MFJ-269 Instruction Manual HF/VHF/UHF SWR Analyzer
4
1.3
A Quick Word about Accuracy
The following text details several common problems and reasons they occur.
The most likely source of false readings, when measuring antennas, is
unintentional external voltages applied to the antenna port of this unit. An
optional HF filter, MFJ-731, greatly reduces external interference without
modifying impedance or SWR measurements a significant amount.
Measurement errors.
Unreliable readings are rooted in three primary areas:
1.) Signal ingress from external voltage sources, usually strong AM broadcast stations.
2.) Diode detector and A/D converter errors.
3.) The impedance of connectors, connections, and connecting leads.
Broad-band voltage detectors.
Narrowband detectors are expensive,
since narrowband detector systems must have at least one selective gain-
stabilized receiver. Narrowband detectors would price antenna and impedance
analyzers far outside the price range of most hobbyists.
Broadband detectors are sensitive to out-of-band external voltages, and
solutions to most out-of-band interference are not simple. C
ommon low-pass or
band-pass filters behave like transmission lines of varying impedances on different frequencies. Low-pass or
high-pass filters change impedance and SWR readings, just as an additional section of transmission line would.
This modification of impedance caused by filters severely limits their usefulness when used with impedance
measurement devices.
Most RF interference problems occur on lower frequencies, since high power AM broadcast signals and other
external voltage sources couple better into large antennas (especially 160 meter verticals). The MFJ-731 is an
adjustable filter that attenuates all off-frequency signals. It also contains an adjustable notch covering the AM
broadcast band. Properly used on amateur bands between 1.8 and 30 MHz, this adjustable filter reduces external
interference and has almost no effect on system measurements.
Note: A solution often suggested by users is to increase internal generator power. Unfortunately the
power required to operate a low harmonic-distortion broadband VFO system is the single
largest drain on the internal battery. In this unit, more than 70% of the total battery drain (-150
mA) is used to produce the low harmonic-distortion test signal. We have selected the best
compromise between battery life and harmonic-distortion.
Component limitations.
At low voltage, detector diodes become very non-linear. The accuracy of the MFJ-
269 is enhanced by the use of special microwave zero-bias Schottky detectors with matching compensating
diodes. Each unit is individually compensated to provide the best possible detector linearity.
Connection lengths.
Connection lengths both inside and outside the bridge upset readings, especially when
impedance is very high or very low. The MFJ-269 minimizes internal problems by using surface mount low
capacitance microwave components with nearly zero lead length. Remember any external leads you add, even
short leads, modify the impedance of the load at radio frequencies.