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Re: phase shift with coax



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
fine, so I know it works. The other nice "feature" about this matching
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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