The comments on ground systems are very interesting and I have some
contributions:
For a single vertical LF transmitting antenna, the most natural path for
a radiating current is perfectly radial to the insulated base of the
vertical, across the surface of the ground. Placing bare wire on or
just under the ground, as so-called "radials" is a proven way of
improving radiation efficiency. The problem is in the number and length
of radials, which generally gets beyond the scope of an amateur antenna.
For a typical amateur LF vertical antenna, the first wires in are the
"best value" and they generally improve efficiency no matter how ideal
or non-ideal they are placed. Metal stakes at the far ends are also
likely to "earn their keep". On adding more and more "ground
connections" there is a situation of diminishing returns (probably
logarithmic?) and also situations can arise where natural current paths
(perfectly radial) may conflict with forced current paths by wire
radials installed in practical ways that involve corners, bends, etc,
such as dodging a house or concrete yard!
I have a practical situation to report where actual measurements were
witnessed by myself and Kevin ZL4MD, when I was visiting his Central
Otago QTH in February. Kevin has a farm, so has opportunity to have a
bigger T antenna and longer radials than many others who experiment on
LF. He had run lengths of surplus power line out as radials, from a
central earth node at the base of the vertical. The radials were on the
surface or slightly below ground. He also could connect to the mains
earth via a low loss large value polypropylene capacitor, with the
intention to connect the electricity mains earth in parallel with the RF
earth. We did tests for antenna resistance (with current and voltage
probes on a directional coupler, as well as line current checks) and the
result we got was not what we expected. Connecting the mains earth to
the RF earth INCREASED THE RF RESISTANCE OF THE ANTENNA. We re-checked
several times, and also checked that the voltage across the capacitor
was low (which it was). So there was no mistake in the experimental
result. In that case, it was not a good move to connect the mains
earth.
The explanation is most likely that the path of the mains earth is back
to Kevin's house (the shack is some 50 metres away), with driven earth
there, then taking an oblique path back via the powerline to the
distibution power line, far from being in a radial direction. This
means that RF forced to flow via that mains earth is far from the ideal
radial direction, and conflicts with the current departing via the
intended RF earth with radially laid bare wire. Nature's answer of the
current conflict is to "increase the system resistance". This could
also be thought of as a sort of "shorted turn" in the ground system.
This type of event arises only when the radial system is getting fairly
good anyway, but serves as a reminder that it is not always best to bond
all grounds together.
Regards,
Bob ZL2CA
|
