# Would an RF amplifier be required for an indoors antenna connected via coaxial cable to an outdoor directional antenna (900/1800 MHz)

Sorry, couldn't make the question title any shorter without losing meaning.

So, I'm looking to build an outdoors yagi antenna that's connected to another indoors antenna via coaxial cable. The purpose is to get better cellular coverage inside the house. The outdoors signal strength is about -81 to -73 dBm, while the indoors signal strength goes as low as 103 dBm to no coverage.

I've looked at several commercial solutions, but all of them have a bidirectional RF amplifier between the two antennae.

I was wondering if this would work without an RF amplifier between the antennae. I've looked at several DIY projects that are related to (but not exactly) what I want to achieve. Here's one: http://my-homemade-diy.blogspot.com/2012/07/diy-cell-phone-gsm-3g-signal-booster.html

The above linked project is delivering the cellular signals only to a small area inside the house. I want to do essentially the same thing except with the signals spread out over a larger area, say, 20 cubic feet. Is it doable?

Apologies if my question is missing any information that I should have provided. Any response is appreciated.

• Very nice wooden dog house!! Jan 5, 2015 at 19:37
• Don't forget that cellphone signals are two-way. You also need to pay attention to what the phone transmits as well as what it receives. Jan 5, 2015 at 20:05
• @GRTech I don't understand your comment. Jan 6, 2015 at 6:49
• @DaveTweed Indeed, I hadn't thought of that. Unfortunately, it's a moot point, since it seems this isn't doable, according to tomnexus's answer. Jan 6, 2015 at 6:50

Good question, but unfortunately this doesn't work very well for covering any useful volume.

The problem is that the path loss between the indoor antenna and the phone is surprisingly large, even though you're only a few feet away. Put another way, the fraction of the transmitted signal that is intercepted by the receiver is very very small.

I'll try some numbers to check this. If the signal outdoors (on your phone) is about -75 dBm, then you need to add the following:

• Good Outdoor antenna: +8 dB
• Coax cable: -6 dB
• Indoor antenna: + 2 dB
• Indoor path loss to your phone, 900 MHz, 6 feet: -37 dB

• at 1800 MHz: -6 dB

• Holding phone on the wrong side of your head: -20 dB
• Walk another 6 feet away: -6 dB

The total effect of this system, in the best case, leaves you with 33 dB less signal than you had outdoors, which is -108 dBm, not enough to make a call, and it quickly gets worse.

The exact numbers are full of assumptions and can be debated, but the indoor pathloss is the killer.

Commercially available repeaters have 60 dB bi-directional amplifiers in them, that's a power gain of 1 million times. They also need sharp filters to prevent oscillation, and other circuits to stop them interfering with the cell network.

The site you link to shows a reasonable antenna that can be constructed without measurement equipment. But it depends on coupling directly into the back of the 3G modem or phone. This is probably only a few dB path loss between antenna and 3G stick, which tips the balance. Or you might have a phone or stick with an actual RF connector, first prize.

So I'm sorry but if you want to walk around indoors, on the phone, you really need that 60 dB amplifier.

• Good coax cable has a loss of 0.1db/metre at 1Ghz. a volume of 20 cubic feet is finy, less than 1 cubic metre. '6 feet away' isn't ooing to be a problem Jan 5, 2015 at 22:30
• @Jasen the numbers are all separated so you can try changing them. But in total you can see that sticking the phone right against the indoor antenna (or a foot away, but absolutely stationary) is probably going to work, and holding the phone and talking, isn't. Jan 6, 2015 at 5:17
• @tomnexus Thank you for your answer. You just saved me a lot of time. I guess I'll just have to pony up for the amplifier since this system seems unlikely to work how I want. Jan 6, 2015 at 6:46
• Do plenty of homework and get a demo or trial period if possible... Even with the amplifiers it is tricky and may not work as well as you hope. Also make sure it's approved by your network operator, as it's operating in their band. Imported devices may interfere with them and also break the law. Jan 6, 2015 at 10:41
• Thanks for the precautions. I'll practice due diligence before investing in such an expensive device. Jan 6, 2015 at 16:07