[TowerTalk] Re: Force12

[n4kg@juno.com]

On Thu, 11 Oct 2001  "Mike" <W4EF@dellroy.com> writes:

	BIG  SNIP
 > > 
> Yes, of course, but switch over to a 50 ohm reference and then
> mover your 50 ohm VSWR meter along the length of the line.
> At 1/4 wavelength from the 35 ohm load, the 50 ohm VSWR will
> be 160/50 = 3.2:1. Move another 1/4 wavelength (total 1/2 
> wavelength) from the load and the 50 ohm VSWR will be 50/35 =
> 1:43:1 VSWR.
> 

HUH?    Where did you get 160 Ohms?

The formula for a 1/4 WL transmissionline transformer is

Zin X Zout = Zo^2  (Zo squared)

Zout = Zo^2 / Zin =  50^2 / 35 =  2500 / 35 = 71.4 ohms
at 1/4 WL from the 35 ohm load for a 50 ohm cable.

SWR = 71.4 / 50 = 1.43, same as at the load, ignoring losses.

OH wait, I see, you are assuming a 75 ohm coax.

Then the IMPEDANCE 1/4 WL from the 35 ohm load is
160 ohms but the SWR = 160 /75 = 2.13, same as at
the load (75 / 35 = 2.14).

SWR is a function of the LOAD impedance AND the LINE
impedance.  You can't just arbitrarily change the reference
impedance unless you also change the line impedance.

IF you placed a 50 ohm line after 1/4 WL of 75 ohm line
with a 35 ohm load, THEN the SWR in the 50 ohm line
would be 160 / 50 = 3.2 as you stated, but the SWR in
the 75 ohm line is still only 2.14

IF you place a 50 ohm line after 1WL of 75 ohm coax
the SWR = 50 / 35 = 1.4 in the 50 ohm line and is still
2.14 in the 75 ohm line.  

Using a REFERENCE impedance that is different from
the LINE impedance is MEANINGLESS.

Tom  N4KG


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