I enjoyed your ground loss comments, and mostly agree with
> The Sigma eliminates 1) by just not using the ground as 1/2 the
> circuit. Feeds the top half against the bottom half.
I'm not speaking of any antenna brand in particular, but only of
system in general. there is no magic bullet. But here are some
Making a dipole out of a Marconi-feed structure of given dimensions
quadruples the amount of loading reactance required, all things
equal. That quadruples loss in a loading system if that system has
equal Q! An additional shortfall is bandwidth becomes narrower
while efficiency of the loading system decreases for a given system
efficiency! Bandwidth can actually become more restricted while
For example, a 30 foot tall one inch diameter vertical has a loop
radiation resistance of 5.2 ohms (zero ground resistance
considered for this example) and a feedpoint reactance of 400
ohms at 3.6 MHz.
The same structure center-fed and isolated from ground has a loop
radiation resistance of 4.78 ohms and a feedpoint reactance of
about 1600 ohms.
Since we need four times the reactance, for a given Q we would
have four times the loss resistance. We would also have about 1/4
the bandwidth even though loss increased four times!
We must be VERY careful to keep loading system Q high in a
ground-independent vertical of the same height as a Marconi-fed
system. Linear loading would be a poor bet for doing that, because
Q's are normally in the double-digit numbers even with large
A few west coast amateurs learned that playing with linear loaded
yagi's on 75 meters.
> If you also want to run high power on such a solution, some vertical
> solutions have ohmic losses in the antenna (due to design and/or
> aging) and have been known to literally melt down (been there, done
> that...). The Sigma is efficient, and won't melt down. It's not the
> only one.
"Melting down" is a poor example of loss, unless you confine the
heat to a small area where you can measure it. One way to do this
would be to enclose the whole antenna in a thick styrofoam box
and measure temperature rise, or we could measure field strength
and input power over perfect ground.
Distributed loading systems (such as linear loading) CAN be very
inefficient and not show the slightest sign of heating. Loss is
distributed over a large air-cooled area with good thermal
A small inductor such as a traditional 2" #12 gauge miniductor can
"cook" from as little as 30 watts of heat. A physically-long air-
exposed aluminum tube could easily dissipate many hundreds of
watts of heat without any noticeable effects.
The only way we know if efficiency is actually high is to measure
the FS in a A-B test. We won't know that from reading an
advertisement, guessing, or watching for melt-downs.
73, Tom W8JI
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