2 dbm|
|19.7 dbm||15.0 dbm||12.2 dbm||18.5 dbm||16.5 dbm|
|20.5 dbm||16.3 dbm||12.5 dbm||19.3 dbm||16.6 dbm|
|19.7 dbm||15.9 dbm||12.2 dbm||18.6 dbm||17.2 dbm|
|Loss:||3-4 dbm||7-8 dbm||1-2 dbm||3-4 dbm|
Determining which frequency is better for reduced signal interference varies depending on the region of the country, size of the area, and the amount of RF in the area. For example, Manhattan could have more frequency challenges compared to rural Utah. Here are two good options to determine the best frequency for your area:
- Demo a Listen receiver from both the 72 MHz and 216 MHz frequencies. After you verify that the unit is working, press the SEEK button. The receiver will “seek” for any carriers in the area. If the receiver does not stop on a frequency, the area is clear for that particular frequency.
- Survey your local area. Look for nearby public facilities that may also be using an FM system for assistive listening or language interpretation (houses of worship, theaters, corporate offices, schools, etc.) Generally, when disturbances are found, the culprit is a place nearby using the same channel. Obviously, if an area is inundated with frequencies on one particular band, use the other band – the great benefit of being able to use (2) two different bands.
In conclusion, both frequency bands have advantages. Follow this simple guide when selecting a frequency or contact the Listen technical support team
for more assistance.
|Frequency||Range||# of Simultaneous Channels |
in a Venue
|System Frequency Response|
|72 MHz||1,500 ft. (457 m)||Six (6)||50 Hz - 15 kHz|
|216 MHz||3,000 ft. (914 m)||Three (3)||50 Hz - 10 kHz|