Ummm...
a very novel idea to say the least...
I'd imagine that the best way to make a radio jammer would be to make an all round radio jammer...
the way that radio communications and RADAR are blocked is to fill the spectrum with white noise so that no other waves can reliably use the specturm with any real use...
http://en.wikipedia.org/wiki/IEEE_802.11
802.11b and 802.11g standards use the 2.4 gigahertz (GHz) band, operating (in the USA) under Part 15 of the FCC Rules and Regulations. Because of this choice of frequency band, 802.11b and 802.11g equipment can incur interference from microwave ovens, cordless telephones, Bluetooth devices, and other appliances using this same band. The 802.11a standard uses the 5 GHz band, and is therefore not affected by products operating on the 2.4 GHz band.
you'll also find on that page that there are 14 channels, (though I imagine that you knew that already).
but, what you may not know, (I didn't) is that some channels effectivly overlap...
the example given on the page is that.
802.11b and 802.11g divide the 2.4 GHz spectrum into 14 overlapping, staggered channels whose center frequencies are 5 megahertz (MHz) apart. The 802.11b and 802.11g standards do not specify the width of a channel; rather, they specify the center frequency of the channel and a spectral mask for that channel. The spectral mask for 802.11b requires that the signal be attenuated by at least 30 dB from its peak energy at ±11 MHz from the center frequency, and attenuated by at least 50 dB from its peak energy at ±22 MHz from the center frequency.
Since the spectral mask only defines power output restrictions up to ±22 MHz from the center frequency, it is often assumed that the energy of the channel extends no further than these limits. In reality, if the transmitter is sufficiently powerful, the signal can be quite strong even beyond the ±22 MHz point. Therefore, it is a misconception that channels 1, 6, and 11 do not overlap. It is more correct to say that, given the separation between channels 1, 6, and 11, the signal on any channel should be sufficiently attenuated to minimally interfere with a transmitter on any other channel. However, this is not universally true; for example, a powerful transmitter on channel 1 can easily overwhelm a weaker transmitter on channel 6. In one lab test, throughput on a file transfer on channel 11 decreased slightly when a similar transfer began on channel 1, indicating that even channels 1 and 11 can interfere with each other to some extent.
So... in short...
whilst you may think that the best way to build a jammer would be to mask the waves, (this could be with a directional antena (comparable with putting ear muffs on someone to stop them hearing your conversations). (note directional antena would also boost the signal in the direction of the antena).
the best way to build a wireless jammer would be to flood the spectrum with meaningless data, that would mask the usefull data,
this is somewhat comparable to putting a lound stereo next to someone so that they cannot overhear your conversations because it's masked out by noise.
your transmitter would not have to transmit on all 14 channels (as mentioned above some channels are so close as to almost overlap). but it would be better to make a jammer for all channels, (perhaps with a way that you could make them turn on or off depending on the particular channel you wanted to block?). (14 channels are spaced 5MHz appart across a centre of 2.4 GHz)
your jammer would simply transmit on the 2.4Ghz band, with a power greater than that of the wireless base station, or wireless cards connectiong to the base station.
The channels that are available for use in a particular country differ according to the regulations of that country. In the United States, for example, FCC regulations only allow channels 1 through 11 to be used. In Europe channels 1-13 are licensed for 802.11b operation but only allow lower transmitted power (only 100 mW) to reduce the interference with other ISM band users.
I'm not sure of the exact power used in US rtansmiters, (though I imagine it's the same as UK), but your 5W of transmitter power should be more than fine to flood with a more powerfull signal.
Here comes the next part of design... and where you have to consider what you actually want to do...
Wireless base stations are omni directional, meaning that they transmit in all directions equaly, (this is obvious right?)...
to jam all clients trying to connect to the base station you should build an omni directional antenna (
http://flakey.info/antenna/omni/quarter/) that transmits at a higher power ration than the base station, (or at least transmits at a higher power than the clients trying to send info back to the base station. (i.e either block out going of in comming from the base station).
the other type of antena you could use would be a directional antenna, this kind would have a center omnidirectional antenna with a parabolic dish to direct that radio waves forward... this would effectivly enable you to send your data flood to a single laptop, (possibly a single base station). whilst letting other clients even in the same room connect to the network without trouble...
there are various warnings that should come with a directional dish,
firstly, it might just be a bit of plastic or metal, but it is effectivly a forward amplifier, and you're transmitting in the microwave spectrum... (i.e don't stand in front of the dish whilst it's transmitting, cause your 5 watt can be effectivly amplified to a single beam at hundreds of watts...).
also because of this fact you'd probably be breaking the FCC licensing laws...
the very last thing that I have to say is...
I thought that the clause of the FCC radio equipment was that...
"This device must accept inteferance and this device must not cause inteferance."
The idea of using existing hardware may work... however, you have to imagine that that existing hardware has already passed FCC test and doesn't intefeere with existing equipment, so it may not be possible to block the frequencies.
also I'm not sure as you'd be able to get the router to broadcast on all channels at the same time...
(in which case your best bet is the somewhat less portable PC loaded with 7 wireless cards, all fitting with directional antennas..., (actually you could group all the antenna into a single directional dish).
IMO the best way to biuld this is to start from scratch... build a 2.4GHz radio transmitter... it's not cheap and it's not easy... but it is pretty much guarenteed to work...
the next best way is the PC laded with cards to block all channels at once...
again not cheap. but a lot easier...
cheapest, and easiest is to use the base station,
power boost the signal. transmit meaning less data and perhaps fit it with a directional anetnna to further boost the signal...
but I don't think that will work as well, or have the versatility or the other solutions in terms of jamming capabilities...
and besides, if you are using that, you may as well use a regular PCMIA card in a laptop, or a cheap USB adapter that you could powerboost and fit an antenna to...
that would make the unit a lot more portable.
at the end of the day your only goal is to decrease the signal to noise ratio to a point where the channel. (or entire band of 14 channels), is completly unusable...
