Vectronics SWR-584B - Manual (Page 19)

SWR-584B Instruction Manual HF/VHF SWR Analyzer
19
Most antennas are adjusted by varying the length of the elements. Most home made antennas are simple verticals
or dipoles that are easily adjusted.
6.1 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 feedline SWR.
Typical heights result in SWR readings below 1.5 to 1 in most installations when using 50 ohm coaxial cable.
In general, the only adjustment available is the length of the dipole. If the antenna is too long it will resonate too
low in frequency, and if it is too short it will resonate too high.
Remember feedline length, when the antenna is not exactly the same impedance as the feedline, modifies the
impedance along the feedpoint. The SWR will remain constant (except for a small reduction in SWR as the
feedline is made longer) if the feedline is a good quality 50 ohm cable. If feedline length changes SWR at any
one fixed frequency, the feedline either has common mode currents that are detuning the antenna or the feedline
is not a true 50 ohm cable. Common mode currents are caused by lack of a balun or other installation errors.
6.2 Verticals
Verticals are usually unbalanced antennas. Many antenna manufacturers incorrectly downplay the need for a
good radial system with a grounded vertical. With a good ground system, the SWR of a directly fed quarter-wave
vertical can be nearly 2 to 1. SWR often improves if the ground system (and performance) is poor.
Verticals are tuned like dipoles, lengthening the element moves the frequency lower, and shortening the element
moves the frequency higher.
6.3 Tuning a simple antenna
Select any mode that indicates SWR. Tuning basic antennas fed with 50 ohm coaxial cable can be accomplished
with the following steps:
1.) Momentarily short the feedline center conductor and shield, then connect the feedline to the
SWR-584B .
2.) Adjust the
SWR-584B frequency to the desired frequency.
3.) Read SWR, and adjust the
SWR-584B frequency until the lowest SWR is found.
4.) Divide the measured frequency by the desired frequency.
5.) Multiply the present antenna length by the result of step 4. This will be close to the antenna length actually
needed.
Note: This method of tuning will only work on full size vertical or dipole antennas that do not employ
loading coils, traps, stubs, resistors, capacitors or capacitance hats. These antennas should be