A counterpoise is what we do when the full size of a double-ended antenna,
dipole, OCF, etc is too large for us to build, maintain, etc. Very simply,
we want to jam the energy from the shield of our coax into the
counterpoise, and the energy from the center conductor into the radiating
element, the vertical, T, inverted L, etc, the aerial wire. Then we want to
get all that energy back from the counterpoise, none lost if possible, at
the phase reversal. Any you don't get back is mostly outright loss. With
commercial high grade radials you can show that the effective series
resistance of the counterpoise is 1/2, 1/3 or sometimes even 1/10 of an
ohm. That means that the aerial wire is radiating something like 50, 100
times the energy lost/radiated by the radials' connection to ground.
The two current destinations taken together MIMIC a circuit, because the
current into the counterpoise is the same, but opposite polarity as the
current into the radiating part of the antenna. If the currents are equal
and opposite, it looks like a circuit, walks like a circuit, quacks like a
circuit. You can model it with a fake circuit, and use Maxwell's equations
for circuits to predict what is gonna happen. There is no magic
circulation, just the ability to convince the coax it is hooked up to a
circuit. With the massive parallelism of a commercial grade radial field,
the radial's electrons are coupled into the ground as a reservoir, with the
push back from extra or missing electrons that will return the current when
the phase reverses. The more radials, the more even the radials, the longer
the radials, the lower the power lost to current through resistive
materials, lost to dielectric loss in dielectric materials, lost to
resistance in the wire. Not perfect return, but a nice, high percentage
return.
In free space, it is possible to construct a counterpoise that NEC4 can
accurately predict will radiate power to the far field at a rate 30 dB
below the RF current's energy. The essential loss is in the RF resistance
of the wire. You are talking about a counterpoise that is 98 or 99+ percent
efficient in free space.
We are not interested in a counterpoise radiating, or invoking loss in the
environment. Talking to the counterpoise, I'm telling it I'm giving it this
pile of energy. A half cycle from now I want it all back. No skimming off
the top. Maybe just a skoch.
A commercial quality radial field beneath a vertical is deliberately
intended to be non-radiating. Looking at the current around the base of the
vertical, the current to the east is exactly the opposite of the current to
the west, as are to the north and south, as are all opposite radial pairs,
therefore the fields generated are opposite, intended to be net zero in the
far field. That's on purpose, pretty much true, and exactly what the
engineers had in mind.
It is easy to show that there are unfortunate ham designs and
implementations of the counterpoise/aerial concept where not even 10
percent of the power is radiated skyward. That is the 160 meter two ton
elephant in the room that gets ignored an awful lot of the time.
73, Guy K2AV
On Mon, Mar 22, 2021 at 9:42 PM Adrian <vk4tux@gmail.com> wrote:
> Recycling signals at the feedpoint sounds more like a mismatch swr
> situation.
>
> The ground radials form half of the antenna, and that radiation from
> ground, in phase with the vertical radiation determines the field
> strength and take off angle.
>
> vk4tux
>
>
> On 23/3/21 11:31 am, Charles Moizeau wrote:
> > I feel it more appropriate to say that the function of ground-mounted
> radials is to harvest the radiation from a vertical antenna that splashes
> on the ground and return it to the feed point for recycling.
> >
> > 72/73,
> >
> > Charles, W2SH
> >
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