[TenTec] Model 238C Tuner Questions

Bob McGraw - K4TAX RMcGraw at Blomand.net
Thu Oct 18 09:40:26 EDT 2012

Good points.   A sanity check is always worth while.  My intent was to show 
the differences in line loss vs. line impedance under highly mis-matched 

I do find that measurements made by N7WS indicated the actual loss values 
are greater then those obtained from the N6BV calculator.  The measured 
numbers by N7WS do closely concur with your values.  At the same time the 
frequency used by N7WS is 50 MHz.   When this frequency is applied to the 
N6BV calculator the loss figures are quite similar although in every case 
the N6BV calculated values are lower by some 30%.  Same for the Z of the 
line being where N7WS shows the example to be 405 ohms.

In one case the numbers are presumably calculated while the other case the 
results are measured values.   In dealing with very small loss values, the 
difficulty increases proportionally.

In practice and any case, the actual installed loss values for feed lines 
will be greater than the "book values" as these numbers are largely ideal or 
values obtained under highly controlled lab conditions and thus "not the 
real world" of applications.

Thanks for your feedback and input.

Bob, K4TAX

----- Original Message ----- 
From: "Steve Hunt" <steve at karinya.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Thursday, October 18, 2012 3:15 AM
Subject: Re: [TenTec] Model 238C Tuner Questions

> Bob,
> One word of caution - you need to treat N6BV's loss figures for twin-wire 
> lines with a big dose of scepticism; they are often a factor 2 optimistic! 
> It's the case in every ARRL Antenna Book I have, and for the ARRL TLW 
> software.
> Here's an example:
> N6BV's Generic Window Line has a quoted Zo of 405 Ohms and uses #18 wires.
> 100ft of #18 wire has an RF resistance of 14.75 Ohms at 30MHz
> If the line is matched and flows 1A, the power delivered to the line is 
> 405W and 29.5W (2x14.75W) is dissipated as heat
> That represents a loss of 10Log(375.5/405) = 0.328dB/100ft
> Compare that with N6BV's TLW figure of 0.152dB/100ft
> It's almost as if he forgot there are two wires, and so ended up with only 
> half the actual loss ;)
> 73,
> Steve G3TXQ
> On 18/10/2012 02:04, Bob McGraw - K4TAX wrote:
>> 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
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