Listen Technologies LA-82, LA-81, LA-80 User Manual

ListenTech-Note

Doc. # LTN0020 Page 1 of 2 Revision: 05-31-17

Stationary RF Antenna Combining

Overview:

In multiple channel applications, it is often desirable to use one antenna for multiple LT-800
transmitters. This may be the preferred method, rather than mounting multiple antennas
within an area due to space limitations or aesthetics. The purpose of this document is to assist
in determining if antenna combining is a viable option for a multiple channel application.

Considerations:

It is important to note that there is a maximum number of channels or transmissions, limited
by the bandwidth at each frequency band. The maximum numbers of channels are:

 72 MHz: 6 Channels
 216 MHz: 3 Channels

The main disadvantage of combining signals is the loss of power and range. For every two
signals that are combined, generally half the power is lost (3dBm). This can equal a loss in
transmission range between 25 and 50%. The table below shows the typical loss for different
frequencies and number of channels.

Frequency

Band

Number of Channels Combined

2 3 4 5 6

72 MHz Loss

3dBm

6dBm

6dBm

9dBm

9dBm

216 MHz Loss

4dBm

7dB

N/A

N/A

N/A

NOTE: Variances in loss between +/- 1dBm are expected.

If this loss in range is acceptable for the intended application, combining multiple signals for
the use of one antenna is an option. There are still considerations to note that will ensure that
the combined system will work efficiently as possible without any intermodulation problems or
other undesired results. The following points should be kept in mind:

 Verify that all the transmitters are on the same RF power level setting. This power level

setting is on the rear of the transmitter. If the power for one channel is greater than
another, you are more likely to hear intermodulation artifacts and two transmissions on the
same channel.

 Verify that the coaxial cable from each transmitter to the combiner is kept equal in length.

This is needed so the RF power level is the same for each input to the combiner.

ListenTech-Note

Doc. # LTN0020 Page 2 of 2 Revision: 05-31-17

 If combining an odd number of transmitters, it will be necessary to terminate the unused

combiner input. A 50-ohm terminating resistor is included with the 4 and 6-way combiners
for this purpose. An unterminated connection is susceptible to noise which can be induced
on to the antenna coaxial line and heard on the receiving end.

 Choose channels that are spaced evenly apart in the frequency band and at least 300 kHz

apart from one another. This should be done whether a combiner is being used or not. See
table below for recommended channels.

Frequency Band

Channel Recommendations

72 MHz

A, C, E, I, J, H

216 MHz

2A, 2K, 2V

It is important to determine how many transmitter outputs are to be combined prior to
ordering. Use a combiner that is appropriate for the amount of transmitters installed. For
example, if a 6-way combiner is used with only 2 transmitter inputs to the combiner, the loss
would still be equivalent to combining 6 transmitters.

Listen Technologies sells three models of RF antenna combiners for 72 and 216 MHz. The
available combiners are:

LA-80 2-Way RF Antenna Combiner
LA-81 4-Way RF Antenna Combiner
LA-82 6-Way RF Antenna Combiner

Should you have any further questions or concerns, please contact Listen Technologies’
technical services team at 801.233.8992 or [e-mail] for further assistance.