Thanks to James M0BMU for publishing the results of measurements of UR67
coaxial cable at 100 kHz. The very low Q readings are convincing
evidence that using coaxial cable for LF inductive stubs would be rather
inefficient. I can confirm that well made loading coils can have a Q of
at least 300 at 180 kHz.
The tests also verify that the use of coaxial cable with open circuit
far end (as a capacitor) would be high Q at LF, however there are not
many LF transmitting applications where the voltage rating would be
sufficient. For a loaded vertical antenna, the voltage on the "upwire"
(at the "hot" of a loading coil) can be rather high. For a given
antenna capacitance it is generally better to add more turns to the
inductor to resonate the antenna, rather than add parallel capacitance
(added capacitance draws some reactive current, but does not assist
radiation). The inductor can be wound to have fairly high volts per
turn, so the whole inductor can withstand much higher RF voltage than
could a capacitor made of coaxial cable. If a given loading coil does
not quite have enough inductance to resonate with the antenna, it would
be better to add more wire to the top loading.
Bob ZL2CA
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