Jim,
Those are important points of clarification that you raise!! I
frequently hear folk commending ladderline-feed for a multiband doublet
on the basis that "ladderline handles high SWR much better than coax" -
that tells me there is still much misunderstanding about the fundamental
causes of loss in HF transmission lines.
I would add just one thought to what you've explained:
A typical multiband doublet might exhibit feedpoint impedances anywhere
from 40 Ohms to 4000 Ohms depending on the band. To minimise SWR
excursions across the bands we would want to choose a transmission line
characteristic impedance that is the geometric mean of those figures:
Zo=SQRT(40*4000)=400Ohms. Choosing a commercial ladderline would limit
the SWR to around 10:1 at worst; whereas using 50Ohm coax we would have
1.25:1 at one extreme and 80:1 at the other.
So, not only does the high impedance line benefit from flowing lower
current for a given power level, it can also minimise the worst-case SWR
experienced.
73,
Steve G3TXQ
On 17/10/2012 01:57, Jim Brown wrote:
On 10/16/2012 5:32 PM, Robert Mcgraw wrote:
The feed impedance on my 160M 1/2 wave wire is about 38 ohms as I
recall.
Some clarifications. First, feedline loss is directly related to
feedline current and conductor resistance, and is determined both by
the feedline impedance and the SWR. What gives open wire line it's
low loss characteristic is it's IMPEDANCE, NOT the fact that it is
balanced. Loss is lower in high Z line because for a given power level
and the same conductor size, because the current is lower.
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