MFJ MFJ-266 User manual

INSTRUCTION MANUAL
CAUTION: Read All Instructions Before Operating Equipment
MFJ ENTERPRISES, INC.
300 Industrial Park Road
Starkville, MS 39759 USA
Tel: [phone] Fax: [phone]
COPYRIGHT 2011 MFJ ENTERPRISES, INC.
C
Model MFJ-266
VERSION 1A

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. ii
DISCLAIMER
Information in this manual is designed for user purposes only and is not intended to supersede information contained
in customer regulations, technical manuals/documents, positional handbooks, or other official publications. The copy
of this manual provided to the customer will not be updated to reflect current data.
Customers using this manual should report errors or omissions, recommendations for improvements, or other
comments to MFJ Enterprises, 300 Industrial Park Road, Starkville, MS 39759. Phone: [phone]; FAX: (662)
323-6551. Business hours: M-F 8-4:30 CST.

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. iii
Contents
1.0 Introduction…………………………………………………………………………………...1
2.0 Power Sources…………………………………………………………………………………2
2.1 Internal Batteries……………………………………………………………………..2
2.2 External Power Supply………………………………………………………………..2
3.0 Operating Mode………………………………………………………………………………3
3.1 Display Backlight……………………………………………………………………..3
3.2 Main Menu Screen……………………………………………………………………3
3.3 Frequency Counter Mode…………………………………………………………….4
3.4 Antenna Analyzer Mode……………………………………………………………...4
3.5 RF Signal Source……………………………………………………………………...5
3.6 L/C Measurement Mode……………………………………………………………...5
3.0 Frequency Selection………………………………………………………………………….. 6
4.1 Variable Tuning………………………………………………………………………6
4.2 Range Selection………………………………………………………………………. 6
4.3 HF Band Selection…………………………………………………………………….6
5.0 Accuracy Limits……………………………………………………………………………….7
5.1 SWR Measurements and Local Interference………………………………………..7
5.2 Checking for Local Interference……………………………………………………..7
5.3 Detector Linearity and Accuracy…………………………………………………….7
5.4 Calibration-Plane Error………………………………………………………………8
5.5 Sign Ambiguity………………………………………………………………………...8
6.0 Antenna Measurements……………………………………………………………………….8
6.1 Antenna Connectors…………………………………………………………………..9
6.2 SWR……………………………………………………………………………………9
6.3 Measuring SWR ………………………………………………………………………9
6.4 SWR, Bandwidth, and Resonance……………………………………………………9
6.5 Antenna Tuning……………………………………………………………………….10
6.6 Antenna Matching…………………………………………………………………….10
6.7 Matching Antennas Through a Tuner (ATU)……………………………………….10
6.8 Antenna Impedance Readings………………………………………………………..11
6.9 Unpredictable SWR………………………………………………………………….. 11
7.0 Using Advanced Functions……………………………………………………………………11
7.1 Frequency Measurement……………………………………………………………...12
7.2 Field Strength Measurement…………………………………………………………12
7.3 Stimulus Generator as a Signal Source…………………………………………….. 12
7.4 Measuring Unknown Capacitance………………………………………………….. 13
7.5 Measuring Unknown Inductance…………………………………………………… 13
7.6 Tuning a ¼-Wave or ½-Wave Stub…………………………………………………. 13
7.7 Determining Velocity Factor (Vf)…………………………………………………... 14

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. iv
7.8 Testing RF Transformers…………………………………………………………… 14
7.9 Checking HF Baluns………………………………………………………………… 15
7.10 Checking Coax Cable…………………………………………………………… 15
8.0 Quick Guide to Analyzer Controls and Functions………………………………………… 16
9.0 Technical Assistance
9.1 Technical Assistance…………………………………………………………………. 17
9.2 Limited Warranty……………………………………………………………………. 19

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 1
1.0 Introduction
Important: Read Section-2 before attempting to use your analyzer -- applying incorrect
operating voltages could result in permanent damage! Also, never apply a DC voltage to the
antenna connector.
General Description: The MFJ-266 is a self-contained handheld RF analyzer that performs the
following diagnostic functions:
SWR (1:1 to 9.9:1)
Complex Impedance (Z = R + jX)
Impedance Magnitude (Z = Ω)
Capacitance (pF)
Inductance (uH)
Relative Field Strength (mV)
Frequency (MHz)
The MFJ-266 also generates a 2-dBm RF signal that may be used to check receivers, networks,
amplifiers, and antenna patterns. Operating range is:
HF: 1.5 - 71 MHz in six HF bands
VHF: 85-185 MHz continuous coverage
UHF: 300-490 MHz continuous coverage
A 10:1 vernier drive provides smooth tuning. Measurements are displayed on an easy-to-read
LCD screen with optional backlighting. Power is supplied by internal AA cells or by a regulated
12-VDC external power source (not included). Weighing just over 1.3 pounds, the MFJ-266
package fits comfortably in one hand for convenient bench work or on-the-fly testing in the field.
Operation is simple, but you will need to read the manual to learn all of the unit's features and
functions. The more you know, the more valuable it will become as a diagnostic tool.

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 2
2.0 Power Sources
The MFJ-266 may be powered with internal AA batteries or with an external DC supply. To
avoid needless damage and ensure top performance, please follow the guidelines below when
choosing a voltage source:
2.1 Internal Batteries
Battery power requires eight (8) AA-size 1.5-volt alkaline cells. Rechargeable 1.2-volt cells
should not be used because the combined voltage will be too low to meet the unit's 10.8-volt
minimum threshold. Batteries are installed in two fully encased 4-cell plastic trays mounted
inside the analyzer enclosure. To access these, remove all four screws securing the analyzer's
back cover and carefully open the case. Slide the battery box covers sideways to unlatch, then lift
vertically to expose the cells.
Slide and
Lift
Never change batteries with an external power supply connected or when the power switch is
turned on. Doing so could damage the unit. Note that alkaline cells are not rechargeable and the
analyzer has no built-in charging circuitry. When replacing old batteries, be sure to follow the
manufacturer's environmental guidelines for safe disposal. For longest battery life, always
replace with a matched set of factory-fresh cells.
2.2 External Power Supply
Powering the MFJ-266 externally requires a well-filtered DC supply capable of delivering 10.8
to 12.5 VDC under varying load conditions. Current drain ranges from 30 mA to 180 mA,
depending on operating mode, frequency range, and whether or not the display backlight is on.
Many low-cost 12-volt "wall-wart" adapters are very poorly regulated and may deliver damaging
voltages under variable load conditions. If in doubt, check your supply with a voltmeter before
connecting to confirm it delivers no more than 13 volts or less with no load connected. The unit's
external power jack is located on the front panel and accepts a standard 2.1-mm power plug.
Positive voltage (+) must be applied to the connector's center pin.

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 3
+
-+
-2.1 mm
Inserting the plug into the power jack automatically disables the internal AA battery pack and
switches the analyzer over to the external voltage source.
Important Warnings: Reverse polarity or excessive voltage could permanently damage the
MFJ-266! To avoid damage:
1. Never apply more than 13 Volts
2. Never connect an AC transformer or positive-ground power source
3. Never install or remove batteries with external power connected
4. Never change batteries with the PWR switch on
3.0 Operating Mode
Once you have suitable power (battery or external), you're ready to explore the analyzer's basic
operating features. Begin by pressing the red PWR button on.
BAND-MODE
SELECT
Up Down
ANT Counter
Bk Lite
PWR ON
OFF
3.1 Display Backlight
When the analyzer comes on, the screen displays a brief 1-second prompt before automatically
switching to the main menu. This prompt allows you to turn on the optional display backlight. If
you elect to use it, press the Up button immediately, before the screen changes to the main menu
and hold it down until the backlight comes on. If you ignore the prompt, the backlight will
remain off. Off is the default setting to reduce battery drain.
3.2 Main Menu Screen
The main menu screen has two purposes:
(1.) Power Supply Voltage: Appears on the right side of the screen. If it falls outside the
10.8 to 12.5-V operating window, be sure to change batteries or make power supply
adjustments.

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 4
(2.) Operating Mode Prompt: On the left side of the screen. This prompts you to select
between the two primary operating modes (see below).
D >FC DC:12.00V
U >ANT Analyzer Supply Voltage"Down" for Counter
"Up" for Antenna Analyzer
(1.) (D >FC) Press the “Down” button to select Frequency Counter mode.
(2.) (U > ANT Analyzer) Press the “Up” button to select the Antenna Analyzer
mode.
3.3 Frequency Counter Mode (D -> FC)
In this setup, the MFJ-266 functions as a 1-500 MHz frequency counter. Note that the BAND
SELECT switch Bmust be "up" in the HF position for the counter mode to activate. If switch B
is down, an error message will prompt you to change the band setting to HF.
AB
BAND SELECT
BAND A B
HF X
VHF
UHF
Counter X
UP
When a signal is applied to the Antenna jack, the frequency is displayed in MHz. Two gate
speeds are available. The default gate speed is Fast (or Fg -- see the top right-hand side of the
display). The fast gate provides 1-kHz resolution. The alternative gate speed is Slow (or Sg),
which provides 100-Hz resolution. To change the gate speed:
(1.) For Fast Gate, press the UP button.
(2.) For Slow Gate: press the DOWN button.
Fg f: 010.000 MHz
REF FS: 100mV
Gate Speed Frequency Readout
Relative Field Strength
The Counter mode also provides relative Field Strength (REF FS). This feature is useful for
conducting relative field-strength tests, estimating input levels to the counter, and detecting local
signals that could impact SWR accuracy (see Section 7.2).
3.4 Antenna Analyzer Mode (U -> ANT)
In this mode, the analyzer's built-in stimulus generator drives a bridge circuit and the unit
functions as a network analyzer. The top line of the screen displays band selection (a single
letter) and the operating frequency in MHz (see Section-4). The bottom line simultaneously

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 5
displays complex impedance (Z = R+JX), impedance magnitude (Z = Ω), and SWR for any
load connected to the antenna jack. Note that only SWR is displayed in the UHF operating range.
50+j 0 50 1.0
Stimulus Frequency
Band (Frequency) Selection
SWR (1.0:1)
10.000MHz D SWR
Impedance MagnitudeComplex Impedance
3.5 RF Signal Source
RF output from the MFJ-266’s built-in stimulus generator is available at the ANT connector in
Analyzer mode. This signal is a +2 dBm continuous carrier. When using the analyzer as a signal
source, the operating range, band, and frequency are selected in the normal manner and will be
displayed on the screen (see Frequency Selection, Section-4).
3.6 L/C Measurement Mode
The MFJ-266 may be used to measure the value of unknown capacitors and inductors. To
measure L/C values, connect the device to be tested to the antenna jack and follow the procedure
outlined below:
Measure Capacitance
Turn the analyzer off, then press and hold the Up button while turning PWR back on. The
screen will display the value in pF along with the stimulus frequency being used for the
measurement.
Measure Inductance
Turn the analyzer off, then press and hold the Down button while turning PWR back on. The
display will show inductance in uH along with the stimulus frequency.
5.000MHz D C=
122 pF
Capacitance
Frequency Band 5.000MHz D L=
10.200 uH
Inductance
Frequency Band 5.000MHz D C=
Xc>1.5k
Frequency Band
Reactance out of Range
Change Stimulus Frequency

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 6
4.0 Frequency Selection
The MFJ-266 covers the HF region (1.5-71 MHz) in six bands, plus VHF (85-185 MHz) and
UHF (300-490 MHz). Tuning and band-selection are electronically switched for high reliability.
4.1 Variable Tuning
The TUNE control uses an instrumentation-grade potentiometer with a 10:1 drive reduction to
ensure gradual tuning on each band. Note that a mechanical lock is provided on the side of the
TUNE knob assembly. The lock is used to prevent accidental changes once a desired frequency
is set.
Dial Lock
TUNE
4.2 Range Selection
BAND SELECT buttons Aand B are used to toggle between the HF, VHF, and UHF ranges.
Follow the up/down position prompts shown in the table printed next to the buttons to select
ranges.
AB
BAND SELECT
BAND A B
HF X
VHF
UHF
Counter X
Xmeans the VHF/UHF switch can be in either position. Tuning is continuous in the VHF and
UHF ranges. The frequency readout line of the display shows both the operating frequency in
MHz and the letter of the selected range.
Band V: 85 to 185 MHz (FM, Airband, 2 Meters, 2-Way)
Band U: 300 to 490 MHz (Military, 70-cm, 2-way)
4.3 HF-Band Selection
The UP and DOWN buttons are used to step or scroll through a selection of six HF bands. To
step up or down in one-band increments, quickly tap the appropriate switch to initiate each
change. To scroll, press and hold the switch down. Most users find it easier to watch the letter
designation for the desired band rather than watch the frequency display when making a
selection:

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 7
A: 1.5 to 2.7 MHz (160 Meters)
B: 2.5 to 4.8 MHz (80/75 Meters)
C: 4.6 to 9.6 MHz (60,40 Meters)
D: 8.5 to 18.7 MHz (30,20,17 Meters)
E: 17.3 to 39 MHz (17,15,12,10 Meters)
F: 38.7 to 71 MHz (6 Meters)
Before moving on to the next section, take time to review the MFJ-266’s basic set-up
procedures. Operation becomes second nature quickly, but should you need it, there’s a
supplemental "quick guide" in the back for reference (Section 8.0). The remainder of the manual
will focus on general instructions and helpful tips for making accurate measurements.
5.0 Accuracy Limits
The MFJ-266 will serve as your “eyes and ears” when working with RF systems, and it can
deliver results that rival units costing thousands of dollars. However, all handheld analyzers
share certain limitations, and being aware of them will help you to achieve more meaningful
results.
5.1 SWR Measurements and Local Interference
The MFJ-266 (and other hand-helds) use a broadband diode detector that is open to receiving
signals across the entire radio spectrum. Most of the time, the unit's built-in stimulus generator is
powerful enough to overcome any lack of front-end selectivity and override stray pickup.
However, a powerful transmitter located nearby could inject enough RF energy into the detector
to disrupt readings. If this condition occurs, performance will become erratic and SWR readings
may appear higher than they really are.
5.2 Checking for Local Interference
Unlike many analyzers, the MFJ-266 has an onboard function for identifying local interference.
Simply switch to the analyzer's Frequency Counter (FC) mode and note the readings you
obtain with the antenna connected. If a strong signal (>100 mV) registers on the field-strength
display (FS) and the counter shows the frequency of a known local broadcast station or radio
service (f:), then suspect interference. If the interfering source can't be turned off or your antenna
can't be moved to a different location, you may need to use a station transceiver and a thru-line
directional Wattmeter to complete the adjustments.
5.3 Detector Linearity and Accuracy
Diode detectors typically become non-linear at very low voltages. Because of diode non-
linearity, it's not uncommon for two identical analyzers to show slightly different readings when
checking a load with very low SWR (or low RF-return voltage). For example, one analyzer may
read 1.2:1 while another reads 1.1:1 when checking the same antenna. The MFJ-266 is

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 8
electronically compensated to minimize detector error, but be aware of the potential for minor
differences.
5.4 Calibration-Plane Error
The analyzer’s calibration plane is the point of reference where all measurements have the
greatest accuracy (gain reference=0 dB, phase shift = 0-degrees). For basic hand-held units like
the MFJ-266, the calibration plane is fixed at the antenna connector. As such, any measurement
made through a cable will displace the load from the calibration plane and introduce some
amount of error. For SWR readings, error is mainly caused by losses in the cable. Specifically,
SWR will read somewhat lower through a length of cable than with the analyzer connected
directly to the direct load because the forward and reflected stimulus signals are attenuated in the
feedline. The more loss there is in the cable, the greater the error. Most of the time, this
inaccuracy isn’t a problem because the SWR you measure with the analyzer is the same SWR the
radio will encounter when connected. However, if you wish to know the antenna’s actual
feedpoint SWR for documentation purposes, the analyzer should be connected directly to the feed
point through a short pigtail.
Calibration-plane error has a much more significant impact when attempting to measure
impedance values because of phase rotation in the cable. In fact, impedance readings can swing
dramatically, depending on the cable’s electrical length and the severity of the load’s mismatch
with reference to 50 Ohms. For accurate impedance data, always connect the analyzer directly
to the antenna or device you’re testing using the shortest lead possible.
5.5 Sign Ambiguity (± j)
Most hand-held analyzers (including the MFJ-266) lack the processing capability to calculate the
reactance sign for complex impedance (Z = R ± j). By default, the MFJ-266 displays a plus sign
(+ j) between the resistive and reactive values, but this sign is merely a placeholder and not a
calculated data point. Although the analyzer’s processor can’t calculate sign, it can often be
determined with a small adjustment of the TUNE control. To determine sign, TUNE the analyzer
up-frequency slightly --
(1.) If reactance decreases, the sign is likely to be ( - ) and the reactance capacitive (XC).
(2.) If reactance increases, the sign is likely to be ( + ) and the reactance inductive (XL).
6.0 Antenna Measurements
Excellent tutorials are available in ARRL Handbooks and other League antenna publications to
help you master the art and science of constructing and adjusting effective antenna systems.
Informative introductory material may also be found on line, but choose carefully. Not all web
material is well edited or accurate (especially items discussed in chat rooms and forums). Here
are some general guidelines to help you get started.

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 9
6.1 Antenna Connectors
The MFJ-266 uses a type-N female (or NF) connector to ensure reliable signal connectivity up
to 500 MHz. It also comes with a SO-239-female to N-male (UHF-NM) adapter for
transitioning to popular PL-259 connectors. When purchasing additional adapters, look for N-
male rather than UHF transitions. Stacking multiple adapters together places unnecessary stress
the analyzer’s NF connector and increases the possibility of measurement error. Avoid using PL-
259 connectors above 2 Meters because they may contribute significant mismatch to your
measurements. Finally, when installing N-male connectors on patch cables and feedlines, pay
close attention to pin depth. If the tip of the connector pin extends more than a few mils above
the surrounding contact fingers, the pin shoulder could damage the analyzer's NF connector.
6.2 SWR
Standing Wave Ratio (SWR), sometimes referred to as VSWR, is the most widely used format
for checking tuning error and impedance mismatch between antennas and radios. The MFJ-266
is calibrated to work on the 50-Ohm impedance standard used by amateur and commercial two-
way equipment (Zo=50). Unless a different cable impedance is specified by the antenna designer
for matching purposes, always use 50-Ohm cable of known quality when making up
transmission lines and patch cables.
WARNING: Never apply external dc voltages or strong RF signals to the analyzer’s antenna
connector or permanent damage will result. Also, never connect the output of a transmitter to
your analyzer.
6.3 Measuring SWR
Here is the recommended procedure for the checking antenna SWR with the MFJ-266:
(1.) Turn the unit ON and select the ANT Analyzer function (Section-3).
(2.) Select the desired Range, Band, and Frequency (Section-4).
(3.) Connect the antenna to the analyzer (Antenna connector)*.
(4.) Rotate the Tune knob to find the lowest SWR reading and write it down.
(5.) Rotate Tune to either side of minimum SWR and note the 2:1 SWR points.
*When testing large ungrounded antenna systems such as HF dipoles, momentarily short the
feedline center pin to ground to bleed off static buildup before connecting to the analyzer.
6.4 SWR, Bandwidth, and Resonance
The amateur-radio industry's standard for maximum SWR is 2:1 (1.5:1 for commercial 2-way).
Most modern transceivers operate safely and deliver full power within this mismatch range. The
antenna bandwidth is the frequency interval between its two 2:1-SWR points. This specification,
along with minimum-SWR and minimum-SWR frequency, is often included on antenna
specification sheets. Note that minimum-SWR is sometimes wrongly confused with resonant

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 10
frequency. The technical definition for resonance is the frequency where inductive and
capacitive reactance cancels, leaving a purely resistive load ( ± j = 0). The minimum SWR and
resonant frequencies may be close, but they rarely coincide.
13.9 14.0 14.1 14.2 14.3 14.4
1.5
2.0
1.0
Min. SWR Freq.
Bandwidth
Lower 2:1 Freq. Upper 2:1 Freq.
Min. SWR
6.5 Antenna Tuning
Where possible, make adjustments to your antenna that will yield SWR readings under 2:1 over
the frequency range where you normally operate. If the minimum-SWR frequency measures low
in the band (or below the band edge), your antenna is probably too long and will need to be
shortened. If the Minimum SWR frequency is too high, it should be lengthened. To calculate the
required change in length:
(1.) Write down the desired minimum-SWR frequency (ex: 14.200 MHz)
(2.) Use the analyzer to measure the present minimum-SWR frequency (ex: 14.050 MHz)
(3.) Divide the present frequency by the desired frequency (ex 14.050 ÷14.200 = .989)
(4.) Multiply the present length by the result (33.3 feet x .989 = 32.94 feet)
Note that this formula applies to full-sized antennas, but not to elements shortened by coils,
traps, or capacitive hats.
6.6 Antenna matching
If your antenna doesn’t exhibit 1:1 SWR at the minimum SWR frequency, then some mismatch
is present relative to 50-Ohms. For simple dipoles and ground-independent verticals, mounting
height above ground may be the primary cause. Generally, it’s best to ignore SWR readings
under 2:1 and mount these antennas as high as possible where they’ll perform best. For antennas
that feature adjustable matching networks (Yagis etc), SWR can usually be improved by
following the manufacturer’s antenna setup instructions. Note that matching and tuning settings
may interact, so readjustment of both the antenna’s element length and matching network may be
needed to obtain best results.
6.7 Matching Antennas Through A Tuner (ATU)

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 11
If your antenna can’t be tuned or matched to an acceptable SWR level by making physical
adjustments, then an external antenna tuner (ATU) should be installed. The MFJ-266 may be
used in conjunction with the tuner to make adjustments without the need to transmit test signals
over air. Simply connect the analyzer to the tuner input (radio side) through a short patch cable.
Select the Analyzer mode, set up the Band, and Tune for the desired frequency of operation.
Then, adjust the antenna-tuner's controls following the manufacturer's recommendations until
SWR approaches 1:1. Remove the analyzer, reconnect the radio, and the load will be pre-
matched to the radio's 50-Ohm operating impedance.
6.8 Antenna Impedance Readings
The MFJ-266 displays complex impedance and impedance magnitude readings on the same
screen with the SWR reading. However, when measuring through coax, remember that the
impedance readings are phase-shifted values appearing at your end of the cable and not the
actual feedpoint impedance of the antenna itself (Section-5.4). As a “work-around” strategy, it’s
possible to measure the antenna’s actual impedance remotely if the feedline is cut to an exact
electrical half wavelength. In a half-wavelength line, the phase shift is a full 360 degrees, which
electrically rotates the analyzer’s calibration plane back into alignment with the load. However,
this strategy only works at one frequency and errors compound quickly if your cable is multiple
half-wavelengths long. As a practical matter, unless you have an advanced working knowledge
of transmission-lines, Smith charts, and impedance matching theory, it’s best to ignore
impedances and rely on SWR for routine antenna-system adjustments.
6.9 Unpredictable SWR
A change in feedline length shouldn't shift your antenna's minimum-SWR frequency or have
much impact on the SWR readings. If it does, suspect a significant mismatch between the antenna
and coax, or more likely, poor isolation between the feedline and the antenna. Isolation problems
typically occur when unbalanced coax line is connected directly to a balanced element such as a
dipole or a loop, and the outer surface of the coax shield literally becomes a part of the antenna.
If the length of the shield happens to presents a low impedance path, it can load the element
significantly and shift the minimum-SWR frequency unpredictably. It will also introduce needless
mismatch, divert transmitted RF back toward the operating position, cause RFI problems in the
residence, and increase unwanted noise pickup in receive mode. The best way to decouple the
outer surface of the shield from the antenna element is with a balun. Current-type baluns work
best because they have higher power-handling capability and less loss than other types. An
effective current balun could be as simple as a few loops of coax taped together at the feedpoint,
but for best common-mode rejection, a Guanella-style balun wound on a ferrite core is
recommended.
7.0 Advanced Functions
Here are some of the MFJ-266 advance functions. Note that some of these procedures involve
connecting component leads to the unit's N connector. For these connections, we suggest making

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 12
up a very short N-male coaxial pigtail or obtaining a type-N dual binding post adapter to prevent
damage the center-contact of the analyzer connector.
7.1 Frequency Measurement
The MFJ-266 features a precision counter that accurately measures the frequency of RF signals
between 1 and 500 MHz with up to 100-Hz resolution (see Section-3.3 for setup). To conduct
measurements, connect your signal source to the unit’s antenna jack. The counter will typically
lock onto any signal 30 mv (-20 dBm) or stronger, with an upper signal measurement limitation
of 1 volt (or +10 dBm). Any input signal exceeding +10 dBm automatically triggers a Danger –
High FS warning on the analyzer display (high field strength). This is a notification to reduce the
signal level as quickly as possible.
Warning: Avoid connecting any external RF source more powerful than 10
dBm (or 10 mW) to the analyzer's antenna connector.
7.2 Field Strength Measurement
The field strength function works in conjunction with the counter mode to display Relative RF-
input Level (REF FS). Any external antenna that yields a usable signal level may be connected
to the analyzer’s antenna jack to serve as a pickup device. The usable signal range is around 30-
dB (30-mV to 1-V rms). Note that the display reading is a RMS level taken directly from the
MCU detector, so it must be multiplied by 1.414 to obtain a peak AC value (V-rms x 1.414 = V-
p, conversely V-p x .707 = V-rms). Also, despite the display's 0.1 mV resolution, readings will
be approximations rather than precise values because of detector non-linearity. The Danger -
High FS display warning means a dangerously strong signal is being applied and the level
should be reduced immediately. Also, interference greater than 100 mV will almost certainly
cause inaccurate SWR measurements when in Analyzer mode.
7.3 Stimulus Generator as a Signal Source
When operated in Analyzer mode, the MFJ-266 generates a +2 dBm CW carrier (2 mW). See
section 3.6 for setup procedures. Output will vary slightly, depending on frequency and operating
voltage, but typically holds to within 1-2 dB of the rated power level over the analyzer's
frequency range. Second-harmonic suppression averages -20 dBc. A quarter-wave stub or low-
pass filter may be installed if greater harmonic suppression is required for a specific application.
Frequency stability and carrier purity are sufficient for testing filters, mixers, low-power
amplifier stages, and for checking antenna patterns when a range antenna is connected to the
analyzer output. The stimulus generator may also be used for producing lower-level signals with
a suitable precision RF attenuator installed in line. When connecting the generator directly to
active circuitry, always insert a coupling capacitor to prevent DC voltages from back-feeding
into the bridge circuit and destroying the detector diodes. Also, avoid connecting the stimulus
signal directly to sensitive preamps or receiver circuits that could be damaged by an un-
attenuated 2-mW signal.

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 13
7.4 Measuring Unknown Capacitance
To measure capacitance, connect the unknown component to the Antenna connector (usable
range is from approximately 15 pF to 1200 pF). To enter the Capacitance mode, begin with the
analyzer turned off, then press and hold the Up button while pressing the PWR switch (see
Section 3.6). The screen will display the approximate value of the unknown capacitor in pF
along with the stimulus frequency where the measurement is being made. Note that you will not
have full control over stimulus-frequency Band selection in this mode. Of the bands offered, the
best accuracy is typically obtained on Band C,which may be selected using the Up/Down
switches. Note that any capacitor and lead combination that approaches self-resonance at the
stimulus frequency will trigger the C = Xc >1.5K Ω message and will be un-measurable. Attempt
to re-measure at a lower frequency.
7.5 Measuring Unknown Inductance
To measure inductance, connect the unknown component to the Antenna connector. To enter the
Inductance mode, begin with the analyzer turned off, then press and hold the Down button
while pressing the PWR switch on (see Section 3.6). The screen will display the approximate
value of the unknown inductor in uH along with the stimulus frequency where the measurement
is being made. You will not have full control over the stimulus-frequency Band selection and the
best accuracy is usually obtained on Band B or C (selected using the Up/Down switches). Note
that any inductor approaching self-resonance at the stimulus frequency will trigger the L = XL
>1.5K Ω message and will be un-measurable. Try to re-measure at a lower frequency.
7.6 Tuning a ¼-Wave or ½-Wave Coaxial Stub
To accurately tune a coaxial stub, begin by calculating the free-space length at the stub's
intended operating frequency:
For 1/4-λin inches = 2951 ÷ MHz For 1/4-λin feet = 246 ÷ MHz
For 1/2-λin inches = 5902 ÷ MHz For 1/2-λIn feet = 492 ÷ MHz
Next, multiply the free-space length times your cable's velocity factor. Finally, add at least 10%
to this length for a margin of error (better too long than too short). Cut the cable to this initial
length. Connect one end of the cable to the analyzer's Antenna connector. For a 1/4-λstub, leave
the far end open. For a 1/2-λstub, short the far end. Next:
(1.) Set the MFJ-266 to Analyzer mode (Section-3)
(2.) Initially, set the Range, Band, and Tune for the desired stub frequency (Section-4)
(3.) *Tune down in frequency to find lowest impedance-magnitude reading (the load is a
short).
(4.) Write your measured frequency down.
(5.) Divide the measured frequency by the desired stub frequency to obtain a correction
factor
(6.) Multiply the present stub length by the correction factor to get the desired stub
length.

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 14
(7.) Re-cut the cable to that length.
*Note that the impedance value may not drop to zero, but it will begin to increase again
as you continue to tune past the null. If the null reading is broad, choose a frequency at
the center.
7.7 Determining Velocity Factor
If you have coax cable with an unknown velocity factor, you can determine it quickly using the
following procedure:
(1.) Set the MFJ-266 up in Analyzer mode (Section-3)
(2.) Set the Range to HF and the Band to E (Section-4)
(3.) Make a 1/4-λstub from 9 feet of the unknown cable and connect it to the
analyzer (open end)
(4.) Rotate Tune for minimum impedance magnitude reading. Write down the
frequency (MHz)
(5.) Divide 246 by this frequency to find the free-space 1/4-λwavelength in feet
(L = 246 ÷ f MHz)
(6.) Divide 9 (actual length) by free-space 1/4-λwavelength to get the Velocity
Factor (VF =9 ÷ L)
Note that there is nothing magical about the 9-foot stub length, other than it falls conveniently
within the limits of Band E’s tuning range. Other lengths could be used. Shorter stubs will yield
poorer accuracy and long ones may needlessly waste useful cable.
7.8 Testing RF Transformers
Broadband HF-matching transformers wound for the 12.5 to 200 Ohm range may be tested using
the MFJ-266. Connect the 50-Ohm (primary) side to the analyzer connector using a short pigtail
and attach the appropriate resistive load across the secondary side (always use a non-inductive
resistor). Next:
MJ-266
Load
Transformer
(1.) Set the MFJ-266 up in Analyzer mode (Section-3)
(2.) Set the Band Select to HF and the Band-Mode to the desired frequency
range (Section-4)
(3.) Rotate Tune across the frequency range and note SWR. Change bands, as
needed.

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 15
At the low and high ends of the transformer's frequency response range, SWR and reactance will
climb to unacceptable levels (< 1.2:1 is ideal). HF, VHF, and UHF tuned transmission-line
transformers may be tested in similar fashion by connecting one end directly to the analyzer and
terminating the far end. However, only precision RF terminations with known impedance
characteristics should be used above 50 MHz. Set up the analyzer for the desired range and
sweep the band of interest using the Tune control. Transmission-line transformers are “frequency
specific” and have much more limited frequency response.
7.9 Checking HF Baluns
A well-designed balun will have low SWR and good balance over its operating range. The MFJ-
266 can test both of these qualities using the setup shown below. Configure the unit to operate in
Analyzer mode in the HF range. Connect the input (unbalanced) side of the balun to the
analyzer’s antenna connector. Connect a center-tapped resistive load to the balanced side (R1,R2
= 25Ωfor 1:1 baluns, R1,R2 = 100Ωfor 4:1 baluns). Using the Tune and Band controls:
B
C
Balun
MJ-266
A
Short Test Lead
R1
R2
(1.) Sweep the balun for SWR with the test lead disconnected from the load.
(2.) Connect the test lead to the mid-point (A) and re-sweep. There should be
minimal change.
(3.) Connect it to either side (B) (C). SWR will go up, but should go up equally on
both sides.
7.10 Checking Coax Cable
To check a length of coax cable for impedance error, connect one end to the analyzer and
terminate the far end with a precision (non-inductive) 50-Ohm resistive load. Set the MFJ-266 up
for analyzer mode and select the VHF tuning range. Rotate Tune across the VHF range while
watching the Impedance Magnitude reading. If the cable is 50 Ohms and in good condition, there
should be little change in the impedance magnitude readings. If there are significant fluctuations,
the cable is either not 50 Ohms or is badly contaminated. If readings cyclically swing between 25
Ohms and 100 Ohms, the cable is 75-Ohm coax.

MFJ-266 HF/VHF/UHF Antenna Analyzer Instruction Manual
©2011 MFJ Enterprises, Inc. 16
8.0 Quick Guide to Analyzer Controls and Functions
Power: Use only 1.5-V Alkaline Batteries. External power must be 10.8-12.5 Vdc, well
regulated. Power plug: 2.1-mm, positive (+) to center pin.
Power Up: Press PWR, wait for the Main Menu to come up.
Power Up + Backlight: Press PWR then UP, hold until screen lights and Main Menu comes up.
Main Menu Screen: D >FC DC:12.00V
U >ANT Analyzer Check Supply VoltagePress Down button for Counter Mode
Up button for Antenna Analyzer Mode
Counter Mode Screen: Fg f: 010.000 MHz
REF FS: 100mV
Set Gate Speed: Up = Fast, Down = Slow Incoming Signal Frequency Readout
Relative Field Strength
Analyzer Mode Screen:
50+j 0 50 1.0
Stimulus Frequency Band (Frequency) Selection
SWR (1.0:1)
10.000MHz D SWR
Impedance MagnitudeComplex Impedance
L/C Mode:
Measure C: Press and hold Up, press Power On
Measure L: Press and hold Down, press Power On
5.000MHz D C=
122 pF
Capacitance
Frequency Band 5.000MHz D L=
10.200 uH
Inductance
Frequency Band 5.000MHz D C=
Xc>1.5k
Frequency Band
Reactance out of Range
Change Stimulus Frequency
Set Up Stimulus Frequency:
(1.) Press A/B Combination to Select Operating Range
AB
BAND SELECT
BAND A B
HF X
VHF
UHF
Counter X
HF: 1.5 - 71 MHz
VHF: 85-185 MHz
UHF: 300-490 MHz
(2.) Press Up/Down to Select HF Band