Paul,
The coax used for comparison in the QST article was belden 8214; matched
loss is quoted as 1.64dB/100m @ 10MHz. Belden say the cable is RG8/U type.
As I mentioned earlier, the ARRL assumptions about ladderline loss are
very optimistic. It's easy to do a calculation based on the RF
resistance of the gauge of wire used in ladderline to show that to get
their loss figures the line would need to be constructed with something
thicker than #12 gauge solid copper. If you take a more typical figure
of #18 gauge you find the losses are *double* the ARRL figures.
That leads to the mischievous thought that whoever did the ARRL
calculations worked out the loss for one wire, but forgot that there are
two wires making up the ladderline ;) The error is present in the
Antenna Book and the associated software.
Here's the simple maths based on the ARRL TLW figures for " 450Ohm
Window Ladder Line":
Assumed wire size = #18 gauge
Quoted Zo = 405 Ohms
RF resistance of 100ft of #18 copper wire at 30MHz = 14.75 Ohms
Assume 1A flows into the matched line. Then Pin = 405W, and the power
lost in the line is 2*14.75 = 29.5W
So line loss = 10*log (375.5/405) = 0.328dB
Compare that with the ARRL TLW figure of 0.152dB/100ft at 30MHz
73,
Steve G3TXQ
On 01/12/2010 16:41, Paul Christensen wrote:
> Steve,
>
> I just ran the numbers referred to by Owen, only I used RG58 instead of
> LMR400:
>
> http://vk1od.net/transmissionline/VSWR/aldv.htm
>
> Since the QST author did not specify the coax type, the outcome for RG58 is
> closer to the midpoint of their respective results. Here's the loss I get
> with RG58:
>
> 40 ft of RG58: 9.6 dB
> 300 ft of Wireman 450 ohm windowed ladder line: 3.5 dB
>
> Difference: 6.1 dB
> VK1OD's difference: 2.4 dB (I get the same result with LMR400)
> QST author's difference: 10.6 dB
>
> I suspect the author got that result from: (1) relying on the graph; (2)
> using figures from lowloss nonwindowed or tubular 450 ohm line where loss
> is less; and (3) perhaps RG58 was used for the QST comparison rather than a
> lowloss coax type like LMR400.
>
> Just for fun, I then ran the calculation with 300 ft of RG58 for an
> applestoapples comparison of line length for a long run. Total loss is a
> whopping 19.1 dB. In fact, the return loss is so high, the VSWR at the
> transmitter is 1.66:1. With that much loss, darn near anything you do to
> the end of the line would probably keep the SWR under 2:1. With LMR400,
> 300 ft of loss is 13.5 dB versus 3.5 dB for somewhat lossy windowed line.
>
> Paul, W9AC
>
>  Original Message 
> From: "Steve Hunt"<steve@karinya.net>
> To:<towertalk@contesting.com>
> Sent: Wednesday, December 01, 2010 10:57 AM
> Subject: Re: [TowerTalk] tuners and power rating
>
>
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