Steve,
I just ran the numbers referred to by Owen, only I used RG-58 instead of
LMR-400:
http://vk1od.net/transmissionline/VSWR/aldv.htm
Since the QST author did not specify the coax type, the outcome for RG-58 is
closer to the mid-point of their respective results. Here's the loss I get
with RG-58:
40 ft of RG-58: 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 LMR-400)
QST author's difference: 10.6 dB
I suspect the author got that result from: (1) relying on the graph; (2)
using figures from low-loss non-windowed or tubular 450 ohm line where loss
is less; and (3) perhaps RG-58 was used for the QST comparison rather than a
low-loss coax type like LMR-400.
Just for fun, I then ran the calculation with 300 ft of RG-58 for an
apples-to-apples 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 LMR-400,
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
> Paul,
>
> I checked that page of Owen's earlier for a chart which shows the losses
> at various distances from a mismatched load; it was the epitomy of "a
> picture paints a thousand words"! Unfortunately Owen seems to have
> removed it as part of his recent site re-vamp.
>
> 73,
> Steve G3TXQ
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