Dear LF Group,
Over the last few days I have been doing some experiments with ground
systems with my LF/MF antenna. The purpose was partly to see whether it is
possible to reduce the overall loss resistance significantly, but also to
test out some convenient types of ground for antennas for portable
operation, in particular radial wires. The antenna is currently a top-loaded
vertical about 10m high, with about 80m in total of horizontal wires in a
narrow, asymmetrical "Y" shape about 50m long, giving a capacitance at 136k
of around 500pF, and around 580pF at at 500k due to the increased effect of
the distributed inductance of the wires. As a reference, I used the normal
ground system, consisting of 6 x 1m ground rods, distributed within a couple
of metres of the base of the antenna tuners.
The measuring technique was to initially set the normal series loading coils
to resonance at 136kHz and 502khz with the normal ground system, and then
measure the antenna resistance between the "cold" end of the loading coil
and ground with an RF bridge. The total loss resistance of the antenna
(including the loading coils and the normal ground system) was 57ohms at
136k and 24ohms at 502k. Then an alternative ground system was connected
instead of the normal ground, the bridge re-tuned to measure the new
resistance, and any change in reactance determined from the change in
resonant frequency. The reactance measurement is only approximate, but good
enough for these purposes. The bridge equipment was all battery operated,
and sat on a wooden table, so had minimal effect on the impedance of the
ground system. When I set up a similar antenna with similar ground rods in a
field a while back, the loss resistance of the antenna was only about 8 ohms
in total, so the component of the loss resistance due to the normal ground
system is probably less than 8 ohms. The much higher total loss resistance
is due mostly to the environmental factors affecting the antenna.
I first tried combinations of insulated radial wires laid on the ground,
including 4 x 20m radials, 4 x 40m radials and 8 x 20m radials. It did not
seem to matter much how the radials were laid out, whether underneath the
top load wires or in completely different directions. Some had bends or
doubled back - there is not really room at my QTH for 40m radials! Some were
laid on the grass, others along the concrete driveway that runs down one
side of the plot. None of this made more than a few ohms difference to the
loss resistance, provided the wires were reasonably well spaced apart. All
configurations had significant capacitive reactance compared to the normal
ground rods:
4 x 20m radials, 136k: R = 80ohm , X = -j466ohms
4x 20m radials, 502k: R = 28ohm, X = -j113ohms
4x 40m radials, 136k: R = 66ohms, X = -j248ohms
4 x 40m radials, 502k, R = 25ohms, X = -j61ohms
8 x 20m radials, 136k: R = 62ohms, X = -j283ohms
8 x 20m radials, 502k: R = 23ohms, X = -j76ohms
So it would seem that having the total of 160m of wire as 8 shorter radials
is slightly better than 4 longer ones as far a loss goes. I also tried
elevating the radials above ground about 0.3m on lengths of cane:
8 x 20m radials, elevated, 136k: R = 80ohms, X = -j622ohms
8 x 20m radials, elevated, 502k: R = 23ohms, X = -j158ohms
The loss resistance at 502kHz is slightly reduced, but the extra capacitive
reactance is now getting very high, especially at 136k. I tried adding a
"metallic" ground connection by adding a 200mm long steel tent peg to the
end of each radial, and driving it into the ground:
8 x 20m radial + ground spike, 136k: R = 80ohms, X = 0
8 x 20m radial + ground spike, 502k: R = 69ohms, X = -j11ohms
So this is effective at reducing the reactance, but does not help loss
resistance; in fact, it leads to a drastic increase in loss at 502k.
Instead of insulated radials, I tried a 15m x 0.6m strip of wire mesh (a
roll of chicken wire that was lying around), held down flat on the grass
with bricks:
15m x 0.6m wire mesh, 136k: R = 82ohms, X = -j102ohms
15m x 0.6m wire mesh, 502k: R = 31ohms, X = -j9ohms
So in spite of having a large area of metal in reasonable contact with the
ground, there is still relatively high resistance, and some capacitive
reactance. Since the insulated radials in general resulted in lower loss
resistance, I tried insulating the wire mesh from the ground using polythene
sheet:
15m x 0.6m wire mesh, insulated, 136k: R = 65ohms, X = -j320ohms
15m x 0.6m wire mesh, insulated, 502k: R = 26ohms, X = -j63ohms
Since the wire mesh would be quite a convenient earth system to use for an
antenna over a paved area, or on rocky ground, I tried laying it on the
concrete driveway:
15m x 0.6m wire mesh, on concrete, 136k: R = 57 ohms, X = -j320ohms
15m x 0.6m wire mesh, on concrete, 502k: R = 23 ohms, X = -j65ohms
The loss resistance is now as low as the normal ground rod system, although
the reactance is higher.
Looking at these results, the ground rods win for the fixed station antenna.
The ground here is a permanently wet clay soil, which is probably quite good
for ground rods. They give practically the lowest loss resistance (a couple
of the other ground systems give a marginally lower loss resistance at 502k,
but in practice the actual resonant frequency was increased due to the
additional reactance, and the loss resistance of the antenna decreases at
higher frequency). The reactance is also lower. Most of the alternative
ground systems effectively behave as capacitive coupling to ground, with
significant capacitive reactance as well as some additional loss resistance.
For the fixed station, if the ground system has a high reactance, most of
the ground current will flow through the lower impedance path via TX
chassis/mains earth, instead of the ground system. This is possibly why
adding ground radials to an existing system often has no effect. One way
round this would be to have an auxilliary tuning inductor connected between
the ATU ground, and the ground radial sytem. You could tune the inductor for
maximum RF current in the radials, or minimum current via the TX ground.
For portable antennas, it is often difficult to drive long ground rods into
the earth. It seems that short ground spikes, like the tent pegs, or
metallic conductors laid on the surface of the ground, do not make a very
good ground connection. The insulated radial wire systems generally give the
lower loss resistances for the antenna system, and are quite easy to set up.
The effect of the unwanted ground system reactance for a portable station
with no other earth connection is that the ground terminal of the ATU is not
at ground potential, and so probably also the cases of all the station
equipment. For 500kHz stations with fairly low power (say <1A antenna
current or so), where reactance is quite low, this probably does not matter
much so long as the equipment (and operator) are reasonably well insulated
from ground. For a QRO 136kHz station, it is likely to be a different
story - with 1kW I can get about 4A antenna current - if I was using the 8 x
20m radials, the 283ohms reactance would mean the equipment ground would
have more than 1kV of RF relative to ground! The capacitance of the
insulated radials works out very roughly to 28pF per metre of wire, and one
wants to make the total capacitance large enough to keep the voltage down to
a reasonable level. More and longer radials would clearly be desirable,
which are not really feasible in my garden, but might be quite easy at a /P
location.
The insulated wire mesh ground required less area than the radial wires, and
had low loss resistance with similar reactance, so could be quite promising,
especially where the ground is already insulated; on concrete or tarmac for
example.
Hopefully some time this summer, I will be able to borrow a field again, and
do some further experiments with more space available.
Cheers, Jim Moritz
73 de M0BMU
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