On Thu, 4 Feb 2010 06:32:47 EST, TexasRF@aol.com wrote:
>200ft of new RG213 measures right at 4.5 dB/100ft at 432 MHz so 200ft is
>almost 9 dB and a return loss of 18 db with short or open at the far end.
OK. But one other VERY important thing. RG-numbers are generic. They used
to be a spec, so they used to tell you what the cable was. That hasn't been
true for at least 50 years. As an example, there are dozens of cables in
the Belden catalog described as RG8, and some are quite different from each
other. All the RG-number tells us is the APPROXIMATE size of the cable and
its impedance. So RG-213 is "like" RG8, which is 50 ohms and approximately
0.4-in diameter. But I've got some RG8 made by Commscope rated for plenum
use with an o.d. of 0.35-inch, and an RG6 made by Belden with two copper
braid shields that's 0.33-in o.d.
How do these cables vary? First, the amount of copper, which is what
determines the loss at HF. The center conductor of RG8 can be anywhere
between #14 and #10, and it may be solid copper, copper-coated steel, or
copper-coated Al. The braid can be anything from Al foil to foil plus
copper braid, to two copper braids. The lower the total DC resistance, the
lower the loss at HF. Copper coated steel and copper coated Al have more
resistance at DC, so below about 5MHz, they have more loss than if the
conductor were solid copper. On the higher HF bands, there's more skin
effect, so the Al or steel is no longer part of the picture. The Al plus
light braid cables designed for CATV use are great for VHF/UHF, but can be
quite lossy at HF, especially if they have a copper coated steel center.
As you get up into the VHF/UHF range, the dielectric increasingly
contributes to loss, and above 1,000 MHz it dominates. The outer jacket may
or may not be UV resistant. These cables aren't designed to carry
transmitter power at HF, so their thin shield and copper coated steel
center work fine. They're also much lighter and cheaper than cables with
more copper designed for transmitting at HF, or for carrying baseband
Some other ways these cables differ is their combustibility. Remember "The
Towering Inferno?" That was based on a true story of a fire that was spread
by cables running up through a high rise building, burning and spreading
the fire, and killing some people with the noxious fumes from combustion.
After that fire, building codes changed, requiring cables in parts of
buildings where this could happen to be made with materials that are much
less likely to burn or create noxious fumes.
The lowest loss RG8 cables I know about are less than 3 dB/100 ft at 400
MHz -- LMR400UF, Belden 9913, Commscope 3227 are examples. Commscope 2427K
is the same copper as 3227, with a plenum jacket and plenum dielectric.
It's the same as 3227 at HF, but has more loss at UHF, thanks to its
fireproof dielectric. Both 3227 and 2427K stand up to UV.
And if you're feeding a 75 ohm load, like a high dipole, consider using a
75 ohm cable. Yes, a bit of mismatch to a 50 ohm transmitter, but MUCH less
loss than a comparable 50 ohm cable. My 80/40 dipoles are fed with Belden
8213 (low loss RG11), so they work great on 6 meters!
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