Newsgroups: comp.dcom.cabling
Subject: Re: Fishing tips for finished basement without drop ceiling
References: <[email protected]> <[email protected]> <[email protected]>    <[email protected]> 
[email protected] writes:

> Ed Nielsen  wrote:
> > 
> > If a house connected to a CATV system 75 miles away from TV transmitters 
> > has ghosting on locals due to a loose connector at the pole (ingress), 
> > why would one think that there would be no ingress with UTP (which has a 
> > whole lot less shielding than a loose connector).
> 
> Because CATV is unbalanced and thus highly susceptible to ingress
> whereas UTP is balanced and thus highly immune to ingress?

CATV signal is normally transported as unbalanced signal ovet 
75 ohm cable. It can be transported through balanced medium as 
well if the medium has suitable characteristics (low enough 
high frequency attenuation, good enough shielding / balance etc.) 
and you have a suitable aapter to convert 75 ohm signal to 
that other form (match impedance and do unbalanced-balanced
conversion). 

In the old days (and maybe even sometimes nowadays) twinlead wire 
is used to transport TV antenna signals (at least at VHF
frequencies). That twinlead wire is 300 ohm balanced wire pair. 
75 ohm to 300 ohm balun transformers (for example 
Radio Shack cat. #'s 15-1140 and 15-1253 or MCM #33-050 and #33-010)
are use when this kind of cable needs to be connected to 
something that uses 75 ohm coaxial interface (modern equipment, 
cable TV network etc..)

In the same way 100 ohms UTP wiring can be used to carry 
antenna signal in the balanced format. All you need is 
low ernough loss cable (CAT5 for short distances, CAT5e and CAT6 
have lower losses) and suitable 75 ohm to 100 ohm balun adapter. 
The CAT5 or higher UTP wiring can carry RF signals 
quite well. The wires are quite near each other, tightly 
twisted together and reasonably well balanced (very similar 
resistances and capacitances), which means that properly 
balanced signal does not leak out mich from the cable and 
the cable is not easy to pick up interference 
(if you try to push unbalanced signal directly to UTP 
those "shielding" properties of balanced pair are lost, 
meaning cable will radiate considerable amount of 
RF signal out and pick up RFI noise from outside sources 
considerably). The loss of the typical CAT5 cable is considerably 
higher that with the cables orginally designed to carry 
RF signals (RG6 coax, RG59 coax, twinlead), but still 
in range that it can be used to carry antenna signal 
tens of meters, 100 meters might be too much for highest 
frequencies to be useable anymore. So for cable TV connection 
you need a sutiable 75 ohm to 100 ohm balun converter. 
Here is one product that claims to do the needed conversion:
http://www.svideo.com/coaxbalun.html
"VideoEase CATV Balun allows traditional 75-ohm coaxial cable to be
replaced by one-pair Unshielded Twisted Pair (UTP) in the CATV, VHF
and FM environments in certain applications."


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/