[mythtv-users] Attic antenna

[Brian Wood]

On Friday 28 August 2009 16:06:52 David Brodbeck wrote:
> On Fri, August 28, 2009 2:46 pm, Tom wrote:
> > The TV and the inputs on the TV cards are 75 ohm impedance. Twinlead is
> > 300
> > ohms and won't work. Not without a matching transformer-- another place
> > you
> > can lose signal. Most antennae for UHF are already 75 ohm F-connectors,
> > so why transform the signal from 75 ohm to 300 and then back again when
> > you get it into the house?
>
> I don't think I've ever seen an antenna with a 75 ohm feed -- usually
> antennas are balanced and need a balun anyway to connect to coax, although
> sometimes the balun is internal to the antenna if it's in a housing.
>
> Loss in the balun will probably be less than 1 dB.  Making an efficient
> one is not much of a trick.
>
> > Yes, coax has loss, but it's so far simpler than using twinlead...
>
> No argument there.
>
> Twinlead has more loss than the "window line" used by hams but is still
> quite a bit less lossy than RG-6.  Adding an amp would make the point
> moot, of course.

All the comparisons between coax and balanced twinlead of course assume that 
the coax is operated "flat", ie: properly terminated with no significant 
standing waves, and that the twinlead is also properly installed and operated 
essentially "flat", or even "tuned" for a specific frequency.

There are more opportunities to screw up twinlead than there are for coax, so 
in the real world of practical installations you may well actually have less 
loss with coax.

Coax also shows large differences in loss depending on the frequency. A length 
of coax with a 10db. loss at 54 Mhz. (channel 2) will have 20 db. of loss at 
216 Mhz. (channel 13). This results in significant "tilt" in long runs of 
coax, something the cable companies have to deal with using dual-pilot AGC 
amplifiers or even thermatic equalizers (long ago). This effect is less 
pronounced with twinlead.

The end result is that for most people in most cases coax is a better way to 
go than twinlead, and the extra loss can be made up with an amplifier. 

Cable TV *must* use coax, because they use frequencies not assigned to 
over-the-air broadcast to carry "extra" channels, and they must prevent such 
signals from leaking out of the cable system and causing interference to 
things like aircraft radios. Twinlead systems, especially if not perfectly 
balanced, can make pretty good transmitting antennas, definitely not what the 
cable companies want. In fact, you can make a pretty good folded dipole using 
300-ohm twinlead.


-- 
Brian Wood
beww at beww.org