MFJ-269 Instruction Manual HF/VHF/UHF SWR Analyzer
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the antenna and the MFJ-269. If the line is 50 ohms (or the impedance selected in
advanced mode 3), this unit will always display the feedline’s true SWR, with the
exception of a reduction in SWR present in feedlines having appreciable loss.
1.)
RESONANT FREQUENCY
is where reactance is zero ohms, or in some cases as close to
zero ohms as the MFJ-269 indicates. Lowest SWR is often
not
at the point of lowest
reactance, or resonance. That’s because the resistance may be wrong where reactance is zero
(resonant). The most desirable load is almost always the lowest SWR, even though it may not
be resonant.
2.) An
IMPEDANCE
of 50 ohms can be composed of both resistance and reactance. If the
impedance is 50 ohms (or whatever the meter measures), but the SWR is not 1.0 to 1,
reactance is probably making up part or all of the impedance. Contrary to popular
misconception, it is impossible to obtain a 1:1 SWR when the load is reactive. This is true
even if the complex impedance is exactly 50 ohms.
A good example is a 50 ohm load with almost pure reactance and almost zero resistance. The
MFJ-269 LCD will indicate R=0 X=50 while the impedance meter reads 50 ohms or the Z
display indicates a 50 ohms impedance. The SWR would overflow (SWR>25) because the
nearly-pure 50 ohm reactance and impedance load absorbs almost no power from the source.
It has a nearly infinite SWR, despite having an impedance of 50 ohms.
On the other hand if resistance is near 50 ohms and reactance near zero, the impedance
would remain 50 ohms. SWR would be 1:1 in this case, since a dissipative resistance readily
accepts power from the source.
3.)
Electrical Half-wave lines
only “repeat” the far-end impedance over a narrow frequency
range. The line is only “impedance transparent” when lossless and an exact electrical
multiple of 1/2 wavelength. On other frequencies, the line will not repeat the true feedpoint
impedance of the antenna. The longer the transmission line is when measured in
wavelengths, the “more length and frequency critical” it becomes. A longer line has larger
errors in repeating load impedance when operated slightly off-frequency, and also has
additional errors due to line loss.
4.)
Resonance
at the feedpoint only repeats when a mismatched feedline is an exact multiple of
1/4 wl. If the line is not an exact multiple of 1/4 wl, the resonant frequency of the antenna
might be shifted higher or lower by the transmission line. A mismatched line that is not an
exact multiple of a quarter-wavelength adds reactance that can either cancel antenna
reactance at frequencies where the antenna is not resonant, or add reactance at frequencies
where the antenna is resonant.
Multiple antenna-and-feedline-combination resonances commonly occur with antennas,
where reactance crosses zero (indicating system resonance) at frequencies other than the
antenna’s actual resonant frequency. This is a normal effect.
5.)
Line length does not change SWR
if the line is a 50 ohm line (or matches the Zo of the
instrument), has no radiation or parallel currents, and if the line has minimal loss. If the line
is not perfectly matched, impedance and resonant frequency normally change from line
transformation effects but the true SWR will not change.