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 would assert that if you're running a dipole, you're not obsessing
about the last milliBel of gain.
So then, you're just at the problem of feeding a reactive load with
30-300j impedance, and a high Q inductor will help nicely with that. A Q
of 100 should be easy (3 ohms out of 300), and would be 10% loss (0.5
dB?) Maybe that's where the "1dB less gain for the short antenna" comes
from: 1/2 dB from the pattern being slightly broader and 1/2 dB from
loss in the matching/loading components.
You could probably tap the inductor to get a 50:30 impedance transformation.
The resulting antenna WILL be narrower band than a dipole (but again, a
75% length antenna isn't hugely reactive... it's not like a 1 meter
compact loop or a 7 ft whip on 40 meters, where the VSWR bandwidth is
10s of kHz.
NEC doesn't model that very well, treating the inductor
as a lumped element, with no change in the current distribution across
it. NEC probably would model linear loading pretty well if the model
accurately described the antenna.
True enough, and linear (or distributed) loading might get a more flat
topped current profile, which will have MORE gain than a dipole (just as
a aperture with uniform illumination has more gain than with a tapered
illumination)
73, Jim K9YC
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