Hi Joe,
> The 31' antenna would be about 21 degrees. Based on the chart in the
> NAB Handbook, it would have a radiation resistance net of any matching
> network (folded diploles, linear loading, stubs, etc. are matching
> networks) of around 5 Ohms. With the heavy top loading that might
> increase to around 10 Ohms (without looking it up, I believe W2FMI
> showed one could approximately double the radiation resistance of a
> short vertical with heavy top loading.
Accelerating charges cause radiation. If an antenna has half the
length, we need twice the charge acceleration to radiate the same
power. If we move twice the charges past a given point, we have
twice the NET EFFECTIVE CURRENT flowing in the radiator. Twice the
current at a fixed power means HALF the voltage. Twice the current
and half the voltage means 1/4 the radiation resistance and four
times the power loss for a given base radial resistance.
If the current shape is triangular (base loading and small uniform
cross section radiator) the top of the radiator contributes nothing at
all. The effective height is HALF as tall as the same radiator with
uniform current. The radiation resistance is 1/4 as much, since the
effective height is 1/2 as tall. Ground terminal loss, for the same
height, is four times greater.
Folding the wires does NOT change any of this. The net current
flowing into the ground has to equal the total charges moved upwards
in the antenna.
Standing back away from the radiator, you would have to "see" twice
the charge acceleration. It is the vector SUM of the movement in all
the wires that causes this effect. The radial system has to carry the
effective net current of ALL the wires returning to the ground
system, not the current at the one point in the system where the
feedline connects.
If I parallel four vertical wires (each carrying one ampere), and
feed only one, the feedpoint resistance increases 16 times. But ALL
FOUR WIRES return to and couple current to the earth connection. The
total earth connection current is FOUR amperes, not one ampere. And
it remains four amperes no matter what I do with the feedpoint.
If I tie these same wires in parallel and feed them as one, earth
connection current is four amperes even though the feedpoint
resistance is now 1/16th as large!
Any claim to the contrary is pathological science.
With a top loaded uniform current 21 degree tall vertical loop (or
base) radiation resistance will be about 5 ohms (you can find this on
page 19-4 of Jasik "Antenna Engineering Handbook".
With base loading the loop (or base) radiation resistance is about
1.3 ohms. Since this is a physical spatial area problem, and current
distribution problem, there is NO way to violate the 5 ohm limit.
> >What if I told you that a 39' antenna would produce a 292 millivolt
> >per meter field at 1 Kw at 1 kilometer? Would you believe that? The
> >FCC did, and has type certified the CTSVR for commercial AM broadcast
> >use.
I see this argument used over and over again to support wild
theories. It is up to the END USER to do a proof of performance. I've
seen salesmen do proofs that could never be duplicated.
The question in my mind is why the BCB antenna needed and used 120
radials, when Hams were (incorrectly) told a folded element reduces
ground system requirements.
Claiming the antenna is "very efficient" and propagating a theory
that increased radiation resistance greatly reduces ground losses,
and then using a different antenna with a much larger ground system
as "proof" really proves nothing at all.
73 Tom
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