On Jul 14, 7:44*pm, Jeff Liebermann wrote:
On Sat, 14 Jul 2012 15:48:53 -0700 (PDT), "hr(bob) "

wrote:
You don't want to tape 300 ohm twinlead to the mast, it killls the
signal. *Use standoffs or coax cable.

Agreed. *However, I would recommend double shielded RG-6/u coax cable
instead of twinlead.

300 ohm has lower losses on
average than coax, but does have to be kept at least a couple of
inches away from anything metallic and thus is more of a pain to
install.

Yep. *I dunno about the low loss part.
For the feed lines by themselves:
* * * * *300 ohm * * * * *RG-6/u
MHz * * *twinlead * * * * *coax
100 * * 1.1dB/100ft * * *2.8dB/100ft

Most antennas are 300 ohms, while modern TV's are all 75 ohm. *Add a
300 ohm to 75 ohm transformer for an additional 0.5dB loss to both
transmission lines.

However, when I tried to measure the losses at 100Mhz, with a few
impairments added. *Losses are approximate as I can't fine where I
scribbled down the results.

Coiling the entire 100ft of twinlead into about a 2ft dia coil
resulted in an additional 3dB loss due to radiation. *While this is
unlikely to happen in a real install, it does indicate that you cannot
store extra wire in a loop in the attic.

Running the twinlead through a nearby metal object (i.e. my bench
vise) resulted in an additional 2dB loss. *This was intended to
simulate running through the opening in a metal framed window or door.

Running the twinlead, with one wire close to a handy metal object
(10ft of steel rain gutter) resulted in an additional 3dB loss. *This
demonstrates what happens with the balanced line becomes unbalanced by
different capacitance from each wire to ground.

A simulated wet bird, in the form of a plastic bag filled with 2 cups
of water straddling the twinlead, resulted in 2dB loss.

Doing the same with the coax cable, showed no changes in loss.

Bottom line: *Twinlead is lower loss, but only if you have an ideal
installation, which I have yet to see. *Use coax so that you don't
have to worry about such things.

--
Jeff Liebermann * *
150 Felker St #D * *http://www.LearnByDestroying.com
Santa Cruz CA 95060http://802.11junk.com
Skype: JeffLiebermann * * AE6KS * *831-336-2558begin_of_the_skype_highlighting*FREE*831-336-2558

Jeff - I installed antennas in the mid-1950's in Ft Pierce, FL.
Nearest tv broadcast station was Miami @ 125 miles. We used antenna-
mounted signal vacuum tube boosters, and 300 ohm twinlead with
standoffs to get the maximum possible signal to the tv sets. When
West Palm Beach came on the air, only 50 miles away, we were able to
drop the boosters, but kept the 300 ohm twinlead. There was also a
hollow tube 300 ohm twinlead that claimed slightly lower losses in wet
weather, but that required sealing the tube at the top or it could act
like a small pipe and be worse than regular flat twinlead. Coax
losses were just too great at those signal levels/distances.