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FM Broadcast Combiner

An FM Broadcast Combiner, or Diplexer, is a passive device.  It combines signals from two to as many as ten Transmitters into a single coaxial cable and antenna system.  Instead of setting up an antenna for each transmitter system.  All the FM frequencies combine into a single feeder and antenna system.

The antenna system will be able to handle the combined transmitter power from all the transmitter frequencies.  The total power from, for example, 6 x 5kW transmitters will be about 30kW.  If the transmitters are 10kW, then 60kW  is required.  In this situation, the antenna system splits the power through two connectors and antenna halves.

However, the most critical issue with Combiners is the frequency separation of each channel fed into the Combiner.  Ideally, the separation is at least 2 MHz.

FM Broadcast Combiner

This ensures no interference occurs within each channel.  However, with careful multi-stage filtering, we can allow frequency separation to 800 KHz.  A narrow 400 kHz separation between signals can lead to intermodulation, potentially degrading stereo quality.

FM Broadcast Combiner Selection

There are two main FM combiners configurations: Starpoint and Constant Impedance.  They differ in handling impedance, a measure of resistance to current flow in the feeder cable.

  • Starpoint Combiners join the filtered signals at a common point.  They're more straightforward and affordable but can introduce slight signal reflections.  At the connection point, this can potentially affect signal quality at higher frequencies.
  • Constant Impedance: This design employs transformers to maintain a continuous impedance (typically 50 ohms) across the FM frequency band.  This minimizes signal reflections and performs better, especially for longer cable runs.  However, Constant impedance combiners usually require more investment.

Specialist filters in the Combiner maintain the integrity of both FM signals.  They allow them to coexist on the same cable without crosstalk and maintain quality.

Choosing the Right Combiner:

  • A Starpoint combiner might be sufficient for basic setups with short cable runs.  It is ideal to keep the cost down.
  • A constant-impedance combiner is technically the better option if you have long cable runs and experience signal quality issues.

Overall, Sky Technologies can guide you to the best Combination.  We have the support of the world's most experienced Combiner supplier, Jampro Antennas, in the USA.  Combiner systems can be provided that meet your exact requirements.  We focus on keeping the overall cost as low as possible.

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