Re: Coax + Ladder Line
- From: Owen Duffy <none@xxxxxxxx>
- Date: Sat, 13 Jun 2009 01:11:42 GMT
Jim Lux <james.p.lux@xxxxxxxxxxxx> wrote in
news:h0umv9$77d$1@xxxxxxxxxxxxxxxxx:
....
Mythical properties are ascribed to ladder line, some of attributable
to ARRL publications.
Or, I think more properly, to statements in ARRL publications taken
with a different set of underlying assumptions..
The ARRL has long published characteristics of "Generic ladder line"
being #18 conductors spaced 1". They publish loss figures better than
theoretical loss for the same size copper conductors in a vacuum (eg at
10MHz, about 0.27dB/100m vs 0.47dB/100m).
The second factor is, I think, the notion that since one-way loss is
relatively low on open wire lines, that it is safe to consider it
*always* insignificant (irrespective of VSWR), then exagerate the point
by talking about it being essentially lossless at extreme VSWR.
I think that the best characterisation we have of ladder line is that of
Wes, N7WS. Similar measurements at lower frequencies might well reveal
whether the stranded copper clad steel versions have inadequate cladding
depth for copper-like performance at low HF.
For instance, ladder line between a tube amp output tank and a doublet
is probably "reasonably" low loss in most common practical
applications..
Perhaps a better concept is "acceptable loss" where acceptable is
influence by the application. The compromises accepted for multiband
operation might well include 3dB of line loss on the lowest band.
....
Ham applications in any case are kind of an odd thing,
efficiency-wise, since the limit is on RF power at the transmitter
output connector. Pretty much every other user of RF power amplifiers
sets the system measurement plane somewhere else.. in space (EIRP or
ERP) or at the power source (power budgets). For instance, you can
trade increased power consumption against lower feedline loss.
Actually, interesting you mention that.
I recently had cause to probe the meaning of the term "transmitter" in
our (VK) regulatory framework. Whilst we are limited in transmitter power
output, the meaning of transmitter is relevant.
From our legislation:
===quote
8 Definitions of radio emission and transmitter
(1) For the purposes of this Act, a radio emission is any
emission of electromagnetic energy of frequencies less than 420 terahertz
without continuous artificial guide, whether or not any person intended
the emission to occur.
(2) For the purposes of this Act, a transmitter is:
(a) anything designed or intended for radio
emission; or
(b) any other thing, irrespective of its use or
function or the purpose of its design, that is capable of radio emission.
=== end quote
It could be argued that an "emission" exists beyond the antenna ("without
continuous artificial guide"), and that everything up to an including the
antenna could be captured as part of a "transmitter" as defined.
Though there is a well understood ordinary meaning to the word
"transmitter", the drafters of the legislation have provided a definition
that should override any ordinary meaning in the context of that law.
Perhaps we (VK) are entitled to apply our limit to radiated power!
Owen
.
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