[TenTec] Model 238C Tuner Questions

[Steve Hunt]

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
>