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Antennas are a vital part of an FM system. Listen offers many antennas to choose from that should cover just about any application. But, we often field more inquisitive calls about our antennas that may be of interest to others. This blog will focus on many of those questions.

Many of the antenna scenarios are covered in the manuals of the LT-800 Stationary FM Transmitter and antenna manual. After speaking with our technical team, here are a few frequently asked questions with a corresponding answer. Keep in mind, the physics of electromagnetic waves is very complicated. The purpose of this blog post it simplify the scientific response so that all readers can understand.

la-122


The LA-122 Universal Antenna Kit is great for remoting an antenna in another room, but the antenna is too long. How can I remote my antenna in a more discrete manner?
There are a couple different ways to discretely position an antenna in a room. Keep in mind, if you choose to remotely mount a dipole antenna, it should be mounted with the ground radial pointed down. This rule of thumb also eliminates placing the antenna in the ceiling. One way that we recommend is to remotely mount the 90° Helical Antenna (available in 72 MHz, 216 MHz, or 150 MHz). To do this, you must use the LA-130 Remote Antenna Kit. The kit contains 25’ of RG-58 cable that is attached to the mount and the back of the transmitter. This kit was originally designed for our 863 MHz products, but can be used with 72, 216 or 150 MHz products.
LA-130

Another way to discretely mount a dipole antenna is to use a flexible coaxial antenna. With our 72 and 216 MHz products, we have two different antennas that can be used – LA-116 Coaxial Dipole Remote Antenna (72MHz) and LA-117 Coaxial Dipole Remote Antenna (216 MHz). These antennas can be remotely placed in an isolated place in the room.
LA-116
I do not have enough room to vertically mount our LA-122 Universal Antenna Kit, can I mount the antenna horizontally?
An antenna is either vertically or horizontally polarized. Polarization is determined by the position of the radiating element or wire with respect to the earth. Therefore, an antenna that is parallel to the earth radiates horizontally and an antenna at a right angle to the earth (vertical) radiates with vertical waves. Listen’s dipole antennas have been designed to radiate vertically. We recommend that the antenna is kept perpendicular to the earth.

Can we put the antenna behind the transmitter in a metal rack?
No. Metal acts as a reflector for electromagnetic waves. The reaction of an electromagnetic wave when it hits metal is to bend around the metal and escape. In the case of an enclosed metal rack, the wave would be “trapped” by four walls of a metal encasement. It will incur a considerable amount of power degradation. It is not recommended to put the antenna in back of the rack.

What type of materials in the wall (or ceiling) can reduce the transmission range?
Radio frequencies pass though any non-conductive material, regardless of thickness. So, if you do not have conductive reinforced walls or ceilings (such as sheet rock) you should have relatively little resistance from these surfaces.

Walls or ceilings that are metal reinforced are challenging to transmit from. The conductive surfaces within the wall or ceiling will reflect or bend the frequency. The results will be signal degradation at longer ranges and multipath. A steel rebar reinforced concrete room is very difficult to transmit or receive a signal.

Should I use RG-58 or RG-8?

It depends upon the length of the cable run and the frequency being used. The chart below is a tool to calculate the amount of attenuation that will take place with a cable run. We have studied four different transmitter samples for the 72 and 216 MHz frequencies.

Cable Loss – 72 MHz
Transmitter Sample RG-58 RG-8
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 13.0 dbm 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
Transmitter Sample RG-58 RG-8
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

 

Keep in mind that a 3 dB loss is half of the transmitter power. A 6 dB loss is half of the effective range of the system. Our LT-800-072 stationary transmitter is rated up to 1,000 linear feet of broadcasting. The LT-800-216 stationary transmitter is up to 3,000 ft.

Does the antenna need to be fully extended?
For the best range of the system, the answer is yes. An antenna that is not fully extended effectively de-tunes the system. Listen uses four different frequencies to broadcast on: 72 MHz (North America), 216 MHz (North America), 150 MHz (Australia), and 863 MHz (Europe). Each will have a different length of antenna to properly tune the transmitter.

Here is a bit of trivia for determining antenna lengths: Electromagnetic wavelengths can be calculated by dividing the speed of light in centimeters per second by the frequency in cycles per second.

  1. Determine what the speed of light is in meters per second. The speed of light is 299,792,458 m/s.
  2. Determine what the frequency of the wavelength is in cycles per second. For example, the frequency one of our transmitting frequencies is 72 MHz or 72,000,000 Hertz.
  3. Divide the speed of light by the frequency to get the wavelength in meters.
Example: 299,792,458/72,000,000 = 4.16 meters or 13.12 feet.

Instead of putting a 13 foot antenna on our transmitter, which is highly impractical, Listen transmits on a ¼ wavelength. Thus, to determine the length of our fully extended 72 MHz ¼ wavelength antenna, we would divide 13.12 feet by four which is 3’4” or 40”. If you measure the length of the LA-106 Telescoping Top Mounted Antenna (72 MHz), you would see that it is 40”.

Can I use the remote antenna kit in a monopole situation?
Yes. A monopole antenna is less efficient than a dipole. The range will be similar to using a top mount antenna with the added signal attenuation of the coaxial cable (refer to the post above). Please see the chart below for the approximate transmission ranges of our antennas. Keep in mind, the measurements are all linear and direct line of sight from transmitter with no obstructions.
stationary-fm-guide

Can I add another antenna to the system for better range?
No. Adding another antenna in a close proximity would greatly diminish the reception range instead of increasing it. The two antennas effectively cancel each other out. Listen uses what is called a non-diversity system. A diversity system eliminates what is called multipath (when a signal bounces and is received by the receiver at different times). It can do this by using two separate receive antennas. The Listen receivers have only one receive antenna which is the headphone cable, thus diversity is not an option.
We would love to hear your feedback and any additional questions you might have about antennas.
Three separate ListenTALK receivers in a row with different group names on each display screen.
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