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I live right on the edge of mobile reception. I just get a signal with an external antenna, but it is weak. I am thinking of getting a small indoors GSM booster, like this.

I was wondering if it might be possible to connect the booster out-aerial socket directly to the external aerial socket on my phone, instead of using the phone's internal aerial, as is intended with these devices. I think that it might reduce other local radio noise that the phone picks up, and provide a stronger link. But I'm not sure if it's safe to do so (wrong impedance or something?). If that's the case, would there be a safe way to directly connect the devices, with some kind of coupler? Is it going to provide any benefit over just having the GSM booster set up normally?

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  • \$\begingroup\$ What does the manual say? Why don't you ask the eBay seller? EE.SE is not his volunteer support desk. \$\endgroup\$ – Nick Alexeev Apr 11 '15 at 3:04
  • \$\begingroup\$ @NickAlexeev: I haven't got one yet, but I suspect this won't be in the manual anyway - I don't think this is how the device is intended to be used. \$\endgroup\$ – naught101 Apr 11 '15 at 6:08
  • \$\begingroup\$ @NickAlexev: you could consider this a question about the design of a repeater-phone coupler that is safe for both devices... \$\endgroup\$ – naught101 Apr 12 '15 at 1:40
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I don't think this is a good idea.

It might damage your phone to connect it directly. The repeater will transmit a significant signal indoors, to travel to your phone.

It will almost certainly damage the repeater to do this. Your phone produces up to 1 Watt of RF power, the repeater is expecting a much smaller signal.

But most importantly, I don't think a repeater will help with your signal problem. If the signal is too weak when directly connected to the phone, there's no reason to think their amplifiers will be able to do better.

On the receiving side, your phone will probably have a lower noise figure than the repeater - it needs to work through some filters your phone doesn't need. On the transmit side, the power is probably limited by license conditions, to the same as the phone's power.

Repeaters are for convenience, to free you from the direct cable. They do "boost your signal" compared to what would have leaked into the building, but that's not your question. See This Answer for a bit more about the signal levels.

You're already doing the best thing using an external antenna. Can it be improved? A higher gain antenna might help. Can you replace the coax with a much lower loss cable? These would help.

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  • \$\begingroup\$ Thanks for the answer. I'd still be interested to know if a safe coupler could be designed, and if so, how, even if it is not a practical solution. \$\endgroup\$ – naught101 Apr 12 '15 at 1:45

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