a 5/8 antenna is incapable of self resonance (X ≠ 0) because of the excessive level of capacitive reactance present at the feedpoint and requires an L network at the input to dump the reactance and provide an input impedance compatible with 50 ohm feedline. 3/8 wl., 1/2 wl,, 5/8 wl., 7/8 wl, & 1 wl. radiators are all examples of antennas that are not capable of self-resonance.
the radiation pattern of a full size 3/4 wl. vertical gm monopole differs quite a bit from the pattern of a 5/8 wl. vertical.
"which factor defines a resonant antenna, physical length or electrical length ?"
the difference between the two when speaking of a self resonant 1/4 wl. vertical that is electrically 90 degrees is very small, any variations are caused by differences in the length to diameter ratio (physical size) of the conductor, other variables having only minor effects.
here's a real life scenario covering some other variables.
given:
100W into a 3/4 λ gm vertical monopole with a total length of 8.281419 meters and a diameter of 38.10 millimeters designed to operate @ resonance @ a frequency of 27.205 mhz. over 16 radials with a length of 0.7 meters and a diameter of 3.175 millimeters over average ground.
0.752 λ @ 27.205
feedpoint input impedance: 70.7 ohms
feedpoint reactance: jX = 0.0 ohms,
load is purely resistive.
loss in T match coil: 0.6% (not used)
loss in antenna conductor: 0.1%
loss in soil vicinity: 0.0%
loss in radial system: 3.4%
ground electrode system:
dc resistance: 42.1 ohms
rf resistance: 2.4 ohms
rf reactance: 0.0 ohms
power radiation efficiency: 95.96%
loss relative to ideal system:
0.2 dB. or 0.0 S-Units
confirm radiation efficiency:
total resistance - loss resistance = radiation resistance
(impedance at resonance is purely resistive)
70.7 ohms - 2.4 ohms = 68.3 ohms radiation resistance
radiation efficiency = radiation resistance / radiation
resistance + loss resistance (total loss)
2.4 ohms of rf resistance is part of the 70.7 ohms of
total antenna resistance.
68.3 / 70.7 = 0.9660 = 96.60%
96.60 - 95.96 = .64. since the
T match used by default is not
needed the discrepancy between
the two figures is resolved to .04%,
i.e., 96.56 - 96.60%
since the T match is not used the swr
on the feedline @ both the transmitter
and the antenna is 1.414:1 @ 27.205 &
equal to or less than 1.5:1 from 26.915
to 27.465 mhz..
the importance of resonance in an antenna system:
1.optimizes current in the antenna. (see #3)
2.makes the antenna easier to feed.
3.indicates frequenc/y/ies where the load represents a pure resistance.
4.establishes the center of the band in reference to operational bandwidth
5.tracking changes in antenna systems due to changes, weather, etc..
6.provides a point from which antenna "Q" can be calculated
references:
James F. Murray introduction to the concepts of energy resonance
https://duckduckgo.com/?q=antenna+resonance+in+high+performance+receivers+&t=h_&ia=web
