Page 301 - Maxwell House

FEED LINE BASICS 281

again and again from the tube metal walls. That is the effect of the transverse resonance when
= but the energy stops its movement along the line, i.e. = 0 in (6.1). The wavelength

or frequency at which this occurs called cut-off wavelength or frequency.
6.1.4 Power Handling
Commonly, the power handling means the maximum of RF peak or average power that can be
delivered by a feed line without interruption or internal damage like electrical breakdown, line
element deformation or overheating. It is well-known that too high peak E-fields can cause
corona or even electrical arcing, especially around the sharp edges when the E-field exceeds a
critical value. Typically, the electrical breakdown field strength is around 1 - 3 MV/m for air
and depends on its pressure, humidity, initial ionization level and some other factors like
frequency. In a high vacuum, the breakdown threshold is around 20 – 40 MV/m. Many
dielectrics can survive high field strength too. For example, the critical threshold in Alumina
ceramic is 13.4 MV/m, Teflon 19 – 150 MV/m (depends on technology) and Diamond 2000
MV/m. These numbers should be used with caution. In the same manner, the average power
handling can be severely restricted by the excessive heating of line elements (heat breakdown)
especially dielectrics that can melt or even catch fire.

Some additional caution requires the situation when the line and its load and line and source
generator are mismatched. We described and quantized this effect in Section 3.4 of Chapter 3.
In this case, the multiple waves run back and forth along the line forming the standing wave
distribution, i.e. the series of the amplitude minimums called nodes, and the amplitude
maximum called antinodes. Since E-field intensity in antinodes might far exceed the primary
forward wave magnitude, the probability of the electrical breakdown increases that can severely
restricts the line power handling.

6.1.5 Attenuation
There are several main reasons why the energy dissipates in lines: conductive loss in metal
elements, loss in dielectrics, and radiation loss. The universal rule for conductive loss is quite
simple: the lower energy dissipation is inevitable if the electric current spreads uniformly along
bigger highly conductive surface diminishing its surface density. High-quality dielectrics with
low loss tangent (≤ 10 ) or their absence guarantees a smaller loss. The radiation loss depends
−3
on line and EM mode structure and requires special and specific considerations.

Some additional loss occurs when the line and its load are mismatched. We described and
quantized this effect in Section 3.4 of Chapter 3. Apparently, the reflected wave propagates
back to the source and undergoes additional attenuation. In the case of an imperfect match
between the source generator and line, the multiple waves run back and forth along the line
increasing the total attenuation of even more.
The following material in Section 6.2 and 6.3 is mainly introductory and devoted to the
classification of a wide variety of feed lines. So the informed reader can skip them and go
straight to the subsequent sections to pick up more data used for the practical design of
particular feed. Along the discussion, we paid the special attention to the images of EM-field
in line because they carry the relevant information about line dissipation and power handling.
As well, the field pattern knowledge is critical for the successful design of transitions between