Re: What your mother didn't tell you about transmission lines

Posted by Jim Brown-10 on Sep 12, 2011; 4:26am
URL: http://elecraft.85.s1.nabble.com/What-your-mother-didn-t-tell-you-about-transmission-lines-tp6780383p6782247.html

On 9/11/2011 5:20 AM, Bill W4ZV wrote:
> There is a loss graph on page 2 below but I believe ET3PMW's losses were
> much higher.  Of course this could have been due to poor quality wire
> available in Ethiopia.

At HF, loss is essentially due to the resistance of the conductors, plus
whatever loss there might be due to mismatch. For matched loads, the
current is less with higher impedance lines, so the loss is less.  Most
zip cord uses relatively small conductors.  BTW -- have you ever noticed
that the Red/Black glorified zip cord sold at hamfests by CB radio
dealers is often smaller than it is marked?  BIG insulation, very small
copper. Advertised #12, but try stripping it and taking a micrometer (or
ohmmeter) to it.

Let's compare it to RG58 can be pretty lossy too, depending on how much
copper was used to build it.  At 10 MHz, the published loss data for
Belden RG58 cables varies from 1.1 to 1.5dB/100 ft. That loss is ALL due
to copper, and at 10 MHz, skin effect dominates. The shield in these
RG58 cables is a MUCH greater diameter than in the zip cord, so the loss
in the shield is a lot less than for the zip cord, which can be #18 or
smaller.

Another suggestion to our friend in ZL.  Here in North America, cable TV
companies use millions of feet of 75 ohm 1/2-inch hard line.
Installation companies have no use for short lengths of cable, so it
goes in the trash, and short lengths to these guys is often several
hundred feet. If we make friends with our local CATV company, we are
often able to replace the word "trash" with the words "back of my
pickup."  Loss data for this cable is roughly the same as for 50 ohm
hard line.

About two years ago, I inherited a very large spool of this stuff from a
SK. I am currently installing two monobanders for 20M and 15M on a new
tower, and have cut lengths of 1/2-inch hard line to feed both of them.
One of them is already operational and works great. If you google, you
will find excellent information about practical and technical details
for using this stuff in your ham station, including how to easily fit
PL259 connectors, and how to cut the cable to a whole number of half
wavelengths so that the impedance is not transformed by the cable. Some
very useful websites I found are by W9XT and N1GUN. There's also a nice
piece that ran in the VHF column of QST in 2000. Search the ARRL website
to find it.  I'm in the process of putting together something myself
that shows loss data and an alternate method of installing PL259
connectors.  I found another piece on the internet that correctly
observes that this coax can make stubs that are a bit higher Q than most
other available coax.  Higher Q means greater attenuation, but also
narrower bandwidth for that attenuation.

73, Jim Brown K9YC
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