On Aug 6, 8:06 am, "Richard Fry" wrote:


A NEC-2 model of a 1/4-wave vertical monopole (base at earth level) in the
AM broadcast band using just four, 1/4-wave radials elevated 12-15 feet
above a perfect earth shows a peak h-plane gain within tenths of a decibel
of the theoretical peak value for a 1/4-wave monopole over a perfect ground
plane, ie, at least 5 dBi.


Dunno. I'll have to ponder that a while.. But something doesn't seem
right to
me.. My red flag is going off.. By "perfect ground plane", I assume
you mean
120 radials? I have a hard time seeing 4 low elevated radials within 1
db
of 120 buried radials over real ground. I realize the ratio should be
equal
if converted to "perfect ground" but still, this just doesn't seem
right to me.
I know I've never seen results like that here on real earth. I've
tried four
slightly elevated radials quite a few times on 160m, and I've never
had the
illusion of performance nearly equaling 120 on the ground.
In fact, I've heard of a few others that complained how lousy that
type
of system worked overall, and went to many more buried radials with
better results.
Lets say the two were over poor earth. I would think the 120 radial
system would still be pretty low loss, but the 4 radial system quite
stunted
in comparison. I have a hard time seeing them within a few tenths of a
db
of each other. I dunno if I trust that particular modeling... :/
MK

wrote
Dunno. I'll have to ponder that a while.. But something doesn't seem
right to me.. My red flag is going off.. By "perfect ground plane",
I assume you mean 120 radials?

No, this assumes that the earth is an infinite, flat, perfect conductor --
in which case no radials are needed for a monopole radiator to reach its
theoretical performance. But careful measurements done by Brown, Lewis and
Epstein in 1937 in New Jersy (conductivity 4 mS/m or less) show that a
1/4-wave monopole using113 buried radials about 0.41 wavelengths each will
produce a groundwave field that is within a few percent of that when using a
perfect ground.

A 1/4-wave monopole using such a perfect ground has a peak h-plane gain of
5.15 dBi, and will generate a groundwave field of about 314 mV/m at 1 km
with 1 kW of applied power.

As a point of calibration, the FCC requires Class B regional AM stations to
generate a groundwave r.m.s. field of at least 282 mV/m for 1 kW at 1 km.
Using 120 each 1/4-wave buried radials with a 1/4-wave (or somewhat shorter)
monopole can do that for almost all real ground conditions.

Class A (50 kW full time) AM stations need to generate at least 362 mV/m
r.m.s. for 1 kW at 1 km -- which means they must use a radiator longer than
1/4-wave. Most of them use 195-degree radiators.

I have a hard time seeing 4 low elevated radials within 1 db
of 120 buried radials over real ground.

Understandable, but when properly done this is the case, as described
in the paper I linked to earlier. Buried radials behave much differently
than elevated ones. NEC models also show this.

RF