Antenna hints:
Display readings (of the analyzer display)* are the SWR, impedance and resonant frequency of the antenna system at the point in the
system the MFJ-259B is connected. The impedance and resonant frequency (frequency where reactance crosses
zero) at the point where this unit is connected might not be the resonant frequency of the antenna itself.
This unit (or any other impedance measuring device) displays the antenna’s impedance, 50 ohm SWR, and
resonant frequency as modified by transmission line “transformer” actions of the feedline and other components
between the antenna and the MFJ-259B. If the line is 50 ohms, this unit will always display the antenna’s true
SWR, with the exception of a slight reduction in SWR with longer or more lossy feedlines.
1.) RESONANT FREQUENCY is where reactance is zero ohms, or in some cases as close to zero ohms as the
MFJ-259B indicates. Since resistance has nothing to do with resonance, the resonant frequency is NOT
always at the point of lowest indicated SWR (although they certainly can be the same). The most desirable
load is almost always the load with lowest SWR, even though it may not necessarily be the point of no
reactance (resonance).
2.) An IMPEDANCE of 50 ohms can be composed both resistive and reactive components. If the impedance is
50 ohms, but the SWR is not 1.0 to 1, the likely cause is reactance makes up part or all of the impedance.
Contrary to popular (but very incorrect) misconceptions, it is impossible to obtain a perfect 1 : 1 SWR when
the load is reactive, even if the complex impedance is 50 ohms.
A good example is a 50 ohm nearly pure reactance load. The MFJ-259B LCD will indicate R=0 X=50, while the
impedance meter reads 50 ohms. The SWR would overflow (SWR>25), since the reactive 50 ohm impedance
load absorbs almost no power from the source and has a nearly infinite SWR.
3.) Even if a perfect transmission line is cut to an exact electrical half-wave (or a multiple thereof ) it is a true
half-wave multiple only on one frequency in that band. On a slightly different frequency the line will not
represent the true feedpoint impedance of the antenna. The line is only “impedance transparent” when
lossless and when an exact multiple of 1/2 wl. The longer the transmission line in wavelengths, the “more
length critical” it becomes and the less accurate measurements become.
4.) If the feedline 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 non-quarter wave multiple feedline adds reactance that can
cancel antenna reactance at frequencies where the antenna is not resonant.
Multiple antenna and feedline combination resonances commonly occur with dipoles, where reactance
crosses zero (indicating resonance) at some frequency other than the antenna’s actual resonant frequency.
This is a normal effect.
5.) If the line is a 50 ohm line, has no radiation or parallel currents, and if the line has minimal loss, moving the
analyzer to another point on the line will NOT change SWR reading. Impedance and resonant frequency
might change from line transformation effects, but the SWR will not change.
6.) If SWR changes with coaxial line length, line placement, or line grounding (any distance away from the
antenna) changes, the feedline has one or more of the following shortfalls:
a.) The feedline is carrying common mode current and radiating.
b.) The feedline is not a 50 ohm line.
c.) The feedline has high loss.
A common misconception is that changing a feedlines length will change SWR. If the
impedance of a feedline is 50 ohms and the load impedance is 25 ohms the SWR will remain
2:1 as the feedline length changes. If line loss is low it is perfectly acceptable to make SWR
measurements at the transmitter end of the feedline. The feedline does not have to be any
particular length. However, as line loss increases, and as SWR increases, more error is
introduced into the SWR reading. The error causes the measured SWR reading to appear
better than the actual SWR at the antenna.
