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©2003-2012 W8JI

Revised 8/24/03

Revised May 18, 2005 R84 named in error R89 in harmonic adjustment

Major revision June 24, 2012

History: My amigo JB was the primary MFJ259B designer, and I helped with firmware algorithms and RF and analog hardware.  This information is here because it is the correct way to calibrate the MFJ-259B analyzer. This work is all donated. K1BQT took an instruction set supplied by MFJ, originally developed by JB, and re-wrote it. I reviewed, modified, and edited that work. This page is the most current result. It is best that no one copy this, and start handing it out in mass. The only reason for this request is there must be a point of control of information, so it can be corrected or expanded as modifications, errors, or omissions show up. I am not aware of any other source that gives correct calibration procedures. It is important that the MFJ259B be calibrated by these steps, even if they sound complex. Without following these steps many special functions may not work correctly, even if the unit tests properly on calibration loads!

Operating Tips and Quirks

Please, try to read the manual!

Impedance readings are least accurate
when near 1:1 SWR. When adjusting a normal antenna, lowest possible SWR is
always lowest reactance. There might be exceptions to this, but the would only
occur if antenna or load changes resistance (real part) much faster than
reactance. I doubt this will happen.

If you see 1:1 VSWR, the impedance
has to be 50 j0. Do not waste time trying to make the analyzer read R50 X0 if
SWR says 1.0:1 or some acceptable SWR number. Even a few bits of error, or a
very small stray voltage on the connector, will affect the algorithm that
determines reactance.  The firmware was supposed to contain an algorithm
that weighs the VSWR with priority over Vs and Vz used to determine impedance.
As sometimes happens, there is no assurance everyone stayed on the same page. I
did not write the code, I only suggested changes to minimize error. SWR readings
should be used to smooth R and X readings around 1:1 SWR, but I do not have a
high level of confidence that guideline was followed.  

Operating Defects or Failures

The most common simple failures are dirty band switches, broken
antenna connector pin connections (this is fairly new, caused by a manufacturing
change in the circuit board), and voltage impressed on the antenna connector
from the antenna or load (not just broadcast stations).

Broadcast RFI Test

This is an inexpensive bridge directly coupled to the feedline.
There is no RF or dc isolation from the connector to the bridge. This makes the
bridge sensitive to dc, low frequency ac, and RF voltages on virtually any
frequency from dc to light.

TEST: Broadcast RFI, or even low
frequency AC or DC voltages on the cables will produce errors. The easiest way
to check for these errors is to put the analyzer in Frequency Counter mode and
carefully observe the SWR meter. If the SWR meter deflects at all in the
Frequency Counter mode, the analyzer is being biased from the antenna port by
something.

I do NOT suggest using a low pass or high pass. I suggest using
the MFJ device specifically designed as a bandpass filter. The MFJ device,
properly used, will not seriously affect other readings like low pass or high
pass filters do from filter passband ripples and phase shift.

  Connector Pin Break

At some point after lead free solder was used,  someone
thinned down traces where the connector pin solders to the board. This was an
idiotic mistake. Instead of just using a proper size and temperature iron to
solder the pin with proper training, someone altered the board. While this
allows solder to flow better, the traces are too weak to support the mechanical
stress on the pin. There have been, off and on, attempts to use a jumper wire.
That also was a really bad idea, the wire breaks.

This is the board area they changed. Thinning this trace down to
improve soldering or using a solid wire is a serious mistake.

Good original board as engineered:

Correct pin soldering on good board:

 

Revised defective pin area. Instead of teaching people to solder with correct
tools, they thinned this trace. This ruins the connection life.

 

 

 

 

 

 

 

Dirty Switch

Another common  issue is a dirty band switch. This shows
specifically as a really jumpy frequency, even to the point the frequency
reading goes way out of band or stops. This is a problem with switch grease and
switch manufacturer quality control. The switch needs a little polishing and
wetting of the contacts. Don’t get all hyper about what cleaner to use. WD40
will work fine. Lay the analyzer on it’s back, remove the switch knob, and
spritz just a ting bit of normal WD40  on the shaft, allow it to run down
into the shaft bushing. Run the switch back and forth rapidly. Do not soak the
switch, but use enough to wet the switch internals and soften the internal
grease. 

Note: A dirty switch shows as unstable or major
erratic frequency readings. Minor jumping or drift in low digits is normal.

   

 

How This Type of Device Works

This type of analyzer contains an RF oscillator, a very linear
amplifier to increase power, and an internal resistor bridge in a modified Whetstone bridge configuration.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Since it is designed to be inexpensive and simple, and since the
design is aging now, there are a few
pitfalls with this system. 

The bridge is dc-coupled from an internal resistor bridge to the antenna port.
Each leg of the bridge has a diode detector. This is the weak point for
accuracy.

The bridge detectors are NOT
frequency selective, and respond to anything from minor dc offsets through microwave
signals. This causes inaccuracies if any voltage over a few
millivolts appears
across the antenna port. (This is also true for competing
analyzers from other manufacturers.) There are multiple reasons why, at
the time of design, these units were dc coupled with broadband detectors.
Hopefully someday a higher cost-design with selective detectors will become
available, but for right now this is all that is available for amateur use
from any manufacturer.’

The MFJ259 series RF power level is about 10 dBm, although this
varies with the load impedance. Since the bridge depends on nulls, any external
voltage will throw off readings.

The second shortfall is the internal amplifier must be linear
and have very low total harmonic content. Total harmonic power, at the lowest
load impedance, must be down at least 25dB and preferably 35dB. This is true for
ANY antenna analyzer, since you do not want the analyzer to
measure the load at two frequencies! 

Because the detector is
broadband and because it is dc
coupled to the antenna, any external voltage across the antenna input port causes
measurement errors. It is the accumulated
voltage of multiple sources that is most important, not the strength of any individual signal.
Because of that, large antennas should be tested at times when propagated
signals in the range of the antenna’s response are at minimum strength. 

A definite RFI improvement occurs with a special
parallel-tuned bandpass
filter, but multiple-section bandpass, low pass, or high pass filters cause impedance measurement problems.
Multiple-section filters behave like transmission lines of random line
impedances, loss, and electrical length as frequency is varied. The best solution is to
use a single-stage bandpass filter
and dc isolation on large arrays or with long feed lines. I often use a good
1:1 isolation transformer for measurements, and often find a parallel L/C
filter (like the MFJ-731 Filter) useful.

Where Do the Impedance Bits Come From?

The bridge can be thought of as a simple voltage divider.

 

 

 

Voltage across Vz is R2/(50+R2) * 255 = bits

Voltage across Vs is 50/(R2+50) * 255 = bits

 

 

With 12.5 ohms R2 we have 12.5/50+12.5   *   
255 =  51 bits Vz 

and 50/12.5+50   * 255 = 204 bits Vs

Using this, it is possible to calibrate the 259B with
higher values of load resistance. This may provide better high impedance
accuracy.

This circuit is expanded to a bridge:

 

 

 

 

 

 

    

 Most Likely Failures

Other than manufacturing errors, the detector diodes clearly stand out as the most
common problem. They are the most easily damaged
devices in the analyzer. If you have a sudden problem, it is most likely a
defective detector diode. Diode damage almost always comes from accidentally
applying voltage on the antenna port. 

Why are the diodes so sensitive?

In order for the detectors to be accurate within a
fraction of a percent (one bit), detector diodes must have very low
capacitance and very low threshold voltage. This means the diodes, through
necessity, must be low-power zero-bias Schottky microwave detector diodes. The
same characteristics that make them accurate and linear also cause the diodes to be
especially sensitive to damage from small voltage spikes. ALWAYS
discharge large antennas before connecting them to the analyzer! Never apply
external voltages greater than 3 volts to the antenna port!

Technical Support Errors

Measuring Stub and Fault Distance

I developed the distance to fault and stub length functions.
The theory is frequency spacing between impedance minimums, when converted to
half wavelengths, is the distance to an open or short. This requires the open or
short be a reasonably good open or short, and not an antenna or load. This
system works well, when applied properly. I successfully find opens and cuts in
my trunk cables, some cables are 3000 feet long, within a few feet.   

For a short period of time, with the best of intentions, someone rewrote various manuals.
Unfortunately, they arbitrarily
changed manual instructions for stub length and distance-to-fault measurements.
For a period of time, as a direct result of this error, MFJ support
instructed customers to ignore the older, original, and correct manual. The new
manual, now long out-of-print, advised tuning for the second impedance dip on the next
band-range up or down from the first dip. This is absolutely wrong.

The
original manual was correct. Whatever your particular manual or verbal instructions might
say, this is the only proper stub and/or distance to fault tuning method: 

• You MUST tune for lowest X and/or minimum impedance. Whichever you
  choose to focus on when dipping, stay with that specific observation method
• Store that frequency point 
• Move the analyzer up or down in frequency to the VERY NEXT frequency that
  provides minimum X and/or lowest impedance reading. This may be on the same
  band, or you might have to change to the next band. What is critical is you
  sweep up (or down), and pick the very next dip frequency.  
• When you store that immediately adjacent frequency dip point, the correct
  length will appear. This assumes you set cable velocity properly.

Note: Measurement
errors in stubs and cable lengths will occur if the
harmonic null is not adjusted correctly in the 259B or 269! Setting a test point
to a certain voltage, like 3 volts, is not fully reliable.

Bias Adjustment Errors 

I designed the simple linear amplifier in the MFJ259B. The bias adjustment
was never intended to be set to a fixed voltage at a test point. Some instructions tell users to set
amplifier bias, which minimizes output distortion,
to a certain test amplifier test point voltage. This method can be unreliable, and can cause
stub and DTF (distance to fault) errors.

Proper adjustment should be accomplished by watching distortion, the best
indicator of which are harmonics. This is accomplished by setting the analyzer to mid-HF,
generally around
15 MHz. The analyzer is terminated in a low impedance, which places the highest
load on the RF amplifier. A spectrum analyzer is  bridged across
the lower-than-normal load resistance. Bias is adjusted for minimum harmonic content, consistent with second harmonic
being at least
25 to 30 dB below fundamental. This assures maximum accuracy with narrow band
loads. If you use a receiver for adjustment, be sure the receiver is tuned to
the second harmonic of the MFJ259B, and that the receiver is not being
overloaded by the 10-15 MHz fundamental signal.   

MFJ259B Schematic

How This Unit Works

This is a
rough outline of how this unit works:

The MFJ-259B, and other digitized MFJ antenna analyzers,
compare three major voltages in a 50-ohm bridge circuit. They are:

Vz= Voltage across the load. This is called “Z” in the alignment display menu,
because it is across the load impedance.

Vr= Voltage indicating bridge balance. This voltage
is called “R” in the alignment display menu, for SWR

Vs= Voltage across a series 50-ohm resistor between the RF
source and the load. This voltage is called “S” in the alignment display
menu, for series voltage drop 

All voltages are converted through an eight-bit A-D
converter to a 256-bit digitized output with a test-display range of 0-255
bits. By knowing the ratio of these voltages, as compared
to the AGC regulated RF source voltage, many different load parameters can be
calculated.

An antenna analyzer could calculate everything (except sign of
reactance) from measuring only Vs and Vz, but at certain impedances any small error in
either Vs and Vz becomes critical. This is especially true when voltage is
digitized into a 256-bit format (~0.4% steps). At certain impedances, an almost immeasurable 
voltage change will cause a sudden large jump in the measured impedance
parameters.

When a load is reactive, the theoretical total of Vs and Vz
exceeds 255 bits. Consequentially, if the 259’s total Vz and Vs exceeds 255, the
display indicates reactance. Although any calibration pot can affect readings,
large reactance errors at impedance extremes commonly occur from improper
setting of low-bit adjustments. Low-bit adjustments compensate diode linearity
at low voltages.  

To reduce display impedance jumping, SWR is weighed into the
calculation of reactance and resistance at low SWR values. (An SWR
bridge is most accurate when the load is closest to 50 ohms, which is a
primary measurement area where  impedance measurements through Vz and Vs
become critical.) By factoring in a direct SWR measurement from an internal
bridge, the analyzer can
check and “correct” any small level errors in Vs or Vz. This reduces the
impedance jump that would occur with a one-bit jump in voltage. This also why bits
must be calibrated for near-perfect accuracy. A one-bit error can cause a
resistive load to appear reactive (total of Vs and Vz must always be below 255
bits for a load to be considered resistive).

Calibrating the MFJ-259B Antenna Analyzer

This
calibration procedure is the correct procedure for later MFJ-259B’s. Take
any other information with a grain of salt. Since MFJ-259B firmware has several versions under the same
model number, you may find some final performance or function verification steps
invalid. These steps will involve parameters that do not appear
on the display.

Before proceeding, be sure you have printed a copy of the board layout
showing adjustment points, have read all this, and have suitable loads.  

Adjustments

This unit has tracking and gain adjustments for Vz, Vs,
and Vr. Detector system gain is set at high
detector voltages or high-bits, by R53 (extreme SWR), R72 (Vz high load
voltage bits), and R73 (Vs low load impedance, high-Vs series bits).
Linearity is set at low voltages, by R90 (low load impedance), R88 (high
load impedances), and R89 (low VSWR readings). Together, the
low-bit and high-bit adjustments compensate diode linearity, making detector
system output voltages closely track actual RF voltages appearing across
bridge resistors. 

Control	Detector	Primary Load Calibration Function	determines
R73	Vs high bits series load current	low load impedances, detector gain, high S bits	R and X low Z load
R90	Vz low bit voltage across load	low load impedances, detector linearity, low Z bits	R and X Low Z load
R72	Vz high bits voltage across load	high load impedances, detector gain, high Z bits	R and X High Z load
R88	Vs low bits series load current	high load impedances, detector linearity,  low S bits	R and X High Z load
R53	Vr high SWR bits	high SWR readings, detector gain, high SWR bits	high SWR readings
R89	Vr low SWR bits	low SWR readings and low reactances, SWR detector linearity	low SWR readings

This unit also has meter calibration adjustments. The
analog meters suffer from some scale-linearity problems, so they
will be somewhat less accurate than the digital display in a perfectly
calibrated unit. The metering adjustments, R56 (SWR) and R67 (Impedance), only affect
analog meter readings. These meter adjustments
do not affect the digital display, but digital detector adjustments will
affect analog impedance meter readings.

Quiescent current (bias) in the RF amplifier section is
adjustable. This adjustment directly affects output signal harmonic content.
Harmonics are worse with low supply voltages, and with low impedance loads. Be sure
you check the harmonics as outlined below, with a 1/4 wl open-circuit stub!!

Excessive harmonics can cause severe errors in measurement of
frequency-selective loads, even when dummy-load SWR tests appear perfect. Loads most sensitive to harmonic-induced errors
include, but are not limited to, antenna tuners, tank
circuits, very short resonant antennas, and distance to fault and stub length
measurements. If you notice something “funny” going on with a stub
measurement, it may be a fault of incorrect bias. 

Warning: Never calibrate around a sudden “problem” that appears. If a detector suddenly shifts voltage, the problem is almost certainly a defective detector diode. If the meter is recalibrated with a defective (leaky) diode, the meter will probably NOT track correctly with frequency.

Alignment

Tools and
Equipment:

#2 and
#1 Phillips-head screwdrivers

Digital
meter or accurate analog meter for
checking supply voltage

Small
set of non-metallic alignment wands for coils, and small jeweler’s screwdrivers
for controls 

Well-filtered stable power supply, adjustable to 12-volts,
or as specified

General-coverage receiver with level meter, or a spectrum
analyzer

For stub testing and adjustments, a ~10 MHz
1/4wl open-stub. 15’ of good-quality
solid-dielectric RG-8, with a UHF connector at on end, open on the other end,
will work.

2.2-ohm 1/4 or 1/2 watt film resistor  

Accurate
load set
to include:

A.    
Short

B.    
12.5-W
load

C.   
50-W
load

D.   
75-W
load

E.    
100-W
load

F.    
200-W
load

Quick-connect type-N or BNC loads can be made with surface mount 
resistors on a BNC male chassis mount connector, with the bayonet removed. This makes a “quick connect” connector that will slide directly into
a type-N female, or a BNC female.  In this
case, use a good UHF to BNC female adaptor, or UHF to female 50-ohm N, for the
MFJ259 units. With a 269, the
load will plug directly into the unit’s N-female. 

Note 2: The power source should be the LOWEST expected operating voltage. DO NOT use a standard “wall-wart” or batteries! You can reduce voltage from a conventional 13.8v regulated supply by adding a few series diodes. Silicon diodes will normally drop about 0.6volts or so per diode. Three or four series diodes will reduce voltage below 12 volts.

 

WARNING: The MFJ-1315 AC adapter or other “wall-warts” should NOT be used to power the unit for most alignment steps. It is best to calibrate at typical lower expected battery voltage.

Step 1, look at things carefully.

Visual
Inspection: Before, during, and after calibration, be mindful of physical condition.
Watch for missing or loose hardware. Do not tug, stress, or repeatedly
flex leads, or carelessly
flop or toss things about. Unlike my bench, keep your workbench clean. Follow these rules the entire time
you have the unit apart!

Step 2, prepare the unit.

Battery
Tray Removal: This step provides access to trim-pots and most inductor
adjustments.

[  ] Remove
last two batteries at each end of the tray.

[  ] Remove
two battery holder screws (right side) and extract the tray.

[  ] Always
position the battery tray to minimize strain on wires. 

[  ] Do not
reinstall batteries. If the holder or leads get shorted, you can melt things.

Refer to the
board layout below for specific adjustment
locations.

 

 

 

R90 Load Z low-bit

 

 

 

 

R89 swR low bits

R73 Series high bits

R88 Series low bits

 

R72 Load Z high bits

 

 

R53 swR high bits

 

R84 Amplifier bias null harmonic

 

R67 SWR meter

R56 Impedance meter

 

 

 

 

 

 

 

L1 Lowest range

L6 Highest range

 

 

 

 

 

 

 

 

Figure 1

 

Step 3, verify VFO range

Band Overlapping: Each
band should overlap the next by a small amount to ensure gap-free coverage
from 1.8 MHz to 170 MHz. While viewing the LCD
Frequency Display, wiggle the bandswitch from side-to-side gently. Watch
for any display or meter dropout. Starting from the highest frequency band, check each band as follows:

114-170
MHz: L6 oscillator squeeze-spread tunes from below 114.0 MHz to above 170.0 MHz.
Check tune for dead spots.

70-114
MHz: L5 oscillator squeeze-spread tunes from below 70.0 MHz to above 114.0 MHz

27-70
MHz: slug oscillator tunes from below 27.0 MHz to above 70.0 MHz.

10-27
MHz: slug oscillator tunes from below 10.0 MHz to above 27.0 MHz.

4-10
MHz: slug oscillator tunes from below 4.0 MHz to above 10.0 MHz.

1.8-4
MHz: slug oscillator tunes from below 1.8 MHz to above 4.0 MHz. Check tune for
dead spots.

While verifying band overlap, check the lowest and
highest bands carefully for dead spots. The LCD
Display will indicate 000.000MHz if a dead spot occurs. Dead spots
generally indicate a defective tuning capacitor (TUNE). 

If wiggling bandswitch causes a dropout, the switch
may have dry or dirty contacts. Less likely are poor solder joints, but check solder joints first.
If you must clean and lubricate the switch, be aware it is a difficult task. The entire
board needs to be lifted from the case front. Dirty band-switch contacts may be restored with
spray tuner-cleaners, or WD-40. The best place to spray the switch is from
the front side (shaft side), right below the nut. You must remove the switch
indexing tab retainer nut and the
metal switch retainer (stop)
under the nut. Be sure the stop goes back exactly as removed.

To correct overlap problems, locate and retune the appropriate VFO
coil (see pictorial for coil locations). Note that L1-L4 are slug-tuned and
require an insulated hex-head tuning wand. Using the wrong size or
worn tuning tool may stress and crack a tuning slug. 

Inductors L5 and L6 are located on the
component side of the board and are compression-tuned (press turns closer
together to lower frequency or spread apart to raise frequency). Make only
very small corrections–especially to L5
or L6–and recheck the band you are adjusting. You should also check the next
lower band after each adjustment to ensure that the lower band hasn’t
moved excessively.

Important Warning: VFO coils MUST be aligned from highest frequency band to the lowest frequency band. All higher ranges affect lower bands, with the adjacent higher band having the largest effect.  Do not attempt VFO coil adjustment unless you are experienced working with VHF-LC circuitry or analog tuned circuit alignment procedures.

 

Step 4, set RF bias levels

Harmonic Suppression/ generator bias level: Connect the analyzer exactly as
shown below. 

•  The impedance of the cable to the measurement device should match
  the impedance of the measurement device.
•  The “T” must be connected either directly to or placed
  within a few inches of the analyzer.
•  The power source must be the lowest expected operating voltage.
•  The measurement device must be well-shielded, and not pick up any
  substantial signal from the analyzer when the “T” is
  disconnected from the analyzer.

 

 

 

 

 

 

 

 

 

 

Step 5 

Generator Bias Level
(R84):
This adjustment determines amplifier bias level, and thus determines harmonic
content and battery life. Excessive harmonics will cause incorrect
readings under many common load conditions, especially stub tuning. We always
want maximum possible battery life, consistent with adequate harmonic
suppression.  

WARNING: 
Incorrect
adjustment of R84  will NOT show with resistive dummy loads!!! The unit will appear to calibrate correctly, but will
produce errors in stub length, distance-to-fault, and other frequency-selective
or resonance functions.

When R84 is set
properly, harmonic suppression of –30dBc or more should be possible across
most of the analyzer’s tuning range.

This particular adjustment
should be made
at the lowest expected operating voltage. Proper alignment requires a 12.0-volt
regulated supply as a power source. NEVER use an AC adapter, or any supply
voltage higher than 12-volts, when making this adjustment. 

A calibrated spectrum analyzer works best for monitoring harmonic
output, but a well-shielded general-coverage receiver with signal-level meter
will also work. The receiver MUST
be “T’d” into the analyzer just as the spectrum analyzer is, the “T”
and resistor must be located right at the analyzer ANT connector. If
you do not have a good-quality receiver or spectrum  analyzer, you
probably should not make this adjustment. If you insist on adjusting bias
without a receiver or analyzer, you can connect a 1/4 wave open stub, tune to
the null in Vz, and watch test-mode Vz while adjusting R89. Vz will roughly indicate
total even harmonic voltage, when the analyzer is set at the stub’s exact resonant
frequency. Entering the test mode is described in Detector Calibration (Step
6).     

[  ] a. Install
either a 15’ RG-8 open stub, or a low impedance load resistor and
spectrum measurement device, and tune the analyzer to approximately 10-15 MHz or exactly to stub resonance. 

[  ] b.
(stub and internal Vz use only) Observing Vz on the data
display (analyzer test mode), adjust frequency until the lowest fundamental
output reading (or lowest impedance) is obtained. You should clearly see the
MFJ analyzer’s fundamental frequency
output voltage (Vz) go through a deep null. 

[  ] c. Observe the analyzer frequency
reading. This is the approximate resonant frequency of the stub. 

[  ] d. Without
changing the analyzer test frequency setting, observe the second
harmonic level. This harmonic will be at twice the MFJ analyzer frequency counter reading.
Alternatively, you can watch Vz on the test mode display.

[  ] e. Adjust R84
for lowest 2nd harmonic meter reading on the receiver, lowest Vz test-mode reading,
or lowest harmonic levels on the spectrum analyzer. Be SURE the fundamental
frequency level remains nulled in the stub, if a stub is used. 

WARNING:  Always repeat steps
(b) through (e) at least one extra time when relying on display Vz. The original null point of
any stub will shift if there is a substantial reduction in harmonics after R84
is adjusted. The original stub frequency, as observed at (c), will probably change slightly. It is NOT necessary to recheck
when doing a resistor load test with a good-quality spectrum analyzer
or receiver. With a resistor, exact test frequency is NOT critical.

NOTE: If you have a poorly performing spectrum
analyzer, or if you have a receiver with limited dynamic range, use a 1/4 wave stub with the spectrum
analyzer or receiver instead of a 2.2 ohm resistor. In the case of the stub,
always be sure the 259B is on the stub’s resonant frequency. If you have a reasonable
quality spectrum analyzer or receiver (at least 50dB dynamic range) use a 2.2-ohm
non-inductive resistor in lieu of the stub. Resistor loaded adjustment is easier and
much more accurate, so it is preferred.

Detector
Calibration

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Step 6: 

This critical sequence calibrates A-D conversion for various load conditions. If you
know your unit has been tampered with, preset trim pots R88,
R89, and R90 to their center positions before continuing. If any
control bottoms-out during adjustment procedures, you either installed an
incorrect load for the control adjustment or the analyzer has a defective detector diode.  

To prepare for detector tracking alignment, place the
analyzer in Test Mode. Entering test
mode may be tricky with some units, and it may take practice. To enter Test
Mode:

[  ] Turn
power off.

[  ] Hold
down MODE and GATE  buttons while restoring power.

[  ] As
display comes up, slowly (about 1 second period) rock between applying
finger-pressure on the MODE
and GATE switches. The
best method is to use two fingers, rocking your hand from side-to-side
to alternate your fingers between the two buttons.

[  ] Confirm
analyzer has entered test mode (it may take more than one try).

[  ] Using
the MODE button, advance display to
the R-S-Z screen (shown below).

Note: If you go past the R-S-Z screen, you can still
see R-S-Z by pushing and holding the MODE button.

 

10.000
MHz

                                               
Rxxx      Sxxx   
  Zxxx

 

For initial adjustments, if the unit has never been aligned, start here.
Otherwise, skip down to the next break.

 
 


[  ] Tune
analyzer operating frequency to approximately 10-15
MHz. This is not critical.

[  ] Leave antenna
connector Open

[  ] Set R72
for Z=255

[  ] Set R88
for S=000, if possible

[  ] Install
the Short

[  ] Set R90
for Z=000, if possible

[  ] Set R73
for S=255

[  ] Set R53
for R=255

The list below is the start for any second or third
run-through points, or calibration touch ups. You have now set initial rough
settings for all three detectors, proceeding to impedance calibration loads

[  ] Install 12.5-W
load

[  ] Set R90
for Z=051

[  ] Set R73
for S=204

[  ] Set R53
for R=153 (for 4:1 digital SWR)

Change Load to continue impedance calibration

[  ] Install 200-W
load

[  ] Set R88
for S=051

[  ] Set R72
for Z=204

Change Load to continue impedance calibration

[  ] Install 12.5-W
load

[  ] Reset R90
for Z=051

[  ] Reset R73
for S=204

[  ] Reset R53
for R=153 (4:1 digital SWR)

Change Load to continue impedance calibration

[  ] Install 200-W
load

[  ] Verify
or reset R88
for S=051

[  ] Verify
or set R72 for Z=204

[  ] 
Verify or set R53 for
near R=153 (4:1 digital SWR). This reading should be compromised with
the 12.5 ohm load.

Change Loads to calibrate SWR

[  ] Install 75-W
load

[  ] Set R89
for R=051 (digital 1.5:1 SWR)

[  ] Set R56
for SWR Meter 1.5:1

Change Loads to calibrate impedance meter

[  ] Install 50
ohm load

[  ] Set
R67
for an Impedance Meter reading of 50-ohms

You have now set impedance tracking at 12.5 and 200
ohms, digital SWR tracking between 1.5:1 and 4:1 SWR, and set the SWR analog
meter for 1.5:1 SWR point. There is not any analog SWR meter tracking
adjustment, so you may want to compromise R56 with several SWR test loads. R56
will not affect anything except the analog SWR meter reading.

After verifying calibration with all
loads, carefully reassemble your antenna analyzer.

Important Notes:

1.) Small single-turn trim pots can be “touchy” to adjust,
and tracking settings are somewhat interactive. If specified readings aren’t
fully obtained on the initial run-through, repeat the sequence carefully a second time. When the sequence is complete, turn power off.
This will remove the analyzer
from Test Mode.

2.) Be particularly mindful of the total bits of Vz and Vs. If the sum of
these bits ever exceeds 255 with a resistive load, the analyzer will indicate
reactance.

3.) The analog SWR meter and the analog impedance meter do not have
linearity adjustments. They have to be compromised for your unit’s particular
meters and the scale area you wish to be most accurate.  

Periodic Verification 

Periodically check your analyzer with test load!

Loads Using Standard-Value Resistors

 

 

 

 

 

Install resistors all the way down in the connector, the goal is zero
lead length

Use precision 1% carbon or metal film 1/8th-1/4 watt resistors

 
 

12.5W
= (4) 50-ohm or a single 15W
and 82W
1% in parallel

                                   
50W
= 49.9-ohm  or 100W
and 100W
in parallel

                                   
75W
= 75-ohm or 150W
and 150W
in parallel

                                   
100W
= 100W

                                   
200W
= 200-ohm or 100W
+ 100W
in series

I use male BNC connectors with the locking sleeve removed,
with surface mount resistors. These connectors will plug into type-N 50-ohm
connectors as “quick connect” connectors:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 Important Note: Many simple HF loads,
inside PL259 connectors, will not be accurate above

30 MHz. Only precision terminations should be used in the VHF
region.

Even then, there can be some errors
from connector and trace lengths inside the analyzer.

The MFJ-259B does not correct for connector impedance bumps, or correct for the electrical
length between an external load and the detectors inside the unit.

 

 

©2003-2012 W8JI