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
>
>
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