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MFJ-1789 Rotatable Mini-Dipole 

                                                 Instruction Manual 

 

 

10 

FREQUENCY AND SWR ADJUSTMENT 

 

An  SWR  analyzer  such  as  the  MFJ-259B  or  equivalent  is  highly  recommended  when  tuning  the  MFJ-
1789.  While a transceiver and SWR meter can be used, an SWR analyzer will significantly decrease the 
time and effort necessary to tune the antenna. 
 
The MFJ-1789 covers wider frequency ranges on the higher bands, and narrower segments on the lower 
frequency bands.  The 40 meter band has the narrowest range of operation (approximately 40 KHz) and 
is the most sensitive to adjustments. 
 
The entire antenna must be accessible during initial coarse tuning and testing.  Any repair or adjustment 
to the antenna after it is installed on a tall support will make adjustments difficult and time consuming.  It 
is  best  to  install  the  antenna  on  a  short  temporary  mast  or  pipe  that  is  located  in  a  reasonably  clear 
location for tuning.  The antenna should be mounted horizontally 6-8 feet above ground to make tests and 
adjustments easy. 
 
During tuning, feed the antenna with a reasonably short length of good quality 50 ohm coaxial cable to 
ensure  proper  results.    If  using  a  transceiver  and  SWR  meter,  set  the  transceiver  to  the  lowest  power 
possible when making measurements. 
 

The  normal  resonant  frequency  of  this  antenna  is  at  or  just  below  the  bottom  of  each  amateur 
band.

    This  allows  the  user  to  "trim"  a  small  amount  off  the  capacitance  spokes  to  raise  the  resonant 

frequency.  Conversely, adding a longer capacitance spoke will lower the resonant frequency of a loading 
assembly.  Spare spokes are included in case you need to lower the resonant frequency of the antenna. 

 

CAUTION

Always  start  tuning  on  40  meters  and  adjust  each  band  progressively  higher  in 

frequency

.  Adjustment of a lower frequency band will always have the most effect on the next higher 

frequency  band.    The  tendency  of  the  interaction  is  that  if  you  move  one  band  higher  ALL  the  other 
bands move higher, but only very slightly.  It is always best to "shoot for" the lowest end of the range you 
intend  to  use  and  "trim  in"  by  adjusting  the  outermost  (40  meter)  loading  coil  assemblies  after  the 
antenna is in its final location. 

 

NOTE:  When you tune the antenna at a height of 6-8 feet, you will find that the resonant 

40  meter  frequency  will  shift  up  in  frequency  30-50  kHz  when  the  antenna  is 
raised to the final height.  However, when the plastic end caps are installed on the 
ends of the spokes, the resonant frequency on all bands is lowered 30-50 kHz 

 

Tuning the Antenna 

 

1)

 

Measure and record the frequency of lowest SWR for each band. The lowest SWR should be at or 
below the bottom end of each HF band. The SWR should be below 2:1 at resonance on each band. 

 

 

If you are using a transceiver and SWR meter, the initial resonant frequency may be lower than you can 
measure.    If  so,  remove  one  spoke  from  each  loading  coil  assembly  to  raise  the  resonant  frequency.  
Measure this resonant frequency and calculate the approximate resonant frequency as if the spokes were 
in place using the chart on the next page. 

 

Summary of Contents for 1789

Page 1: ......

Page 2: ...ket 4 805 1796 7 Fiberglass Stabilizer Insulator 4 807 1796 6 Aluminum Stub Channel Bracket 4 808 1796 5 1 x 12 Fiberglass Rod 1 808 1775 12 1 1 8 x 60 AL Tube 2 810 1796 2 1 8 AL ROD 27 IN 2 EXTRA 10...

Page 3: ...st versatile rotatable dipole Each HF band uses separate efficient end loading coils wound on fiberglass forms with TeflonTM wire and capacitance hats no lossy traps 6 and 2 meters are full length hal...

Page 4: ...ightning protection WARNING Always mount this antenna so that it is out of the reach of adults and children The capacitance elements can cause injury and or severe RF burns Permanent Installation The...

Page 5: ...ent used to install the antenna accidentally contacts any utility lines Never install an antenna near power lines 1 Be careful while climbing and carrying the antenna It is heavy enough to cause you t...

Page 6: ...4 After examining the antenna parts gather the following tools needed for assembly 2 Phillips screwdriver 5 16 nut driver for 6 nuts and hose clamps Small 4 6 adjustable wrench OR o 3 8 wrench for 6 a...

Page 7: ...tube fiberglass assembly and through the balun lugs Add the 20 split lock washers and nuts and tighten securely See Figure 3 8 Slide four hose clamps over each aluminum tube Slide two hose clamps one...

Page 8: ...rings of both loading coil assemblies loosely holding them in place with their associated nuts Position the screws so the threaded ends of the screws are pointed outward towards the two ends of each l...

Page 9: ...clamp onto the aluminum radiator Slide the aluminum radiator tube until it contacts the mounting screw that was just loosened Slide the hose clamp over the tab and tighten the hose clamp to firmly at...

Page 10: ...e the resonant frequency Conversely adding a longer capacitance spoke will lower the resonant frequency of a loading assembly Spare spokes are included in case you need to lower the resonant frequency...

Page 11: ...l sections from the end of the spoke until the antenna resonates approximately 15 KHz below the desired operating frequency Use the chart below to approximate the amount of spoke to be trimmed Trimmin...

Page 12: ...on the outer ends of all spokes Note that the plastic end caps will lower the resonant frequency on all HF bands by 30 50 kHz The plastic end caps should fit snugly on the spokes Should a cap fit too...

Page 13: ...r the barrel of the coax connector The sealer should ONLY be placed near the junction of the threaded part of the chassis connector and the knurled area of the male connector This will leave the botto...

Page 14: ...MFJ 1789 Rotatable Mini Dipole Instruction Manual 14 NOTES...

Page 15: ...cover postage and handling 4 This warranty is NOT void for owners who attempt to repair defective units Technical consultation is available by calling 662 323 5869 5 This warranty does not apply to k...

Page 16: ...MFJ 1789 Manual Version 1B Printed In U S A 300 Industrial Park Road Starkville MS 39759 MFJ ENTERPRISES INC...

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