INSTRUCTION MANUAL
CAUTION: Read All Instructions Before Operating Equipment
MFJ ENTERPRISES, INC.
300 Industrial Park Road
Starkville, MS 39759 USA
Tel: 662-323-5869 Fax: 662-323-6551
COPYRIGHT 2013 MFJ ENTERPRISES, INC.
C
Model MFJ-251
VERSION 1A
Switchable
Dummy Load
Derating Curve
300
200
100
12
3
4
5
6
Introduction
Theory of Operation
Thank you for purchasing the MFJ-251 switchable dummy load. The MFJ-251 will
simulate high and low impedance loads of 1:1, 2:1, and 3:1. These loads are useful
in testing equipment to verify proper operation or troubleshooting station problems.
The MFJ-251 uses three 50 ohm high power resistors that can be switched in or out
of the circuit to form five resistance values at the coaxial input. It does this through a
series of relays that place each of the resistors in series or parallel to create the
resistance values needed for 2:1 or 3:1 matches. The front mounted switch sends
12vdc power to the relays needed for each arrangement of the resistors. When the
unit is in the off position or power is removed from the power jack, the MFJ-251
defaults into the 50 ohm mode to become a standard dummy load.
50 OHM
50 OHM
50 OHM
INPUT
GND
150 OHM
3:1 High Z
50 OHM
50 OHM
50 OHM
INPUT
GND
100 OHM
2:1 High Z
50 OHM
50 OHM
50 OHM
INPUT
GND
50 OHM
1:1 SWR
50 OHM
50 OHM
50 OHM
INPUT
25 OHM
2:1 Low Z
50 OHM
50 OHM
50 OHM
INPUT
GND
16.66 OHM
3:1 Low Z
GND
OFF
POSITION
Connections
Switchable Dummy LoadSwitchable Dummy Load
MODEL MFJ-251MODEL MFJ-251
TRANSMITTER
3:1/
150
3:1/
150
3:1/
16.6
3:1/
16.6
2:1/
100
2:1/
100
2:1/
25
2:1/
25
1:1/
50
1:1/
50
H
i
Z
H
i
Z
L
o
w
Z
L
o
w
Z
OFF
SWR/
OHMS
SWR/
OHMS
MFJ ENTERPRISES, INC.
STARKVILLE, MS USA
MFJ ENTERPRISES, INC.
STARKVILLE,
MS USA
RoHS
MADE IN THE USAMADE IN THE USA
+12VDC power
source
-
+
POWER
12VDC
250mA
POWER
12VDC
250mA
Use a 12 volt dc source to supply power to the MFJ-251.
Use the pig tail supplied connected to a power supply or
a wall wart such as the MFJ-1312D. The source should
be able to supply at least 500ma.
Impedance: 50 ohms +- 10%
VSWR : Max 1.3:1 0-30Mhz
Power: 300 watts for 30 sec
(5 min cool down)
This diagram shows how the MFJ-251 could be used to verify accuracy of a wattmeter.
Use the lowest power setting on the transceiver when testing. Do not transmitted high
power into the dummy load when using the 2:1 or 3:1 settings. Doing this may damage
your transceiver.
MFJ
251
Wattmeter
Transceiver
in
out
Rf out
Rf in
MFJ
251
Automatic
Antenna
Tuner
Transceiver
in
out
Rf out
Rf in
This diagram shows how the MFJ-251 could be used to verify the tuning capability
of a automatic or manual antenna tuner. The MFJ-251 provides a purely resistive load
to the tuner. Use the lowest power setting (5-10 watts) on the transceiver when testing.
Do not transmitted high power into the dummy load or through the tuner when tuning.
Doing this may damage your transceiver or your tuner.
Operation
Input
12VDC
RLY 1
RLY 2
RLY 1
RLY 2
RLY 3
RLY 3
RLY 4
RLY 4
FULL 12 MONTH WARRANTY
MFJ Enterprises, Inc. warrants to the original owner of this product, if manufactured by MFJ Enterprises,
Inc. and purchased from an authorized dealer or directly from MFJ Enterprises, Inc. to be free from defects
in material and workmanship for a period of 12 months from date of purchase provided the following terms
of this warranty are satisfied.
The purchaser must retain the dated proof-of-purchase (bill of sale, canceled check, credit card or
money order receipt, etc.) describing the product to establish the validity of the warranty claim and submit
the original of machine reproduction or such proof of purchase to MFJ Enterprises, Inc. at the time of
warranty service. MFJ Enterprises, Inc. shall have the discretion to deny warranty without dated proof-of-
purchase. Any evidence of alteration, erasure, or forgery shall be cause to void any and all warranty terms
immediately.
MFJ Enterprises, Inc. agrees to repair or replace at MFJ's option without charge to the original
owner any defective product under warrantee provided the product is returned postage prepaid to MFJ
Enterprises, Inc. with a personal check, cashiers check, or money order for covering postage and
handling.
MFJ Enterprises, Inc. will supply replacement parts free of charge for any MFJ product under
warranty upon request. A dated proof of purchase and a personal check, cashiers check, or money
order must be provided to cover postage and handling.
This warranty is void for owners who attempt to repair defective units. Technical
consultation is available by calling (601) 323-5869.
This warranty does not apply to kits sold by or manufactured by MFJ Enterprises, Inc.
Wired and tested PC board products are covered by this warranty provided
Wired and tested PC boards installed in the owner's cabinet or
connected to switches, jacks, or cables, etc. sent to MFJ Enterprises, Inc. will be returned at the owner's
expense unrepaired.
Under no circumstances is MFJ Enterprises, Inc. liable for consequential damages to person or
property by the use of any MFJ products.
MFJ Enterprises, Inc. will repair any out-of-warranty product provided
the unit is shipped prepaid. All repaired units will be shipped COD to the owner. Repair charges will be
added to the COD fee unless other arrangements are made.
This warranty is given in lieu of any other warranty expressed or implied.
MFJ Enterprises, Inc. reserves the right to make changes or improvements in design or manufacture
without incurring any obligation to install such changes upon any of the products previously manufactured.
All MFJ products to be serviced in-warranty or out-of-warranty should be addressed to
and must be
accompanied by a letter describing the problem in detail along with a copy of your dated proof-of-purchase.
This warranty gives you specific rights, and you may also have other rights which vary from state
to state.
1.
2.
$7.00
3.
$7.00
4. NOT
5.
6. only the wired and
tested PC board product is returned.
7.
8. Out-of-Warranty Service:
9.
10.
11. MFJ
Enterprises, Inc., 300 Industrial Park Road, Starkville, Mississippi 39759, USA
12.
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