[TenTec] Model 238C Tuner Questions

Steve Hunt steve at karinya.net
Wed Oct 17 04:03:21 EDT 2012


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 

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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