Several folks responded to my original post, suggesting that I should view
the folded inverted-L construction as a single wire construction fed with a
4:1 transformer. Looking at the system through such a transformer causes
the 37 ohm ground loss to appear as 150 ohms.
I agree that that ground loss is increasing by a factor of 4, as does the
feedpoint impedance. But it’s not clear to me that the 4:1 transformer is
an apt explanation.
A folded antenna isn’t electrically equivalent to a single wire antenna fed
through a 4:1 transformer. A 4:1 transformer doesn’t change the current in
the antenna (assuming fixed power). Yet the current in a single-wire
inverted-L is different than the current in a folded inverted-L, so the
transformer analogy seems to break down.
(With 100W input to a single wire inverted-L over perfect ground (with its
13 ohm feedpoint impedance) causes a base current of 2.77 amps to flow. In
contrast, the same 100W input to a folded inverted-L over perfect ground
(with its 50 ohm feedpoint impedance) causes a base current of 1.41 amps to
flow.)
“Warming the worms” loss increases with return current. And return current
starts out as antenna current. Since the antenna current in the single wire
antenna is twice that in the folded antenna, it stands to reason that the
ground loss is greater in the single wire construction. This is in accord
with my understanding of Sevick's teachings (i.e., low impedance antennas
have more ground loss problems).
Thinking the folded antenna vs single wire antenna might be confusing the
issue, I tried another EZNEC analysis. I modeled a full-size, 135' vertical
over perfect ground and got a feedpoint impedance (i.e., radiation
resistance) of 39 ohms. I then modeled the same size vertical over a simple
radial system (4 90' radials) and got a feedpoint impedance of 48 ohms
(radiation resistance plus ground loss). Following the usual analysis, this
indicates the simple radial system contributes 9 ohms of ground loss to the
feedpoint impedance.
I then tried shortening the vertical to 100', and resonating with a series
inductor. Over perfect ground, this short antenna has an input impedance of
19 ohms. When I resonated the same short antenna over the same simple
system of radials, I expected to see an input impedance of about 28 ohms (19
ohms of radiation resistance, plus 9 ohms of ground loss).
That's not what I saw. Instead, I found an input impedance of 23.5 ohms.
Following the usual analysis, this indicates the radial system contributes
4.5 ohms of ground loss.
Just as with the first EZNEC model (folded vs not-folded inverted Ls), this
modeling with conventional verticals indicates that the effective ground
resistance is not a fixed value, but instead scales proportionately with
radiation resistance. Double the feedpoint impedance, and get double the
apparent ground resistance.
Again, this seems contrary to Sevick's notion that as a vertical is
shortened, it requires a better radial system, since the (fixed) ground loss
becomes larger in comparison to the antenna's radiation resistance.
Instead, EZNEC indicates that the ground loss resistance isn't fixed, but
varies with radiation resistance.
So let's table the discussion about folded antennas. In the case of a
simple vertical, where shorter height corresponds to lower radiation
resistance, why does the effective series ground loss resistance introduced
by a poor radial system appear to decrease with decrease in radiation
resistance, instead of being constant?
tnx/73,
/Bill, K2PO
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