Page 355 - Maxwell House

FEED LINE BASICS 335

are very low. So we made assumption that the magnitude of going through all lines incident
wave stays constant thereby neglecting the multiple wave reflected back and forth in each line
section. Certainly, our analysis can be updated to include all this effects but it then loses its
physical simplicity and appeal. It is worthwhile to note that similar troubles can appear
undetected in any numerical simulation as being approximate by definition.

6.7.3 Coax-to-Waveguide Transition/Adapter
The main idea behind this design is to match first EM fields of two lines in the area of transition
using a small antenna like an electrical or magnetic dipole and then try to diminish the reactive
energy stored around the antenna. Sometimes the latter requires putting addition elements
nearby the dipole. Recall that such approach with extra inductance lets reduce the reflection in
the coax-to-coax adapter.
20
Figure 6.7.4a demonstrates that the extension of coaxial line central conductor plays the role
of electrical dipole inside WR. As such, it excites the partial plane waves required for the
formation of a dominant TE10-mode
(check Figures 6.4.3 and 6.4.4). Roughly
speaking, the dipole (often called probe)
should be put in the middle of the WR
wide wall where the E-field is the
strongest, and EM energy transfer is
most efficient. Since the dipole is
omnidirectional in H-plane (see dipole
patterns in Chapter 4) it creates the
infinite set of partial waves impinging on
WR metal walls at all angles between 0°
and 360° as shown schematically in
Figure 6.7.4b (blue and red arrowed
Figure 6.7.4 Coax-to-waveguide adapter: a) lines).
Schematic, b) Two TE10-modes (red and green In Section 6.4.3 of this chapter, we
lines) presentation through partial TEM-waves demonstrated that single mode WR
works as a filter permitting to propagate
one and only one set of partial waves that according to (6.20) incidents at the angle cos =
2 (red lines). All other waves of different incident angle (blue dotted lines) are evanescent,
⁄
exponentially decay from excitation point (check (6.35)) and are stored as the reactive energy
around the dipole. Looking back at Figure 6.7.4b we clearly see that the dipole generates two
dominant modes: the first runs to the left and the second to the right over WR. The latter one
reaches the end metal wall and reflects back (green lines) moving in the same direction as the
first one. Evidently, we are interested to get these two wave in phase thereby increasing the
power transferred from the coaxial line into waveguide. Loosely speaking, the green dominant
mode at first passes the distance L getting the phase shift = , then reflects from the end metal
wall with the phase shift = (see (6.17)) and finally comes back to the dipole with the extra

20 Public Domain Image, source:
http://www.vectortele.com/product_VT40WCANKPA_WR229_Waveguide_to_Coaxial_Adapter.htm.
The drawing was edited to remove the trace of proprietary information.