16
Physical dimensions of a simple antenna (such as a dipole) can be adjusted knowing
the actual resonant frequency and the desired one.
Other types of antennas may contain more than one element to adjust (including coils,
filters, etc.), so this method will not work. Instead, you may use the
SWR mode
or the
Show all
mode to continuously see the results while adjusting various parameters of the
antenna.
AA-54: For multi-band antennas, use the
Multi SWR
mode. You can easily see how
changing one of the adjustment elements (trimming capacitor, coil, physical length of
an aerial) affects SWR at up to five different frequencies.
5.2. Coaxial lines
5.2.1. Open- and short-circuited cables
Open-circuited cable Short-circuited cable
The above pictures show R and X graphs for a piece of cable with open- and short-
circuited end. A
resonant frequency
is a point at which X (see the dotted line) equals to
zero:
•
In the open-circuited case, resonant frequencies correspond to (left to right) 1/4,
3/4, 5/4, etc. of the wavelength in this cable;
•
For the short-circuited cable, these points are located at 1/2, 1, 3/2, etc. of the
wavelength.
5.2.2. Cable length measurement
Resonant frequencies of a cable depend on its length as well as on the velocity factor.
A
velocity factor
is a parameter which characterizes the slowdown of the speed of the
wave in the cable compared to vacuum. The speed of wave (or light) in vacuum is
known as
the electromagnetic constant
: c=299,792,458 meters per second or
983,571,056 feet per second.