ANTENNA TUNER MATCHING PROBLEMS AND SUGGESTED FEEDLINE LENGTHS
Matching Problems using a tuner and high impedance feedline
Most matching problems occur when
the antenna system presents an
extremely high impedance to the tuner. An
antenna system should be considered everything
from the tuner to the tip of the antenna. High impedance feedline is
usually considered that which has over 300 ohms impedance. Common high
impedance feedlines are 300 ohms, 450 ohms and 600s ohm in impedance. 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.
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 80 meter antenna example above. 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.
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 halfwave 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 center 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 the length in the green area of the chart below.
Suggested lengths for high impedance feedline on dipole type antennas
Good lengths are green shaded area in the chart below.
Some trimming or adding of line may be necessary to accommodate higher bands.
Here are 2 examples:
1. You have a dipole and you want to make it into a
multibander using a tuner.
2. Your dipole is cut for 40
meters or about 66 feet total length and you feed it with 450 ladder line
to a tuner to make it a multibander.
WARNING: 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.
So in a nutshell, if your having trouble matching your antenna system on a particular band using high impedance feedline with your tuner, add or subtract the appropriate amount of feedline according to the chart above and try again......73! Hopefully, you will find that "magic" length!