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Antenna Tuner Tips
The following suggestions will reduce the difficulty in matching an antenna with a tuner:
1. Never center feed a half-wave multi-band antenna with a high impedance feedline that is close to an odd multiple of a quarter-wave long. 
2. Never center feed a full-wave antenna with any feedline close to a multiple of a half-wave long. 
3. If a tuner will not tune a multi-band antenna, add or subtract 1/8 wave of feedline (for the band that won't tune) and try again. 
4. Never try to load a G5RV or cente fed dipole on a band below the half-wave design frequency.    If  you  want  to  operate  an  80  meter  antenna  on  160  meters,  feed  either  or  both conductors as a longwire against the station ground.

To  avoid  problems  matching  or  feeding  any  dipole  antenna  with  high  impedance  lines,  keep  the lines around these lengths 
[ The worst possible line lengths are shown in brackets ]:
160 meter dipole:
35-60, 170-195 or 210-235 feet.
[ Avoid 130, 260 ft]
80 meter dipole: 
34-40, 90-102 or 160-172 feet. 
[ Avoid 66, 135, 190 ft ]
40 meter dipole: 
42-52, 73-83, 112-123 or 145-155 feet.
[ Avoid 32, 64, 96, 128 ft ]
  Some trimming or adding of line may be necessary to accommodate higher bands.
    To  avoid  problems,  a  dipole  antenna  should  be  a  full  half-wave  on  the  lowest  band.   
On 160 meters, an 80 or 40 meter antenna fed the normal way will be extremely reactive with only
a few ohms of feedpoint resistance.  Trying to load an 80 meter (or higher frequency) antenna on
160 meters can be a disaster for both your signal and the tuner.  The best way to operate 160 with
an 80 or 40 meter antenna is to load either or both feedline wires (in parallel) as a longwire.  The
antenna will act like a "T" antenna worked against the station ground

Taking the antenna tuner approach is not a good idea when you are using coaxial cable under high (greater than 3:1) SWR conditions. The tuner may provide the 50 ohm match to your radio, but the mismatch and high SWR still exists between the antenna tuner and the antenna! This translates to high losses in the coaxial cable.

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