Rik, ON7YD wrote:
A question to Dick :
A. Antenna plus counterpoise in free space: radiation resistance 17.8
milli-ohm
Did you calculate the capacitance of this 'free-space' model ?
I ask this because further you mention that the antenna against ground had
a capacitance of 176pF while the antenna against the counterpoise has a
capacitance of 74pF.
I wonder of this 74pF is completely a 'direct' capacitance between antenna
and counterpoise or there is also a 'antenna to ground to counterpoise'
component.
This last component could be very 'lossy' (and unwanted).
The fact that there is so little difference in antennacapacitance between
the counterpoise direct under the antenna and the counterpoise opposite to
the antenna could suggest that the main part of the antennacapacitance is
not direct antenna - counterpoise but first antenna to ground and then
ground to counterpoise.
The antenna plus counterpoise in free space shows a capacitance of 72,4 pF.
The same system over perfect ground has a capacitance of 74 pF.
So one may tend to conclude that 74 - 72.4 = 1.6 pF is added to the
capacitance (antenna - counterpoise) by the two capacitances (antenna -
earth) and (counterpoise - earth) in series. But this cannot explain the
large influence of the ground constants on the total loss as this earth
loss resistance is only in series with the (counterpoise - earth)
capacitance.
I think the mirror effect of the earth completely modifies the field
line pattern, invalidating the conclusion above.
Mike, G3XDV mentioned in his mail that with counterpoise he had much less
antennacurrent. He also said that both antenna and counterpoise had their
own loadingcoil and both seperately were tuned to ground. This procedure
should favour the current from the antenna to ground and then from ground
to counterpoise, instead of a direct antenna-counterpoise current. This
could explain the low current.
Maybe it would be better to tune counterpoise and antenna against each
other (if they each have their own coil) or just use 1 big coil as Dick
suggests.
73, Rik ON7YD
As long as the system of antenna plus counterpoise is not connected to earth
it should make no difference whether antenna and counterpoise each have
their own coil or a single coil is used. The system as a whole is tuned to
resonance. Combining the two coils is advantageous however because a single
coil needs less turns to obtain a certain inductance than when turns are
divided
over two coils. The reason is the mutual inductance between turns that
comes in as extra. So a single coil requires less wire and has lower
loss than two separate ones in series.
As to the points raised by David, G0MRF: "Counterpoise" is already very old
and dates back to the beginning of radio. I think "radial" came into use
when the ground plane antenna appeared on the scene. Both have the same
function of replacing an earth connection. But it in case of the ground
plane the dimensions of radiator and radials are of the order of a quarter
wave, making the system resonant in itself and with a radiation resistance
of tens of ohms, resulting in low Q and relatively wide bandwidth. The
LF-antenna plus counterpoise however is a very high Q system and as a
consequence has a very narrow bandwidth; also very high
voltages are involved.
"Can the system with antenna top wire and counterpoise wire in opposite
directions be considered as a dipole?" David also asked.
One could call it a dipole but it won't radiate like one. On LF it is the
vertical polarized electric field component that is radiated as the far
field and that is generated by the current in the vertical leg of the
antenna.
I checked the radiation resistance of the complete system in case of top
wire and counterpoise wire in the same and in opposite directions. The
values hardly differ (1.8%), which is also an indication that it is only
the vertical leg that does the radiation.
To understand how the system works it may perhaps help to visualize it
as a vertical radiator with a tuning coil to which a top wire and a bottom
wire
(the counterpoise) are added as capacitor plates;
the whole thing operating as a tuned circuit.
73, Dick, PA0SE.
| 