Mike,
"...a 300 ohm TV twin lead dipole is not all twin lead, but the twin lead is
shorter than a half wavelength and at each end a wire continues to make
half wavelength length of the dipole..."
This technique could be applied to shorten a half-wave dipole by using the
twinlead as a T-match. Some quick simulations with NEC2 show that the
slower velocity of propagation in the twinlead allows it to replace an
open-wire structure of substantially greater spacing. The twinlead would be
fed in the middle of the of the lead to which the "extensions" were NOT
connected and series capacitance would be inserted in each arm of the "T"
to compensate for the inductive reactance of the match. Depending on the
construction and power levels, the matching caps could be required to
withstand substantial RF voltages.
Like all such shortened antennas, the SWR bandwidth is reduced considerably
compared to a full-sized structure.
Brian K1LI
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Topband reflector - topband@contesting.com
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