From: email@example.com (Tom Bruhns)
Subject: Re: Transmission line fun--re:"who thinks this?"
Date: 5 Nov 2002 16:30:48 -0800
References: <3DC42479.3040207@BOGUS.earthlink.net> <3DC4A09F.firstname.lastname@example.org>
NNTP-Posting-Date: 6 Nov 2002 00:30:48 GMT
(Thought I had posted a reply to this earlier, but don't see it. My
apologies if this ends up being a repeat.)
Roy McCammon wrote in message news:<3DC4A09F.email@example.com>...
> Even at high freaks, the losses due to R are usually
> larger than the losses due to G, though both are
Since the R losses go up generally as sqrt(freq) and the G losses
directly with freq, eventually the G losses dominate. But the line
may not be very practical by the time you get to that crossover point,
if it uses good dielectric.
> Plastic filled coax is typically lossier than open pair
> lines (more G. air is better than plastic), but
> picks up much less noise.
But the additional loss, if you compare two lines that are identical
except for the dielectric, at least through HF and well into VHF or
beyond, is dominated not by loss in the dielectric, but by increased
loss in the conductors. That's because the impedance of the line is
lowered by putting in the dielectric, and to push the same power
through the line requires more current, and thus higher I^2R loss.
You can put in absolutely lossless dielectric (but with relative
dielectric constant greater than 1) and the line loss will increase
over what it is with air. Foam-dielectric coax of a given outer
diameter is lower loss than coax with solid dielectric, because the
center conductor is larger for the same line impedance.
Of course, the noise pickup has to do with the configuration of the
conductors, not the dielectric; air-dielectric coax and solid
polyethylene dielectric coax are practically identical with respect to
radiation, other things being equal.
> It is practical to make L larger by putting discrete
> series inductors ever so often, or by embedding iron
> filings in the dielectric. That was how they "fixed"
> the early trans Atlantic telegraph cables.
An interesting little tidbit: because of skin effect, dB loss at HF
tends to increase proportional to the square root of frequency. But
in RG-174, with stranded copper clad steel center conductor, the
current is pretty much confined to the thin copper over a fairly broad
range of frequencies in the HF range, and the attenuation is
relatively flat with frequency.
About the reactive Zo at low frequencies: consider what the magnitude
of the reflection coefficient is if you put, say, an inductor of 1+j50
ohms across a piece of RG-58 type coax at 100kHz. You should find
that it's greater than one. But just don't read too much into that.
For such a seemingly simple component, there sure are a lot of things
you can find out about TEM lines.