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Cell Phone Booster at home

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ets960

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I'm trying to develop a cell phone repeater that can be used at home. I've had some other ideas, and I will probably start with just using two antennas, one for outside my home, and another for inside, and then see if that works. However, I am really interested in making it an active device...

Here's my idea: An antenna on the roof specialized for the frequency of my carrier (850 Mhz). This runs through coax cable to my repeater part, which I believe will consist of a couple gain stages and several filters. Then I have another antenna inside the house.

The only problem is the frequency this is running at. Does anybody know of a good op-amp that has high gain at around 1 Ghz? Or do you think I need to make the gain stages using just transistors and using passive filters. Please let me know what you think!
 
My guess is that your amplifier approach would require licensing - and a license would probably not be issued to you. The problem that you appear to be solving isn't uncommon. There might be equipment available from the cell phone company that is allowed, licensed, etc.

You certainly could amplify what you are recieving but the transmit end is the issue. If you plan on re-transmitting then re-transmitting on the same frequency that is received is an issue. Even a slight offset is challenging.
 
I'm basing my information off of this page... There are a lot of companies that offer these products...

http://www.tessco.com/products/displaySkus.do?groupId=030&subgroupId=19


And as far as FCC regulations, they have a statement about home-built devices, stating that they only have to attempt to fit the regulations to the best of their abilities because it would be difficult to measure.

Here's the link:
**broken link removed**


I don't understand why it would be difficult to retransmit on the same frequency... I might be missing a very huge piece of this, but if I simply have an antenna, then I filter out all frequencies that aren't in my range, then I amplify those signals, won't I have the exact same signal I started with, only amplified? And couldn't I just as easily put an antenna on the end of this that will simply transmit that signal again?

Are you thinking that I'm going to have to take the signal, demodulate it or whatever, and then remodulate it on another antenna? Maybe you are, and if so, I would definitely have a problem...
 
The signal level into your cell phone receiver is likely to be very low. If you were to have a transmitter on the same frequency, nearby, the receiver would also hear it and likely see it at an even higher level. I'd think it would oscillate as a result of the feedback. Even if it doesn't it would wipe out the signal you intend to receive. A repeater usually shifts frequency slightly but even with an offset the presence of a relatively powerful signal near your receive frequency could be a problem unless you use incredibly sharp (not cheap either!!) filters - duplexers and helical resonators come to mind. A transponder might fit into the picture but the problems, in some cases remain the same unless there's a profound shift however that is likely to move you right out of the cell phone band. My definition of a repeater - it receives, demodulates, modulates and transmits - what goes in goes out on another frequency. It could receive FM and retransmit on SSB (or VOIP as is done on Echolink). My understanding of a transponder - everything that comes in goes out the way it came in on a different frequency - exception could be reversing of a sideband (or something like that).

We haven't even considered whether the cell phone or cell system could follow any offsets.

I'd consider the FCC references you are looking at carefully. I am familiar with amateur radio regulations and they are technically very clear and explicit. The licensee is responsible to adhere to those regulations regardless of ability to actually measure. It's possible that what you've extracted is only a portion of the regulations that apply. I work with codes, rules and regulations all day long and quite often the apparent broad permission in one part of a code is quickly narrowed by other parts of the code or other regulations. "Do the best you can" just doesn't seem consistent.

Anyway, fun to think about but quite the technical challenge in my opinion.
 
ets960 said:
I don't understand why it would be difficult to retransmit on the same frequency... I might be missing a very huge piece of this, but if I simply have an antenna, then I filter out all frequencies that aren't in my range, then I amplify those signals, won't I have the exact same signal I started with, only amplified? And couldn't I just as easily put an antenna on the end of this that will simply transmit that signal again?

Are you thinking that I'm going to have to take the signal, demodulate it or whatever, and then remodulate it on another antenna? Maybe you are, and if so, I would definitely have a problem...

Yep you're missing a huge piece I'm afraid. If they're on the same freq, the antenna will receive the signal it's trying to transmit, and amplify it in a feedback loop.

The receiver cannot be allowed to see the transmitted signal. It's possible to do this with a directional antenna if there's no reflections off of objects sending it back to the receiver again.

The phone's receiver should also not be allowed to get two signals of similar amplitude, there will be some time shifting between them and they'll interfere. If one's like 20x stronger than the other then it will probably filter out the conflicting signal. At least it does when you've got a neighbor a few houses down on the same channel, but that case is a bit different since the freq are not 100% exactly the same so I'm not sure.
 
ets960 said:
Does anybody know of a good op-amp that has high gain at around 1 Ghz? Or do you think I need to make the gain stages using just transistors and using passive filters. Please let me know what you think!

I think this paragraph best explains why you are struggling!, you don't use opamps for RF - and certainly not at the top of UHF and the bottom of microwaves. There are a few specialised IC amplifiers, which will be surfacemount, and VERY layout critical - it's not a job for beginners.

If your RF knowledge is so limited, you don't have a prayer of building something like this, which would be a seriously difficult task for someone experienced in RF design (which I'm not!).

As also suggested, you CAN'T transmit on the same frequency as you receive, it will just oscillate as the input picks up the output - just like feedback in a PA system.
 
Didn't I give my solution to this problem on another site?
Simply use a good antenna outside, connected through appropriate coax cable to another good antenna inside. Unless your castle is huge it should work fine. :lol:
 
One thing you might try is putting your cell phone in front of a parabolic dish, like the directv ones. This will give you some good gain ive heard. Yeah, this is a bit sloppy, and inconvenient (you cant walk around with a parabolic dish), but it might work.
 
zachtheterrible said:
One thing you might try is putting your cell phone in front of a parabolic dish, like the directv ones. This will give you some good gain ive heard.
Which direction, straight up?
How close/far is the focal point?
Zach, you will need an awful lot of luck to make this trick work!
 
Audioguru's solution is simple and efficient. I've also applied once in a valley for TV.
 
Sebi said:
Audioguru's solution is simple and efficient. I've also applied once in a valley for TV.

And most still refuse to believe that it will work(as a repeater) because it is too easy, there are no amplifier, opamps, electronics or power supply involved.

edited: "(as a repeater)" added
 
eblc1388 said:
Sebi said:
Audioguru's solution is simple and efficient. I've also applied once in a valley for TV.

And most still refuse to believe that it will work because it is too easy, there are no amplifier, opamps, electronics or power supply involved.

The main problem is the incredibly low amount of power it provides, a more useful system (for TV in a valley) is an aerial at the top, running down the hill a little way, to another aerial to re-transmit it. You then add an amplifier in between the two aerials (just a normal masthead amplifier gives enough gain). This gives a higher signal level, and the seperation between the two aerials, plus their highly directional properties, prevent the feedback problem.

However as suggested (with no amp) it will work, as long as you have sufficiently high signal levels at the top.
 
On the other site Ets said he needed the repeater so that a cell phone would work in the basement of his home.
Therefore with a normally strong signal at the antenna outside, it would feed the short distance to the other antenna inside. I think his cell phone is going to be pretty close to the antenna inside, so should work just fine. :lol:
 
may i offer a suggestion?

Put ur cell phone on loud and stick it next to a window. When the phone goes of u can pick it up and talk no problems :D
 
audioguru said:
zachtheterrible said:
One thing you might try is putting your cell phone in front of a parabolic dish, like the directv ones. This will give you some good gain ive heard.
Which direction, straight up?
How close/far is the focal point?
Zach, you will need an awful lot of luck to make this trick work!

This isn't a very "precise" solution, more of a hit and miss :lol: I guess you'd just experiment.
 
With a hands-free (mic and earphone) one might actually make this work though it would be cumbersome. Some fellow ham radio operators did a demo on improved Wi-Fy. Many Wi-Fy channels are within the ham bands. Consumers are restricted to using these wireless devices as-is - hams have much greater freedom, within the ham band channels. The demo included using a wireless communicator with the cover removed - placed in front of a parabolic dish (scrap is what they said). Experiments were done on placement. The effective range was enhanced considerably. Note that this is 2.4 gHz. Cell phones, at least some, are on a different band but the idea might still work. Might also be illegal - possibly fitting under some restriction on modification of equipment.

Some hams have taken the improvements on Wi-Fy to include increases in power though at some point we'd have to call it Wi-Fry.
 
Talking about "fry", some guys at a university sent Wy-Fi for many kilometers between two mountains. They used a small Chinese deep-frying tool at each end as a parabolic dish. I wish I kept the link. :lol:
 
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