On 3/17/16 2:54 PM, jimlux wrote:
On 3/17/16 12:40 PM, Jim Brown wrote:
On Thu,3/17/2016 12:14 PM, jimlux wrote:
There's no big advantage to linear loading: you might as well use a
good low loss inductor at the feed (the "shorty 40" does this).
Are you certain about this, Jim? Both change the current distribution,
but linear loading changes it least at the center, where current is
greatest. The inductor places maximum current in the inductor, which
doesn't radiate.
I agree.. It's like base loading a vertical (with a top hat perhaps) vs
loading it half way up. BUT, a 30m dipole isn't that much shorter than
a resonant 40m dipole, so the current distribution is nicely modeled by
the usual "half a sine wave from end to end". That current distribution
doesn't have significantly different gain (1.93 dBi vs 2.14 dBi).
However, if a infinitely small dipole is 1.5dBi and a full size dipole
is 2.15dBi, the gain is going to be somewhere in the middle.
I just ran a NEC model (using 4NEC2 to compute the matching network)..
the 15 meter long (30m band) dipole with a T network (91nH series, 352pF
shunt, 7.47 uH series) has 1.75 dBi gain (vs 2.14dBi for ideal dipole).
The 2:1 bandwidth is a bit more than 300 kHz; 1.5:1 bandwidth is about
150 kHz.
I leave it as an exercise for the reader to figure out the voltages and
currents on the components (which was the original question)
So, in summary
full size 40m dipole: 2.15 dBi gain
75% size dipole: 1.93 dBi gain
75% size dipole with coil Q 250, cap Q 1000: 1.75 dBi gain (about
80-84 degree 3dB beamwidth)
the 40 m full size antenna has a swr of 1.4:1 at best and a 2: 1 BW of
about 400 kHz (not much wider than the 75% sized antenna with a matching
network). And the match is better with the matching network (50 ohms vs
72 ohms.
The beamwidth of the full size is slightly narrower (76-80 deg)
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