Dr. Gerald N. Johnson
geraldj at storm.weather.net
Sun Feb 1 17:30:24 EST 2009
On Sun, 2009-02-01 at 20:50 +0000, Steve Hunt wrote:
> Yes, I'm sure. According to the VK1OD on-line calculator, at 3.8 MHz
> 100m of RG213 with 14-j1 as a load has a loss of 1.9dB; 100m of Wireman
> 551 with 14-j1 as the load has a loss of 3.1dB. I'm not saying the
> difference is particularly significant - I'm just pointing out that
> ladderline is not always lower loss than coax - it depends on the load
> Incidentally, be wary of those ARRL charts which tell you what the
> "excess loss" is for a particular VSWR - they are approximations. There
> are some particular situations where a 1:1 VSWR produces more loss than
> with a higher VSWR. here's an example:
I didn't use any SWR chart. I used formulae involving the matched loss
and the magnitude of the mismatched reflection coefficient. The
mismatched loss was greater than the matched loss.
> Take 15ft of RG213 with a load of 100+j100. At 3.8 MHz the loss with
> this 4.3:1 VSWR is 0.025dB, whereas the matched loss is over twice this
> at 0.055dB.
The formulae on page 24-9 of the 1994 ARRL Antenna Book would always
show a greater loss than the matched loss of the line. Perhaps the chart
you are referring to is showing the ADDED loss because of the mismatch?
> I know these are ridiculously small losses, but I'm just
> illustrating the fallacy of thinking you can add a fixed "extra loss"
> figure for some particular VSWR value.
> Steve G3TXQ
> Dr. Gerald N. Johnson wrote:
> > Are you sure? Using data and formulae in the 1994 ARRL Antenna Book, I
> > see 100' of RG213 has 0.6 dB loss at 4 MHz when matched, and 1.066 dB
> > with a 3.6:1 SWR. 1" spaced window line has 0.027 dB (had to stretch the
> > loss chart by a factor of ten for that line) per 100' matched and 0.41
> > dB loss with the SWR of 32:1.
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