Topband: Handheld Impedance Analyzer
jim at audiosystemsgroup.com
Mon Mar 28 12:14:04 EDT 2016
On Mon,3/28/2016 4:37 AM, KE1F Lou wrote:
> Is the lowest SWR is the resonant frequency?
> Resonant frequency is when Xl=Xc.
> When Xl=Xc is not necessarily the lowest SWR.
Exactly right. If, for example, the antenna's feedpoint impedance at
resonance is different from the feedline impedance, or if the analyzer
impedance does not match the feedline impedance, the lowest MEASURED SWR
will be different from the resonant frequency. This happens with lots of
antennas, including most dipoles, and with most ground-mounted
verticals. For example, this causes the minimum SWRof my 80 dipole,as
measured in the shack by a 50 ohm analyzer, to be about 100 kHz below
its actual resonant frequency.
This is why swept impedance measurements made by an instrument that can
produce a file that can be transformed by "subtracting out the feedline"
is so important.
The feedpoint Z of my high dipoles for 80 and 40 is in the range of 85
ohms, so I feed them with RG11. This minimizes the ACTUAL SWR in the
feedline (because SWR is determined by the LOAD, NOT the match to the
transmitter), so it minimizes feedline loss. It does, of course, require
matching to the rig or power amp in the shack so that the output stage
is happy, and to minimize distortion in the output stage.
73, Jim K9YC
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