A pair of diplexers allows you to utilize just one set of antennas and cables with TWO wireless amplifiers to cover both 3G and 4G LTE signals. The diplexers (which must be used in pairs) allow you to use a traditional dual-band amplifier for voice and 3G signal along with a 4G-only amplifer for Verizon or AT&T LTE signal, without having to buy another set of antennas/cables.
This is ideal for users who already own a 3G repeater system and want to add 4G LTE capability (simply add the LTE amplifier, two diplexers, and four 2' cables to connect everything) and also for people who want to put together a new 3G/4G repeater system but don't want to spend thousands on the expensive 3G/4G amplifiers.
One diplexer connects to each side of the two amplifiers to connect the exterior antenna on one side and the interior antenna on the other. Below is a sample setup:
exterior antenna -> cable -> diplexer -> 2 x 2' cable -> amplifiers -> 2 x 2' cable -> diplexer -> cable -> interior antenna
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Diplexers can also be used to connect two direct-connect amplifiers (e.g. one 3G-only amplifier and one 4G-only amplifier to connect directly to a 3G/4G modem). The setup would be similar to the repeater setup described above, but instead of connecting the second diplexer to an interior antenna, you'll connect it to your adapter cable (using a 2' cable with N and FME connectors):
exterior antenna -> cable -> diplexer -> 2 x 2' cable -> amplifiers -> 2 x 2' cable -> diplexer -> 2' cable (N/FME) -> adapter cable -> modem