MFJ MFJ-249 - Adjusting Simple Antennas

MFJ-249 SWR Analyzer Instruction Manual
ADJUSTING SIMPLE ANTENNAS
Most antennas are adjusted by varying the length of the elements. Most home made antennas
are simple verticals or dipoles that are easily adjusted.
Dipoles .
Since a dipole is a balanced antenna, it is a good idea to put a balun at the feedpoint. The balun
can be as simple as several turns of coax several inches in diameter, or a complicated affair
with many windings on a ferromagnetic core.
The height of the dipole, as well as it's surroundings, influence the feedpoint impedance and the
line SWR. Typical heights result in SWRs below 1.5 to one on most dipoles.
In general, the only adjustment on a dipole is the length of the an‘ nna. If the antenna is too
long it will resonate too low in frequency, and if it is too short it wil! resonate too high.
Verticals
Verticals are usually unbalanced antennas. Most antenna manufacturers down play the
importance of good radial systems with grounded verticals. If you have a good ground system
the SWR of a quarter wave vertical can be nearly 2 to one. The SWR generally gets BETTER
as the ground system, and performance, get worse.
Verticals are tuned like dipoles, lengthening the element move: the frequency lower, and
shortening the element moves the frequency higher. ,
Tuning an Antenna
Tuning basic antennas fed with 50 ohm coaxial cable can be accomplished with the following
steps:
Connect the feedline to the MFJ-249.
Adjust the MFJ-249 until the SWR reaches the lowest reading.
Read the frequency display.
Divide the measured frequency by the desired frequency.
Multiply the present antenna length by the result of step 4. This is the new length needed.
nN & ow vr =
Note: This method will not work on loaded or electrically shortened antennas. They must be
tuned by adjusting and re-testing until the proper frequency 1s obtained.