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Author Topic: Matrix Comms (Part 2)  (Read 18081 times)

Neil White

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Re: Matrix Comms (Part 2)
« Reply #60 on: February 10, 2013, 02:59:29 pm »

Start with these:

Hi Henry,

Thanks for the detailed links. I will have a read through and I'm sure I will have some further questions.

Neil
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Neil White

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Re: Matrix Comms (Part 2)
« Reply #61 on: February 14, 2013, 04:48:52 am »

I have put together a block diagram for a combiner system for 4 channels of semi duplex radio. Am I heading in the right direction?

N.
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Henry Cohen

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Re: Matrix Comms (Part 2)
« Reply #62 on: February 15, 2013, 03:21:18 pm »

I have put together a block diagram for a combiner system for 4 channels of semi duplex radio. Am I heading in the right direction?

Sort of. The RX side is good. On the TX side, the hybrid coupler is wrong. Hybrid couplers are 2 x 2 (4 port) device; two inputs, two outputs. (There are actually other mult-iport designs, but not relevant here.)

You need a total of three hybrid couplers: The first HC's input ports are connected to TX1 & TX2. The second HC's input ports are connected to TX3 & TX4. One output from each HC is terminated with a load => the total RF power of two transmitters. Each of the other output ports of each HC connects to one of the two inputs of the third hybrid. On the third hybrid's output ports, one port connects to the antenna feed line. The other port has a load capable of => the total RF power of the combined transmitters. (see page 40 of http://www.emrcorp.com/documents/Technical%20Information/hybrid-filter_combiners%2836-44%29.pdf)

The other considerations are:

- RF gain structure on the TX side to make sure the passive losses and any antenna gain ultimately provides sufficient ERP.

- Determining the correct amount of TX to TX isolation, and using the correct isolator(s).

- If the RX frequencies or other adjacent channel users are too close,  or the RF noise floor is rather high, you may want to put a very narrowband, high Q bandpass cavity filters between the splitter outputs and the receiver inputs.
« Last Edit: February 15, 2013, 03:23:32 pm by Henry Cohen »
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Henry Cohen

CP Communications    www.cpcomms.com
Radio Active Designs   www.radioactiverf.com

Neil White

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Re: Wireless Comms
« Reply #63 on: February 15, 2013, 04:58:45 pm »

You need a total of three hybrid couplers

I have updated the drawing below and included the average losses for each element as noted in the EMR technical note, for a total of approximately 7dB loss per channel for a 4 channel combiner. So for example a Motorola GM360 mobile radio used as a repeater base station TX at 25W would have a power output of around 5W after the combiner system?

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The other considerations are:
- RF gain structure on the TX side to make sure the passive losses and any antenna gain ultimately provides sufficient ERP

I would assume it is better to use a higher gain antenna than to introduce any amplifiers to the TX side. If amplification is needed should it be located between each transmitter and the combiner system to avoid amplifying any intermodulation products that have occured in the combiner system, and so as not to create intermodulation within the amplifier itself?

Neil

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Henry Cohen

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Re: Wireless Comms
« Reply #64 on: February 16, 2013, 02:23:52 am »

I have updated the drawing below and included the average losses for each element as noted in the EMR technical note, for a total of approximately 7dB loss per channel for a 4 channel combiner. So for example a Motorola GM360 mobile radio used as a repeater base station TX at 25W would have a power output of around 5W after the combiner system?

Figure closer to 8dB real world loss per channel: Add insertion loss for connectors and coax jumpers. Final ERP will be closer to 4W.


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I would assume it is better to use a higher gain antenna than to introduce any amplifiers to the TX side. If amplification is needed should it be located between each transmitter and the combiner system to avoid amplifying any intermodulation products that have occured in the combiner system, and so as not to create intermodulation within the amplifier itself?

Passive gain is always preferred, provided the gain antenna provides the desired elevation and azimuth beamwidths. An amplifier would go between the transmitter and the isolator. Loads on the isolators will have to be increased accordingly.
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Henry Cohen

CP Communications    www.cpcomms.com
Radio Active Designs   www.radioactiverf.com
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