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any viable ways to make a 2.4 GHz amplifier?

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LAN_403

haxor5354

Sun Jan 30 2011, 07:48PM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

i wanted to make an wifi AP thats gonna have atleast 5 watts of transmitting power
the wifi amplifiers that i see on the internet cost over $200
so are there anyway to homebrew a 2.4Ghz amp?
also, do higher frequencies EMF travel further through air than lower frequencies, and higher the frequency the lower the penetrating power through solid walls vice versa?

Mattski

Sun Jan 30 2011, 08:56PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

You can pretty easily buy a packaged 2.4GHz amplifier from Avago or Skyworks which puts out about 1W maximum. I'm not sure if you can find one that puts out 5W, that's fairly high. But you can always buy several of these amplifiers and power combine them with either a packaged SMT power combiner or a power combiner you design on the PCB.

Homebrewing an amplifier at that power level is doable, but it will take quite a bit of effort to design from scratch if you aren't familiar with RF amplifier techniques.

In general low frequencies will travel further outdoors, and I'm pretty sure that applies to indoors as well.

Ash Small

Mon Jan 31 2011, 05:18AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Not sure if this is relevant to this particular project, but it is possible to reduce the output of a microwave magnetron by reducing the value of the series capacitor. There is an article here that describes how to get one to run CW at around 100 watts:

There are also loads of links here about homebrew radar and GHz transmitters:

As I said previously, I'm not sure if any of this is relavant to this project, but I think it is 'on topic' regarding 'any viable ways to make a 2.4GHz amplifier'.

hboy007

Tue Feb 01 2011, 11:55PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

I've been coming back to this thread several times and now that you got me interested, I've just stumbled upon this HEMT FET:

and this one has a demo board included in the datasheet:

I even feel tempted to start experimenting with these myself

Dave Marshall

Wed Feb 02 2011, 05:26AM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Wifi amps are bidirectional, and that makes them inherently complex and expensive. A 1w amplifier will set you back about $200 if you shop carefully.

Also, you will not find a 5w amplifier that is legal to import into the US, as the maximum permissible power for 802.11x in the US is 4w EIRP, or Effective Isontropically Radiated Power.

At 5 watts actual output power from the amplifier, unless you intend to run it through a rather long length of coax, and still use an antenna with almost no gain at all, you will exceed this limit.

With a well designed antenna, in fact, it is quite possible to exceed that power limit with a standard 250mW wifi router.

-Dave

Conundrum

Wed Feb 02 2011, 06:23AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

hmm, interesting.
I read somewhere that using a cantenna with a cheap wifi "stick" breaks the rules, is this true?

-A

hboy007

Wed Feb 02 2011, 01:28PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

Dave Marshall wrote ...
Wifi amps are bidirectional, and that makes them inherently complex and expensive.

What is the magic behind it? One could use a directional coupler to split RX and TX units - or are there even more exotic circuits involved? Looking at these boards

I assume nothing is impossible...

Mattski

Wed Feb 02 2011, 10:15PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

wrote ...
What is the magic behind it? One could use a directional coupler to split RX and TX units - or are there even more exotic circuits involved? Looking at these boards

I assume nothing is impossible...

Yes it's possible, but a directional coupler is not a great way to go about it. The one on the page you linked to has 11dB coupling ratio, meaning that most of the signal at the antenna would not be coupled to the receive amplifier. Assuming you put the PA on the input port, LNA on the Rev port, terminate the Fwd port and connect the antenna to the output port, then you have the competing problems of low coupling factor to the LNA degrades receive SNR, or large coupling factor gives better signal to the LNA but dumps a lot of the PA's power into the isolation resistor, increases insertion loss from PA to antenna.

I assume wifi signals are time shared meaning you don't need transmit and receive simultaneously, so a T/R switch works well to switch the antenna between PA and LNA. If the amplifier is being grafted onto an existing antenna port for an AP then it can detect a transmit condition and switch appropriately. Otherwise circulators can be used, which are unfortunately rather pricey.

ConKbot of Doom

Thu Feb 03 2011, 05:49PM

Registered Member #509 Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329

Mattski wrote ...

Sometimes you can specify a tx and an rx antenna on an AP. You could then have seperate amplifiers, and as long as too much output from the TX antenna isnt coupling over to the RX antenna and saturating the LNA which may take a little bit to recover, you should be fine. I think Ive seen this used for long range link setups where 2 cantennas/pringles can antennas were used, and were spaced apart because it provided a benefit I cant recall at the moment.

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