RT-1000 Antenna
RHOTHETA Page 6 of 30 User Manual
the plane path to the receiver. Part b) is the vertical projection of part a). The circles represent
the lines of equal phase relations for even waves.
If the distance between transmitter and receiver is adequate, these are practically straight
lines when they reach the receiver location. Such an idealized situation is not to be found in
built-up areas, and especially not in mountainous regions. In such areas, the propagation path
is obstructed by obstacles, mirror reflectors; diffuse reflectors with and without absorption
characteristics, diffracting edges and resonators. Reflectors and conducting rods are effective
as resonators if their size is approximately that of the wavelength to be received. Therefore,
reflections increase as wavelengths become shorter, diffractions at edges however are
reduced and so the effect of shadowing obstacles is more important.
Accordingly, the propagation characteristics of radio waves from approximately λ< 10 m
increasingly resemble those of light.
At a wavelength of 1 m to 3 m, wave propagation requires a direct path and if this is not
available, only reflected waves are received. In urban areas, these may come from several
directions simultaneously. But that is not all: the mostly horizontal or vertical plane polarized
waves propagated by the transmitter are also rotated to a certain degree due to diffuse
reflectors and diffracting edges. When the wave arrives at the receiver, it may be oblique,
elliptically, or even circular polarized. This fact becomes apparent by the often curious
antenna positions which are necessary to obtain the best reception of radio or television
waves.
These points are meant to indicate that in the VHF, UHF range, direction finding of a
stationary transmitter using a stationary direction finder in a built-up area or even inside a
building is practically impossible.
Transmitter Reflection
2
Fig. 2: Field of lines of equal phase relations for two coherent waves
