Hallo all interested,
the conditions which Bob, ZL2CA, has described concerning the QTH of Kevin,
ZL4MD, in principle are the same as at my QTH:
For LF I can use either the house ground, which is also connected to the water
pipe net, or a radial ground made of 28 insulated litz wires with lengths
between 12 meters and 30 meters depending on the available space in that
direction.
When working on this radial system and continuously measuring the feed point
resistance under resonance conditions, however, I could not confirm Bobs remark
that metal stakes at the far ends would give an improvement. At least on my
insulated radial system the feed point resistance increased. Especially with
rather few radial wires the radial ground had been very sensitive against any
ground connection, including a simple earthstake connected to the centre of the
system. After the number of radials had approached 18 the earth stake
(just a few inches deep) connection to the centre could be tolerated, but never
the connection to the house ground. Once I have tried to elevate all my radials
about one foot from the ground: The feedpoint resistance remained the same, I
needed more inductivity in the variometer, and the sensitivity to any connection
to ground even increased! This sensitivity obviously depends of the capacity of
the insulated radial system to ground.
As reported by DJ2LF from a distance of about 160 kilometers there is no
significant difference in signal strength between both grounds.
But paralling both grounds also results in an increased feed point resistance
(Rfp). Similarly to ZL4MD I employ a ferrite matching transformer (for matching
the 50 ohms feed line to the Rfp of about 95 ohms) with two independant windings
to separate the equipment in the shack (connected to the house ground) from the
radial system.
The LF antenna has two top wires spaced 1,5 meters, 25 meters long and 10 meters
high, and a Y-shaped downlead to the variometer (Lv) at the centre of the radial
system. The connection lead to the house ground is about 12 meters long.
Some data measured over the weekend:
Series resistance of variometer Lv about 18 to 20 ohms.
Antenna tuned to house ground:
Lv = 3.681 mH; Rfp = 96 ohms.
Antenna tuned to radial system:
LV = 3.780 mH; Rfp = 89 ohms.
Comparing both measurements, the radial system must have a capacity of 13,69 nF
(Xc = 85 ohms) which must be tuned out by Lv. The Q of the radial system is
roughly 1 (Q = Xc/Rfp).
Antenna tuned to both ground systems in parallel:
Lv = 3.726 mH; Rfp = 160 ohms.
Then I have tried to insert L or C in series to one (or even both) of the leads
to the ground systems, employing the 12.000 pF variable capacitor already
mentioned and a variometer of 68 to 390 microhenries. Both grounds could be
paralleled by inserting either a capacitance of 12 nF (max C, optimum flat) in
series to the house ground or an inductivity of 103 microhenries in series to
the radial system. Both solutions resulted in an Rfp of about 85 ohms. But as
the house ground is also beneficial for safety reasons, the method of inserting
a capacitor in this lead has been discarded.
The best solution has been the 103 microhenry coil in series with the radial
system. Obviously this coil is compensating the capacitance of the radial system
to ground. This solution also resulted in the lowest value for Lv = 3.578 mH,
showing that this solution also makes optimum us of the capacity of the (given)
antenna to ground.
In my case the real "gain" in reducing feedpoint resistance from 96 ohms or 89
ohms to just 85 ohms seems to be of little practical value. May be that,
according to the views of Bob, ZL2CA, the two different ground systems on the
same estate are still conflicting, and that we cannot expect a ground resistance
equal to the values of each ground system in parallel.
But this may be different in other ham installations and should be worth to be
investigated. I will continue to try some non-insulated radials buried a few
inch into the ground and see if I can manage to reduce the house ground
resistance significantly or not, because the insulated radial system in the
garden is somewhat problematic, (especially for my wife).
On the other hand, when thinking of an LF fieldday, an insulated radial system
which may have much longer radials in the field than my own may be very useful
and would avoid digging into the ground. Just an earthstake had to be provided
for safety reasons, with a 5 kOhm lead resistor to the radial system and a spark
gap to the antenna!.
Finally I want to note that I have been impressed by the report of Johan Bodin,
sm6lkm, on the beneficial effects of a ground system extending really deep into
the ground. But few of us would have the chance to realize this on a private
basis, I think. For completeness I have added the essentials of his email to
this email report.
73 Ha-Jo, DJ1ZB
Johan Bodin, sm6lkm, wrote:
Concerning paralleling different LF grounds, I have had the same experience. My
antenna is an inverted L, about 80 - 90 meters long, height varies between 10
and 20 meters (sloping terrain) and the far end is the highest point.
The feedpoint resistance was about 75 ohms when mains protective
ground was used alone (including loading coil etc). When I tried the
6 inch steel tube in our deep drilled water source, the feedpoint
resistance fell to 36 ohms at resonance. The tube is 12 meters long,
11 of which of goes through soil and the remaining 1 meter is inserted
into the solid rock below.
When I connected the mains ground in parallel with the big tube,
feedpoint resistance increased to about 55 ohms.
vernall schrieb:
Hi all,
I missed one other piece of information, which is now added: Kevin
ZL4MD uses link coupling from his LF transmitter, which isolates the RF
output, so there is no path for the mains (safety) earth to travel via
his RF feeder to the LF antenna. That is how he keeps the RF and mains
earth separate. Unless the two are electrically separate, and known to
be safe, the experiment of connecting or removing a link between the two
earth systems is not available.
Bob ZL2CA
vernall wrote (one email earlier):
>
> 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
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