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Author Topic: Adjusting shunt coil w/MFJ-259  (Read 3661 times)
WV4L
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« on: September 09, 2014, 05:56:55 PM »

Greetings!
I need to adjust the shunt coil on my screwdriver antenna. I borrowed a MFJ-259 analyzer. Alan K0BG, site gives instructions on making the adjustment using an analyzer that has X and R readouts. The 259 does not have those. I do not have any local that has one of these analyzers. Is there a work around procedure where I can use the 259 to adjust the shunt?

thanks

Wayne C
WV4L
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KH6AQ
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Posts: 7982




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« Reply #1 on: September 09, 2014, 06:39:11 PM »

A VSWR measurement is all that is needed; VSWR does not change along the length of a (lossless) line. Measuring R + X is meaningless unless you know the electrical length of the line (or it is an integer multiple of 180 degrees) and you calculate the impedance at the antenna bsed on the impedance measured at the impedance meter.

For an antenna such as a screwdriver, minimum VSWR does occur at resonance and this can be seen when the impedance meter is right at the antenna feedpoint. But measuring R + X with a length of coax between the antenna and the impedance meter can fool folks into thinking this is not the case.  

To adjust the shunt coil tune the screwdriver for minimum VSWR. If the VSWR is not as low as you'd like, decrease the shunt coil inductance a small amount. Tune the screwdriver again and if the VSWR is lower than before you're on the right track. If the VSWR is higher than before increase the shunt coil inductance and repeat.  

« Last Edit: September 09, 2014, 06:41:20 PM by WX7G » Logged
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