User Manual

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32
APPENDIX B
The optimal cable length is obtained when the sidebands of a 50Ω antenna have a high resistance,
and low reactance. There are several ways of measuring this, but one popular way is a smith chart.
In Figure 20 a smith chart is shown of a pg45 3-loop antenna with the proper cable length. The green
circle shows the resistance and reactance of the antenna for frequencies from 13.06Mhz to
14.06Mhz. Marker 1 is set on 13.56Mhz, and it can be see that the resistance is ~50Ω and reactance
~j0Ω. Marker 2 & 3 are on the 13.1Mhz & 13.9Mhz sideband, and there the resistance is >180Ω and
the reactance approaches ~j0Ω.
Figure 20 - smith chart of pg45, 3-loop
Figure 21 shows the smith chart for a pg45 8-loop with improper cable length. The 13.1Mhz &
13.9Mhz sidebands are now ~1Ω and their reactances are not approaching j0Ω. Adding coaxial cable
to this antenna will make the circle rotate clock-wise around marker 1. In appendix a we gave a cable
length of about 3m to rotate the circle to correct position.
Figure 21 - PG45 8-loop with improper cable length
When a vector impedance analyzer like the: