[TowerTalk] Re: Force12
n4kg@juno.com
n4kg@juno.com
Thu, 11 Oct 2001 12:28:22 -0600
N4KG additional comments placed at the end.
On Thu, 11 Oct 2001 "Mike" <W4EF@dellroy.com> writes:
> Hey Tom,
>
> See my comments below:
>
> ----- Original Message -----
> From: <n4kg@juno.com>
> 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
> my point.
>
> 73 de Mike, W4EF..........
> >
> > Tom N4KG
> >
Mike,
The SWR *in the 75 ohm cable* is still only 2.14 to 1
The SWR *in a 50 ohm cable*from the end of the 75 ohm cable
to the transmitter depends of course on the load impedance
presented to the 50 ohm cable which of course depends on
the transposed impedance (and therefore length) of the 75 ohm line.
I think most readers on towertalk know to use multiples of
1/2 WL of 75 ohm CATV line for their 50 ohm systems
so that the impedance presented to the 50 ohm line is the
same as the load impedance. That topic has been
discussed many times before.
Even if you use an odd number of Quarter wavelengths of 75 ohm
cable which transforms the 35 ohm load to 160 ohms, the SWR
in the 75 ohm cable remains at 2.14. The 3:1 SWR you mention
only exists in the (short) 50 ohm cable from the end of the 75 ohm
line to the transmitter. YES, you may want to match that back to
50 ohms for narrow range amplifiers with limited tuning range.
FWIW, when I was at Collins Radio in the late 60's, I was told the
rule of thumb for antenna system design was to try to keep the
SWR on 50 ohm cables below 3 to 1. Anything above was matched
at the antenna. Anything below was matched at the transmitter.
Tom N4KG
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