[Fwd: LF: Ground systems and first tuning results]

Andre' Kesteloot akestelo@bellatlantic.net
Tue, 27 Jul 1999 20:33:10 -0400

Hans-Joachim Brandt wrote:

> 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
> > > 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
> >
> >