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75 Ohm feed with 50 Ohm ant. - how to match impedance?

75 Ohm feed with 50 Ohm ant. - how to match impedance?

75 Ohm feed with 50 Ohm ant. - how to match impedance?

I've got a 902-928MHz phone system with a roof-mount 50 Ohm ground plane antenna. However, the feedlines that come down to the basement are all 75Ohm, RG-6 Coax. I really don't want to snake a new 50Ohm  cable, so I'm wondering if there is a way to use the 75Ohm cable and insert an impedance matching network so the transceiver "sees" 50Ohms.  Or will I still lose power between the inserted network and the antenna due to the impedance mismatch?

Any thoughts on calculating/designing/procuring such a network?


RE: 75 Ohm feed with 50 Ohm ant. - how to match impedance?

As antenna matching goes, 75 ohms isn't that far away from 50 ohms.  The antenna might be as far off as that - probably in the other direction - just by itself. You might consider just trying it.

With respect to additional loss due to mismatch, it is typically insignificant. Basically, there will be areas along the cable with slightly increased current and areas with slightly increased voltage - these increases will cause slightly additional loss in the cable as compared with a matched line. But when looked at in dB, the additional loss is typically negligible. A matching network can easily have the same or more loss.

One practical difficulty you'll run into is connecting the RG-6 to any of the standard 50-ohm connectors (which are typically not 'F'). If you're lucky, you'll be able to get your hands on a suitable adapter for each end.

If I were you, I'd actually run a new 50-ohm cable. But if you're not going to do that, then just try it.

RE: 75 Ohm feed with 50 Ohm ant. - how to match impedance?

matching used here may be a lamada/4 transformer t.l , single stub, double stub , this is the matching network
return to pozar he gives you the theory of these ideas

RE: 75 Ohm feed with 50 Ohm ant. - how to match impedance?

I agree - just ignore it; most matching networks will probably have more loss at some point in the band than the 50/75 match, especially if home built. RG6 has a loss of about 2dB every 10 metres at 900MHz anyway. You will save more by making sure the run is as short as possible.

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