3-20. Uniform    capacitance    throughout the  length  of  the  line  is  an advantage   of   which   of   the following     transmission     lines? 1. Coaxial    line 2. Twisted    pair 3. Shielded    pair 4. Two-wire   open   line 3-21. What   is   the   primary   advantage of  a  rigid  coaxial  line? 1. Low    radiation    losses 2. Inexpensive construction 3. Low    high–frequency    losses 4. Easy    maintenance 3-22. The   most   efficient   transfer   of electromagnetic    energy    can    be provided   by   which   of   the following    mediums? 1. Waveguides 2. Twin–lead    flat    lines 3. Single-conductor     lines 4. Coaxial    transmission    lines 3-23. Copper    I²R   losses   are   reduced by   what   physical   property   of waveguides? 1. Small   surface   area 2. Large   surface   area 3. Shape   of   the   waveguides 4. Waveguide    material    being used 3-24. In   a   coaxial   line,   the   current- carrying   area   of   the   inner conductor   is   restricted   to   a small   surface   layer   because   of which   of   the   following properties? 1. Skin    effect 2. Copper    loss 3. Conductor    density 4. Waveguide    material    being used 3-25. Which   of   the   following dielectrics   is   used   in waveguides? 1. Air 2. Mica 3. Insulating    oil 4. Insulating    foam 3-26. Which   of   the   following characteristics    of    a    waveguide cause    its    lower–frequency limitation? 1. I²R  loss 2. Physical    size 3. Wall    thickness 4. Dielectric    loss 3-27. At   very   high   frequencies, ordinary   insulators   in   a   two- wire    transmission    line    display the   characteristics   of   what electrical    component? 1. An    inductor 2. A   resistor 3. A    capacitor 4. A    transformer 3-28. At   high   frequencies,   which   of the   following   devices   works best   as   an   insulator? 1. An   open   half-wave   section 2. An    open    quarter–wave section 3. A   shorted   half–wave   section 4. A    shorted    quarter-wave section 3-29. The   range   of   operating frequencies   is   determined   by which   of   the   following waveguide     dimensions? 1. The    widest    (height/width) 2. The    narrowest    (height/ width) 3. The   shortest   (length) 4. The   longest   (length) 3–30. The   cutoff   frequency   for   a waveguide   is   controlled   by   the physical   dimensions   of   the waveguide   and   is   defined   as   the frequency   at   which   two   quarter wavelengths    are 1. shorter  than  the  “a” dimension 2. shorter   than   the   “b” dimension 3. longer  than  the  “a” dimension 4. longer  than  the  “b” dimension 13