See my comments below:
----- Original Message -----
To: <TOWERTALK@CONTESTING.COM>; <W4EF@dellroy.com>
Sent: Thursday, October 11, 2001 8:56 AM
Subject: Re: [TowerTalk] Re: Force12
> 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.
Not if I am using the 75 ohm coax as a transformer in a
50 ohm system. All my meters are calibrated against
Zo = 50 ohms, yet I have a pile of 75 ohm coax behind
my garage. When I used that 75 ohm coax to feed my
tribander, I still used the 50 ohm VSWR meter in the
shack to determine that match to my equipment. In that
case, the length of the line made a big difference (it
was cut for a integer multiple of lambda/2). That was
73 de Mike, W4EF..........
> Tom N4KG
> GET INTERNET ACCESS FROM JUNO!
> Juno offers FREE or PREMIUM Internet access for less!
> Join Juno today! For your FREE software, visit:
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 100 feet for under $1500!! http://www.anwireless.com
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Administrative requests: towertalk-REQUEST@contesting.com