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Date: Tue, 2 Jun 2009 11:23:18 +0100
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From: lawrence mayhead <laurie.mayhead@googlemail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: 136k/500k Grounding experiments (long!)
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Hi James.
Once again a very interesting set of experiments,which does seem to support
my findings that the Mains/water pipes take most of the current,
making all the work on radials earth rods etc a bit of a waste of time.
However I must try resonating the radial system. Presumably I would need
to series resonate with a capacitor, value depending of course on my
particular system..... but did you say that the reactance was capacitive ?
Regards
73 Laurie

2009/6/2 James Moritz <james.moritz@btopenworld.com>

> 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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Hi James.<br>Once again a very interesting set of experiments,which does see=
m to support my findings that the Mains/water pipes take most of the current=
,<br>making all the work on radials earth rods etc a bit of a waste of time.=
 However I must try resonating the radial system. Presumably I would need<br=
>
to series resonate with a capacitor, value depending of course on my particu=
lar system..... but did you say that the reactance was capacitive ?<br>Regar=
ds <br>73 Laurie<br><br><div class=3D"gmail_quote">2009/6/2 James Moritz <sp=
an dir=3D"ltr">&lt;<a href=3D"mailto:james.moritz@btopenworld.com">james.mor=
itz@btopenworld.com</a>&gt;</span><br>
<blockquote class=3D"gmail_quote" style=3D"border-left: 1px solid rgb(204, 2=
04, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Dear LF Group,<br>
<br>
Over the last few days I have been doing some experiments with ground system=
s with my LF/MF antenna. The purpose was partly to see whether it is possibl=
e to reduce the overall loss resistance significantly, but also to test out=20=
some convenient types of ground for antennas for portable operation, in part=
icular radial wires. The antenna is currently a top-loaded vertical about 10=
m high, with about 80m in total of horizontal wires in a narrow, asymmetrica=
l &quot;Y&quot; 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 distr=
ibuted inductance of the wires. As a reference, I used the normal ground sys=
tem, consisting of 6 x 1m ground rods, distributed within a couple of metres=
 of the base of the antenna tuners.<br>

<br>
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 m=
easure the antenna resistance between the &quot;cold&quot; end of the loadin=
g coil and ground with an RF bridge. The total loss resistance of the antenn=
a (including the loading coils and the normal ground system) was 57ohms at 1=
36k and 24ohms at 502k. Then an alternative ground system was connected inst=
ead of the normal ground, the bridge re-tuned to measure the new resistance,=
 and any change in reactance determined from the change in resonant frequenc=
y. The reactance measurement is only approximate, but good enough for these=20=
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=20=
set up a similar antenna with similar ground rods in a field a while back, t=
he loss resistance of the antenna was only about 8 ohms in total, so the com=
ponent of the loss resistance due to the normal ground system is probably le=
ss than 8 ohms. The much higher total loss resistance is due mostly to the e=
nvironmental factors affecting the antenna.<br>

<br>
I first tried combinations of insulated radial wires laid on the ground, inc=
luding 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 lo=
ad wires or in completely different directions. Some had bends or doubled ba=
ck - there is not really room at my QTH for 40m radials! Some were laid on t=
he grass, others along the concrete driveway that runs down one side of the=20=
plot. None of this made more than a few ohms difference to the loss resistan=
ce, provided the wires were reasonably well spaced apart. =A0All configurati=
ons had significant capacitive reactance compared to the normal ground rods:=
<br>

<br>
4 x 20m radials, 136k: R =3D 80ohm , X =3D -j466ohms<br>
4x 20m radials, 502k: R =3D 28ohm, X =3D -j113ohms<br>
<br>
4x 40m radials, 136k: R =3D 66ohms, =A0X =3D -j248ohms<br>
4 x 40m radials, 502k, R =3D 25ohms, X =3D -j61ohms<br>
<br>
8 x 20m radials, 136k: R =3D 62ohms, X =3D -j283ohms<br>
8 x 20m radials, 502k: R =3D 23ohms, X =3D -j76ohms<br>
<br>
So it would seem that having the total of 160m of wire as 8 shorter radials=20=
is slightly better than 4 longer ones as far a loss goes. I also tried eleva=
ting the radials above ground about 0.3m on lengths of cane:<br>
<br>
8 x 20m radials, elevated, 136k: R =3D 80ohms, X =3D -j622ohms<br>
8 x 20m radials, elevated, 502k: R =3D 23ohms, X =3D -j158ohms<br>
<br>
The loss resistance at 502kHz is slightly reduced, but the extra capacitive=20=
reactance is now getting very high, especially at 136k. I tried adding a &qu=
ot;metallic&quot; ground connection by adding a 200mm long steel tent peg to=
 the end of each radial, and driving it into the ground:<br>

<br>
8 x 20m radial + ground spike, 136k: R =3D 80ohms, X =3D 0<br>
8 x 20m radial + ground spike, 502k: R =3D 69ohms, X =3D -j11ohms<br>
<br>
So this is effective at reducing the reactance, but does not help loss resis=
tance; in fact, it leads to a drastic increase in loss at 502k.<br>
<br>
Instead of insulated radials, I tried a 15m x 0.6m strip of wire mesh (a rol=
l of chicken wire that was lying around), held down flat on the grass with b=
ricks:<br>
<br>
15m x 0.6m wire mesh, 136k: R =3D 82ohms, X =3D -j102ohms<br>
15m x 0.6m wire mesh, 502k: R =3D 31ohms, X =3D -j9ohms<br>
<br>
So in spite of =A0having a large area of metal in reasonable contact with th=
e ground, there is still relatively high resistance, and some capacitive rea=
ctance. Since the insulated radials in general resulted in lower loss resist=
ance, I tried insulating the wire mesh from the ground using polythene sheet=
:<br>

<br>
15m x 0.6m wire mesh, insulated, 136k: R =3D 65ohms, X =3D -j320ohms<br>
15m x 0.6m wire mesh, insulated, 502k: R =3D 26ohms, X =3D -j63ohms<br>
<br>
Since the wire mesh would be quite a convenient earth system to use for an a=
ntenna over a paved area, or on rocky ground, I tried laying it on the concr=
ete driveway:<br>
<br>
15m x 0.6m wire mesh, on concrete, 136k: R =3D 57 ohms, X =3D -j320ohms<br>
15m x 0.6m wire mesh, on concrete, 502k: R =3D 23 ohms, X =3D -j65ohms<br>
<br>
The loss resistance is now as low as the normal ground rod system, although=20=
the reactance is higher.<br>
<br>
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 goo=
d for ground rods. They give practically the lowest loss resistance (a coupl=
e of the other ground systems give a marginally lower loss resistance at 502=
k, but in practice the actual resonant frequency was increased due to the ad=
ditional reactance, and the loss resistance of the antenna decreases at high=
er frequency). The reactance is also lower. Most of the alternative ground s=
ystems effectively behave as capacitive coupling to ground, with significant=
 capacitive reactance as well as some additional loss resistance. For the fi=
xed station, if the ground system has a high reactance, most of the ground c=
urrent will flow through the lower impedance path via TX chassis/mains earth=
, instead of the ground system. This is possibly why adding ground radials t=
o an existing system often has no effect. One way round this would be to hav=
e an auxilliary tuning inductor connected between the ATU ground, and the gr=
ound radial sytem. You could tune the inductor for maximum RF current in the=
 radials, or minimum current via the TX ground.<br>

<br>
For portable antennas, it is often difficult to drive long ground rods into=20=
the earth. It seems that short ground spikes, like the tent pegs, or metalli=
c conductors laid on the surface of the ground, do not make a very good grou=
nd connection. The insulated radial wire systems generally give the lower lo=
ss resistances for the antenna system, and are quite easy to set up. The eff=
ect of the unwanted ground system reactance for a portable station with no o=
ther earth connection is that the ground terminal of the ATU is not at groun=
d potential, and so probably also the cases of all the station equipment. Fo=
r 500kHz stations with fairly low power (say &lt;1A antenna current or so),=20=
where reactance is quite low, this probably does not matter much so long as=20=
the equipment (and operator) are reasonably well insulated from ground. For=20=
a QRO 136kHz station, it is likely to be a different story - with 1kW I can=20=
get about 4A antenna current - if I was using the 8 x 20m radials, the 283oh=
ms reactance would mean the equipment ground would have more than 1kV of RF=20=
relative to ground! The capacitance of the insulated radials works out very=20=
roughly to 28pF per metre of wire, and one wants to make the total capacitan=
ce large enough to keep the voltage down to a reasonable level. More and lon=
ger radials would clearly be desirable, which are not really feasible in my=20=
garden, but might be quite easy at a /P location.<br>

<br>
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 fo=
r example.<br>

<br>
Hopefully some time this summer, I will be able to borrow a field again, and=
 do some further experiments with more space available.<br>
<br>
Cheers, Jim Moritz<br>
73 de M0BMU<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
</blockquote></div><br>

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