Marco,
I believe this scheme is known as the "Asynchronous Transformer" 
asynchronous because it can be inserted at any point along the
75 ohm transmission line. I just did some modeling of this scheme
with a Smith chart program that I have here. It does appear to
work, but it is a bit of a compromise. For a perfectly matched
50 ohm antenna (1.0:1 VSWR), the AT will give a worst case
VSWR of around 1.5:1 in the shack as compared to a worst
case VSWR of 2.25:1 using no special matching. For an antenna
with a VSWR of 2.0:1 at the feedpoint, the AT will give a worst
case VSWR of around 3.0:1 in the shack, whereas the worst
case with no AT would be 4.5:1.
The 1/12 wavelength transmission line lengths in the AT's
provide 30 degrees of rotation on the smith chart. If your random
length 75 ohm transmission line happens to be close to an odd
multiple of 90 degrees (1/4 wavelength)  the worst case for
the length of the 75 ohm line, then the two series 30 degree lines
will tend to rotate you away from this worst case point and pull
the impedance loci back toward the center of the Smith chart.
Its not perfect of course as you can see from the above
examples, it will reduce you VSWR bandwidth. A better
solution would be to use 1.5:1 broadband "Unun" transformers
in the shack and at the antenna. This scheme would have no
reduction in VSWR bandwidth or degradation of feepoint
VSWR. Of course if you have an antenna with a very wide
VSWR bandwidth, the AT's can be a quick way of getting an
"okay" match in shack when using 75 ohm line in a 50 ohm
system.
73 de Mike, W4EF...............
 Original Message 
From: "Marco Spalluzzi" <marco.spalluzzi@orobiacom.it>
To: <cqcontest@contesting.com>
Sent: Thursday, February 21, 2002 11:42 PM
Subject: [CQContest] Strange cable lengths
>
> Hi guys, I've got a question for you: a friend of mine said is possible
feed
> an antenna using low loss 75 ohm cable, through a system like this:
> (antenna)(any length of 75 ohm coax)(piece of length L of 50 ohm
> coax)(piece of lenght L of 75 ohm coax)(rtx or amp).
> The lenght L is, band by band, as follows, assuming vf 0.66 (rg213, rg11):
>
> Band Length
> 80 14' 8''
> 40 7' 5.5''
> 20 3' 9''
> 15 30''
> 10 22''
>
> Why this lenghts? For example, 3' 9'' on 20 are approx. 1/12 wavelength.
>
> Bye, Marco  IK2XSL
> ik2xsl@amsat.org
>
>
> 
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>
>
