On Jan 31, 10:56 am, Jeff Liebermann <j...@comix.santa-cruz.ca.us>
wrote:
> "Charlie" <ctuc...@gmail.com> hath wroth:
>
> >As posted before, we are trying to set up a test bed with a bunch of
> >Linksys WRT54GL routers. I would like to be able to cable the routers
> >together so that they are not using OTA to communicate. Ideally, there
> >would be an adjustable attenuator in the cable as well so we could
> >simulate distance.
>
> Yep. Been there and done that. What you want is a portable RF screen
> room or shielded box. Since you're only trying to block a small range
> of frequencies, this should be easy. See:
> <http://www.smallnetbuilder.com/content/view/26746/100/>
> <http://www.azimuthsystems.com/index.asp?p=198>
> <http://www.ramseyelectronics.com/te/default.asp?page=rften>
> <http://www.saelig.com/miva/merchant.mvc?Screen=PROD&Product_Code=TE00...>
> for examples. I'll spare you my photograph of the mylar foil and
> aluminum duct tape "tent" that I constructed for my first attempt at
> shielding. Search Google for "RF test enclosure".
>
> >In a naive attempt at doing this we tried wrapping the routers in
> >tinfoil (after removing the antennae) to isolate them. This worked
> >pretty well.
>
> Until they overheated. That's why I gave up on foil. Also, there was
> considerable leakage from the CAT5 cables on some routers.
>
> >Then we tried wrapping a cardboard tube in tinfoil and
> >connecting the routers by placing the tube over the antennae RP-TNC
> >connectors on the routers. (the naive part!) This didn't work at all -
> >no signals got through.
>
> Chuckle.
>
> >More research on the internet found me at this site: http://
> >www.wirelessdesignmag.com/ShowPR.aspx?
> >PUBCODE=055&ACCT=0031546&ISSUE=0610&RELTYPE=PR&OR IGRELTYPE=FE&PRODCODE=W0240&PRODLETT=A
> >which shows a test setup of wireless nodes connected with 'RF cable'.
>
> In the future, please wrap your URL's with <> so that they don't wrap.
> <http://www.wirelessdesignmag.com/ShowPR.aspx?CommonCount=0PUBCODE=055...>
>
> >Would this actually work?
>
> Sure. However, there's quite a bit of detail missing. There are no
> combiners shown in the picture. You need some what to linearly
> combine all the tx and rx signals. A simple resistive combiner will
> do, or you can use a power splitter.
> <http://www.hyperlinktech.com/web/signal_splitters.php>
> Be sure to terminate any unused ports. I use an 8 port device.
>
> Getting attenuators that don't leak and are accurate is more of a
> challenge. You'll need to attentuate the maximum tx power down to the
> baseline noise level. That's about:
> +20dBm - -90dBm = 110dBm
> range. That's much more than can be easily found with the common
> switchable attenuator that might have perhaps 60dB of adjustable
> range. Fortunately, you only need half the 110dBm of each leg of the
> power splitter so a mess of common 60dB switchable attenuators will
> barely work. However, I suggest also buying a mess of fixed
> attenuators to make life easy on the attenuator.
>
> One big problem will be calibration. You're going to have quite a bit
> of hardware floating around that may not necessarily be accurate at
> 2.4GHz. I strongly suggest you spend the time calibrating the
> attenuation of the various components at 2.4/5.7Ghz before you
> discover that your switchable attenuators were previously used as
> someone's dummy load.
>
> >Assuming it does:
> >Are attenuators required so that the output from one router doesn't
> >damage the other router?
>
> Oh yes. LOTS of attenuators. The typical wi-fi receiver has a very
> limited dynamic range. My guess(tm) is that they can't really handle
> signals much stronger than perhaps -20dBm before blocking, saturating,
> or overloading. You might damage the receiver with anything over
> perhaps +30dBm (1 watt), but that's not possible. I've done some
> crude damage testing and found that most access points can handle at
> least +20dBm before doing any damage. However, that's because the
> diversity switch PIN diodes effectively limit the input power. I
> don't know what will happen is the PIN switch wasn't there.
>
> >What cable is best to use for this application?
>
> Well, for TNC and SMA connectors, I use RG-400 and LMR-240.
> For N connectors, it's all LMR-400 or LMR-400 ultraflex. The stuff is
> really stiff. Make sure you use decent crimp connectors.
>
> >Any other advice (note - we don't have any experience in
> >this field)?
>
> Yes. Find someone with microwave experience to help. Be prepared to
> buy some rather expensive RF test equipment.
Chuckle.
Anyways we got something to work well enough for our purposes. We
wrapped the routers in tinfoil - 2 layers - and poked holes in it for
ventilation. Of course, this is not a perfect RF shield but we found
if we keep the routers about 1 foot apart they cannot see each other.
A local shop here (<http://mroelectronics.com>) helped us with a
cable. It's simply a RP-TNC to N swedge, a 10 db attenuator, a 5 foot
cable, and then swedge back up to RP-TNC. I realize this is far from
ideal but it does what we want - allows us to direct our RF signals to
a particular router. More importantly, it's cheap; about $35 CDN.
Thanks for all your help Jeff!
charlie