MFJ MFJ-945E, User Manual

MFJ-945E Mobile Tuner

3 

5. 

After minimum SWR is achieved, transmitter power may be increased up to 300 Watts. Your 

MFJ-945E

 will reduce the SWR of most feed systems to 1:1.  In some cases, a 1:1 SWR is 

not possible.  Increase or decrease the length of your antenna to improve SWR. 

6. 

An SWR of 1:1 may occur at more than one set of control settings on your 

MFJ-945E

.  When 

an SWR of 1:1 is obtained, check transmitter power. Ensure that transmitter power is 
relatively high.  If transmitter power has decreased substantially, try another 

INDUCTOR

control setting and repeat 

Step 3

. 

In Case Of Difficulty: 

If this tuner 

fails to tune, 

please 

double check all connections

 and follow the tuning procedures 

again.  Be sure you are using 

enough inductance

  (

lowest letter usable for band

) and have the 

capacitors open far enough

 (

highest front panel number

). 

If this tuner

 arcs

 at the rated power levels, please 

double check all connections

 and follow the 

tuning procedures again.  Be sure you are using the 

least amount of inductance

 and the 

greatest capacitance

 possible to match the load on the operating frequency.  If you are still 

unsuccessful, please read the 

Antenna

Matching Problems

 text below. 

Note
: 

If this tuner 

arcs

 when operating on the 

160 meter

 band, it may be necessary to 

reduce 

transmitter output power

. 

Antenna Matching Problems: 

Most matching problems occur when the antenna system presents an 

extremely high impedance 

to the tuner

.  When the antenna impedance is much 

lower

 than the feedline impedance, an 

odd 

quarter-wavelength

 feedline converts the low antenna impedance to a 

very high impedance at 

the tuner

.  A similar problem occurs if the antenna has an extremely high impedance and the 

transmission line is a multiple of a half-wavelength.  The half-wavelength line 

repeats

 the very high 

antenna impedance at the tuner.  

Incorrect feedline and antenna lengths can make an antenna 

system very difficult or impossible to tune.

This problem often occurs on 80 meters if an odd quarter-wave (60 to 70 foot) open wire line is 
used to feed a half-wave (100 to 140 foot) dipole.  The odd quarter-wave line transforms the 
dipole's low impedance to over 

three thousand ohms at the tuner

.  This is because the 

mismatched feedline is an 

odd multiple

 of 1/4 wavelength long.  The line 

inverts

 (or teeter-totters) 

the antenna impedance. 

A problem also occurs on 40 meters with this antenna example.  The feedline is now a multiple of 
a half-wave (60 to 70 foot) and connects to a full-wave high impedance antenna (100 to 140 foot).  
The half-wave line repeats the high antenna impedance at the tuner.  The antenna system looks 
like 

several thousand ohms at the tuner

 on 40 meters.