efficient, low cost, high gain rf cellular power amplifer

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qwerty15
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efficient, low cost, high gain rf cellular power amplifer

Post by qwerty15 » Thu Oct 11, 2007 8:49 am

hi,

I am looking to design an efficient, low cost, high gain dual band rf cellular power amplifer. does anyone have any schematics they have used in the past? any good antenna designs if any, etc?

maybe i should use a class A amplifier with two diplexers and a lot of filtering? what are your thoughts?

thanks in advance!

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jwax
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Post by jwax » Thu Oct 11, 2007 1:55 pm

You must define a lot of details before anybody could begin to comment-
Frequency, power, duty cycle, power supply, etc.
For what its worth, RF amp design is not for the amateur, especially microwave. JMHO.

Robert Reed
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Post by Robert Reed » Thu Oct 11, 2007 2:36 pm

I agree with J-Wax on this one.
Designing Linear VHF amplifiers is difficult enough , but designing UHF amps in it's upper realm is almost impossible to do for the amateur. It's a task that calls for a radio engineering lab with all the associated equipment. At these frequencys, components take on a whole new look such as resistors becoming more capacitive than resistive just as an example. In these cicuits, the circuit board is more important than most components , in fact the circuit board is the components (many inductors and capacitors printed on board) and all interstage coupling is done in a 50 ohm transmission line environment using a technique called microstrip tecnology. The parasitic capacitance and inductance we have to deal with at lower frequencies on standard printed circuit boards would actully kill the design at microwave frequencies. Couple that with the high cost of individual components required would probably make the project not worth while. I don't know what your experience is along these lines and I don't mean to discourage you, but offer this as more of a warning for what you will get into. On a more optimistic note, I have seen some commercial equipment of what you are looking for on E-Bay at decent prices.

qwerty15
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Post by qwerty15 » Thu Oct 11, 2007 4:28 pm

i understand and appreciate your concern; however, i am not an amateur to electrical engineering. I have a B.S.E.E. , but I have never built an amplifier before and would like to get into it. I have the knowledege of Electromagnetics, Antennas, etc to be able to work on a design and understand it. I am just looking for something that someone has done before, so I can build it myself and tweak it etc to see how I can make it better.

I have done some research to see what i want to do, for example, build a Class A amplifier to achieve high gain/linearity and then from there work on the efficiency part. But if there is an easier way of doing that, then I am open to suggestions.

I have a VNA and a Spec. A. at work. Basically all I want to start with is a platform and build upon that.

I am thinking of just buying an eval kit from Linear Tech and begin with that. This one kit has a 900/1800MHz RF Power Module with a Mux, Diplexer, RF power controller, etc...And with this design I'll always have a perfect impedance match no matter what frequency I am on, due to the 50 ohm shunt resistor.

Thoughts?

Thank you.

Robert Reed
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Post by Robert Reed » Thu Oct 11, 2007 8:43 pm

Hi Querty
The ARRL manual has some good stuff in it concerning UHF equipment. The Amateur UHF bands won't exactly line up with your frequencies but should provide a good springboard for getting ideas and getting you started. They call these boys 'Amateurs' which is a misnomer because they actually are extremely well versed on their subject and do some fantastic design and fabrication right on their home workbench.

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Re: efficient, low cost, high gain rf cellular power amplife

Post by stevech » Thu Oct 11, 2007 9:17 pm

qwerty15 wrote:hi,

I am looking to design an efficient, low cost, high gain dual band rf cellular power amplifer. does anyone have any schematics they have used in the past? any good antenna designs if any, etc?

maybe i should use a class A amplifier with two diplexers and a lot of filtering? what are your thoughts?

thanks in advance!
if you build/buy, note that the FCC licensee is the cellular carrier. Emitting a signal in their licensed spectrum with equipment not provided by or authorized by the licensee is a violation. Unlikely, but if a cellular carrier decides your emissions are unwanted, e.g., causing adjacent channel interference or, in the case of CDMA, screwing up their dynamic power control, they could take action against you.

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haklesup
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Post by haklesup » Thu Oct 11, 2007 10:49 pm

I got the impression that all his signals would be contained in coax cables and emitted into dummy RF loads. Don't need an FCC licence for that. An application for said amplifier might be a different story

I think the evaluation kit is the best way to go. you won't have to worry about construction details and the manufacturer will be able to answer many of your questions while using it.

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MrAl
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Post by MrAl » Thu Oct 11, 2007 11:19 pm

Hi there,

I dont think the FCC nor the cell phone company would complain though
if the amp was only used only for emergencies, as when stranded out
in the boondocks when your plane went down. A standard signal
might not get out, while an amplified one might save lives.
LEDs vs Bulbs, LEDs are winning.

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Post by dyarker » Fri Oct 12, 2007 6:50 am

If it is for remote cellular use, aren't we talking about two amplifiers, transmit and receive?
Dale Y

Robert Reed
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Post by Robert Reed » Fri Oct 12, 2007 8:39 am

"I dont think the FCC nor the cell phone company would complain though
if the amp was only used only for emergencies, as when stranded out
in the boondocks when your plane went down. A standard signal
might not get out, while an amplified one might save lives."

At these frequencies - line of sight is the thing. Merely increasing power alone will not neccessarily give the increased range one would expect. Case in point- My marine radio has dual wattage Tx out put, 1 watt and 25 watts. The difference in range that I have experienced with either is about 1.5:1. And thats only at mid VHF frequencies! The most notable difference is in the fringe areas where the higher power does make a difference. Since Cell frequencies are using line of sight or nearly so, the biggest difference one can make is in antenna height giving the most bang for the buck. Thats why NASA can transmit to Mars and back (antenna height and line of sight). Of course, once the signal being clear of the atmoshere and tavelling through a vacuum helps greatly. Of course if said plane were to force land on a mountain top, you would be in great shape :grin:

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jollyrgr
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Post by jollyrgr » Sat Oct 13, 2007 9:14 am

Please people, forget about if this project is FCC approved. It's already been done. You can BUY these amplifiers already made. See this: http://www.thinkgeek.com/gadgets/cellphone/9823/ There are plenty of jammers coming into the country from the Pacific rim as well as electrical engineers right in the US designing cell jammers. One lady, Limor, based her master's design project on such a device. Read about it here: http://www.ladyada.net/techproj/freshair/index.html

Her design process will give you an example of the difficulties involved with RF design at these frequencies. The problem is not the FCC but the issues involved as some have stated. Namely the circuit board IS the circuit. The phones use laser trimmed traces in some cases to from coils and capacitors.
No trees were harmed in the creation of this message. But billions of electrons, photons, and electromagnetic waves were terribly inconvenienced!

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Externet
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Post by Externet » Sat Oct 13, 2007 11:30 am

Hey qwerty, it won't work, do not waste your time and energy.

Your cellular telephone, as is, with it 0.3Watt or whatever puts out does not reach the telco cell antenna say- 10 miles away. That is the problem you try to solve with the power amplifier.

After building the amplifier your same beefed-up cellular telephone now putting out say - 5Watt will reach that telco cell antenna 10 miles away.
But your beefed-up cell phone won't hear it, because the signal to receive won't be strong enough, plus deafening of the phone receiving circuits due to the much higher RF level at its antenna.
Dual band is double trouble...

If cannot be solved with antenna gain and elevation alone, do not even think of it. :sad:

Miguel
- Abolish the deciBel ! -

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