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

 To: "'Tower and HF antenna construction topics.'" Re: [TowerTalk] Coax Loss -- RG-Numbers Don't Tell us Much "Gary Schafer" garyschafer@comcast.net, "Tower and HF antenna construction topics." Thu, 4 Feb 2010 13:32:48 -0500 mailto:towertalk@contesting.com>
 ```To add to Jim's post, anytime that you can use a higher impedance cable the loss will be less. If you have a 50 ohm and a 75 ohm cable with the same size conductors, the 75 ohm cable will have less loss because the current will be lower in the 75 ohm cable so less I squared R loss. This is why open wire transmission line has so much less loss. It is because the impedance of the line is higher which causes the I squared R value to be lower. The higher impedance line may have the same amount of resistance but the current will be less for a given power level. Examples: Let's say all of our cables have a 10 ohm conductor resistance. 1250 watts will be 5 amps on a 50 ohm cable. Loss will be 250 watts. Same power on a 75 ohm cable the loss will be 160 watts. Same power on a 400 ohm open wire line. Loss will be 31 watts. All lines have the same conductor resistance but the lines with a higher impedance will have less current so loss is less. 73 Gary K4FMX > -----Original Message----- > From: towertalk-bounces@contesting.com [mailto:towertalk- > bounces@contesting.com] On Behalf Of Jim Brown > Sent: Thursday, February 04, 2010 1:05 PM > To: towertalk@contesting.com > Subject: [TowerTalk] Coax Loss -- RG-Numbers Don't Tell us Much > > 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 > 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 > > > _______________________________________________ > > > > _______________________________________________ > TowerTalk mailing list > TowerTalk@contesting.com > http://lists.contesting.com/mailman/listinfo/towertalk _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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