FM/DAB cable

[dave2010]

I'm trying to revamp the aerial systems for my DAB and FM tuner(s). It just occurred to me that the tuners probably have 50 ohm inputs, so 50 ohm cable should be used, not 75 ohm as for TV. I have used 75 ohm before, and it works, but now I would like to get this right if it'll make a difference.

Am I right about the cable impedances for radio? I have a Sony 777 ST-D 777ES DAB/FM/AM tuner and a JVC TX55 FM tuner.

Do DAB and FM tuners have the same input impedances?

I suppose alternatively there may be 50/75 ohm impedance matching devices which might help if cable runs are to take composite signals - for example TV and radio signals. That could be useful if common download cables are to be used from the loft/roof area.

Maybe the matching is just really not that important in practice anyway?

[Reid Malenfant]

You'll need 75 ohm cable for either FM or DAB. 50 ohm is generally used in transmitters such as Ham radio & CB etc.

Every FM tuner i have ever known has used a 75 ohm input or a 300 ohm for indoor balanced aerials. I'm dead sure DAB will be no different. Put it like this, TV needs 75 ohm as does FM (via coax) as does satelite TV via a dish ;)

[dave2010]

Thanks Mark.

I checked the Sony - that's 75 ohm FM, so that's consistent. Not sure where I got the idea about 50 ohms - perhaps there were older ones which had different impedance.

In any case, it seems that the issues about impedance matching are really not very likely to make much difference, providing that reflections aren't a big problem. Attenuation losses due to mismatches in impedance are likely to be quite small, and losses in matching transformers or other matching circuits would probably be greater.

Anyway, it looks as though using low loss 75 ohm cable would be appropriate. I suppose I could use spare satellite cable.

[Barry]

Dave,

All tuners FM or DAB use a 75 Ohm aerial impedance. However, were you to use 50 Ohm cable, you would only suffer 0.18dB mismatch loss.

75 Ohm satelite cable should be fine.

Regards