[TowerTalk] Coax Loss -- RG-Numbers Don't Tell us Much

[Jim Brown]

On Thu, 4 Feb 2010 06:32:47 EST, TexasRF at 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 
video. 

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! 

73,

Jim K9YC