OT: Re: [lug] cheap 802.11b for linux...

[J. Wayde Allen]

On Fri, 22 Mar 2002, Nate Duehr wrote:

> You can't use RG-6.  It's 75 ohm impedance.

At least not without suffering mismatch problems or needing to use an
impedance matching network.

> RG-213 is double-shielded, which is where it gets its lower loss from,

Shielding shouldn't directly affect loss.  At least, I can't see why ...?

> Not all the good stuff is listed there on those pages either.  LMR400
> is excellent and not listed, and there are others.  Some are more
> plentiful and actually cheaper in small quantities than others.

Most definitely true.  I just grabbed those few URL's from a quick web
search.  There are many others, and certainle some that may be much better
sources.  

> Look carefully at the sizes also, you'll be surprised at the diameter 
> size differences available.  Typically bigger *usually* means lower loss 

Yes, this is true.

> because as you move toward "hardline" you move more and more toward air 
> as the dielectric.

I'm not sure I agree with this.  The field density has to be higher in a
smaller cable than in a larger one, and I think that this contributes to
the loss.  Also, the dielectric constant has to be adjusted to maintain
the 50 ohm cable impedance, so the loss tangent in the dielectric material
itself may be less.  That is probably what you mean by moving closer to
air? Anyway, I'll have to think about this a bit more.

I'd be kind of surprised if you guys are really using hardline.  At least
what I know as hardline is a solid copper outer conductor, and is a
probably a bit expensive.  Coaxial lines made from concentric tubes with
air dielectric are usually called airlines.

> The other post from Dan did a good job of explaining 
> that, though.

I have to take a closer look at his post.  Will do that shortly.

> Waaaaaaaay off topic now, sorry all...

Yes and no ... this is one of those related, but not specifically Linux
topics.  Wireless networking is growing and becoming more mainstream.  The
popularity of the 802.11 hardware is evidence of this.  Also, this touches
the core technology of the internet itself whether wired or wireless.  In
either case, you are dealing with RF signals. 

- Wayde
  (wallen at lug.boulder.co.us)