On Sun, 01 Aug 2004 08:42:50 -0700, Peter Larsen wrote:
>I am wondering what effect if any using RG-59
>rather than RG-58 would have.
The inherent Z of a free space dipole at resonance is closer to 75 ohms than 50
ohms. Put that dipole close to ground and the Z is going to be closer to 50
ohms.
My rule of thumb is to use 50 ohm coax on low dipoles and 75 ohm coax on higher
ones. Standing waves and losses on the line are determined primarily by the
match of the antenna to coax, not the coax to the transmitter. The transmatch
matches the line to the transmitter, and can easily handle the 50 to 75 ohm
transition.
>I am on a lot of major construction sites that use RG-59
>so I can scoop 250 to 300 feet for free.
Well, there's RG-59, and there's RG-59. The RG specs are VERY broad, and were
established more than 60 years ago when these cables were rarely used above 30
MHz. In fact, if you look in the Belden book, you'll see more than fifty 75 ohm
coax
cables, all designed for different uses, and the majority carrying the RG-59
moniker. Different shields, different center conductors, different dielectrics,
different jackets. All of which results in performance specs optimized for
those
uses. Some have stranded center conductors, some solid copper, some copper
coated steel. We need transmitting coax, and 160 calls for low R at 2 MHz,
which
probably wants to see solid copper.
It all depends on what that coax on the construction site is designed for. Get
the
type number and search for the catalog data on line. If its MATV coax for
indoor
use, it probably has a foil/braid shield, maybe all Aluminum. Not good for
transmitting. If the shield is copper braid plus foil, that's OK. MATV coax
rarely
sees more than 10 volts. Lots of coax is designed to carry baseband video,
where
1.4 volts peak to peak is the standard. These cables will usually be all
copper, or
copper center, copper braid, and maybe a second foil shield. But is the voltage
rating good enough for transmitting? What power level will you use? How high
will
the VSWR be? A high VSWR can increase the voltage and current significantly.
Then there's the outer jacket. If you're going to use it outdoors, it will get
hit with UV.
That requires a non-contaiminating jacket -- that is, one whose chemicals don't
migrate to the dielectric in the presence of UV and contaminate it, increasing
the
loss. If your scrap coax is designed for outdoor use, it's probably a non-
contaminating jacket.
So it may be free, but it may not be of much use in your ham station. If its
scrap, I
would grab it anyway -- even MATV coax can be useful for QRP and for running
your TV and FM antennas around the house. :)
Jim Brown K9YC
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