# Re: phase shift with coax

• Subject: [amsat-bb] Re: phase shift with coax
• From: "MediaOne" <Leescott@xxxxxxxxxxxx>
• Date: Wed, 11 Jul 2001 11:03:26 -0400

```Domenicl (i8cvs) wrote:
> If the impedance of each antenna is 50 ohm and if i use two 50 ohm
> feedlines of any lenght but one 1/4 wavelenght longer than the other
> i get only the phase shift for circular polarization but not the
> impedance matching.

You almost have it right. What you need to do is use one line that is 1/8
wavelength long connected to one antenna, and the other line is 3/8
wavelength long connected to the other antenna. The difference in length,
1/4 wavelength, gives the 90 degree delay you need. Both lines need to be
RG-62, which is a coax with Zo=93 ohms. Now parallel both free ends of the
two coax cables for a direct connection to a 50 ohm coax.

For a Zantenna = 50 ohms and a 1/8 wave lenght of Zo = 93 ohms coax the
impedance at the open end will be Zinput = 77.576 + j * 51.292 ohms.

For a Zantenna = 50 ohms and a 3/8 wavelength of Zo = 93 ohms coax the
impedance at the open end will be Zinput = 77.576 - j * 51.292 ohms.

When both cables are paralleled the impedance is Zparallel = 55.745 + j *
0.0 ohms. The SWR calculates out to be 1.115 to 1. Not a bad match!

If the cable lengths prove to be too short you can try short-cable = (1/8 +
N * 1/2) wavelengths long. The other one would be long-cable = (3/8 + N *
1/2) wavelengths long. The factor "N" must an integer and most be the same
for both formulas above. As you know adding a 1/2 wavelength of line does
not change the impedance so you can use the above idea to get the line
lengths you need so you can physically connect everything up.

I have used this method to build a 2m turnstile antenna which matches just
method is the antenna impedance can be anywhere from about 35 ohms to about
265 ohms, while the SWR will never be above 1.5 to 1 on a 50 ohm system. The
above method is also documented at http://www.cebik.com/turns.html . The
ideal antenna impedance for a perfect 1 to 1 SWR match is 63 ohms by the
way.

Leland C. Scott
KC8LDO

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