[TenTec] Model 238C Tuner Questions

[Bob McGraw - K4TAX]

One additional point regarding loss is the type of dielectric being used. 
The true open wire line will have less matched loss per given unit and 
frequency than the window type line as long as the spacing between the 
conductors is a very small fraction of the wavelength..

Based on the Transmission Line Calculator by N6BV:
For 600 ohm open wire line to a load of 50 ohms:  100 ft at 30 MHz will have 
a 10.49:1 SWR at the input and a 12.0:1 SWR at the load.  Total loss would 
be 0.584 dB {line loss + SWR loss}  Matched  line loss is 0.105 dB.
For 450 ohm window line to a load of 50 ohms:  100 ft at 30 MHz will have a 
7.11:1 SWR at the input and a 8.10:1 SWR at the load.  Total loss would be 
0.574 dB {line loss + SWR loss}  Matched line loss is 0.152 dB

Although the 600 ohm line does have lower matched loss than the 450 ohm 
line, due to a closer match, otherwise somewhat lower SWR, the added loss of 
the lower SWR value will make for less overall loss.

This seems to confirm what Steve says regarding using 450 ohm line.  Using 
the same calculations for 450 ohm window line, except using a load of 40 
ohms the total feedline loss would be 0.702 Db.  SWR at the input would be 
8.61:1 and at the load the SWR would be 10.12:1.   Then for a load of 4000 
ohms the total feedline loss would be 0.716 dB with the SWR at the input 
being 8.44:1 and the SWR at the load would be 9.88:1


And I agree with Jims earlier comments, using a balanced feedline does not 
necessarily make for a balanced antenna system.  Jim listed other factors 
that can cause a system to be somewhat unbalanced.

73
Bob, K4TAX


----- Original Message ----- 
From: "Steve Hunt" <steve at karinya.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Wednesday, October 17, 2012 3:03 AM
Subject: Re: [TenTec] Model 238C Tuner Questions


> 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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