[Fwd: LF: Comments on Dave's (G3YXM) observations on counterpoise grounding]
Sat, 15 Aug 1998 09:47:29 -0400
Hans-Joachim Brandt wrote:
> Dear all,
> Dave's interesting observations when grounding the barbed fence and the
> insulated 100 mtrs counterpoise during his expedition to Cornwall are fully in
> line with my own measurements! As long as we have no LF licence in Germany,
> apart from listening I do some measurements on my double T aerial (total
> horizontal span 25 mtrs, height 10 mtrs, Y-shaped downleads, C ~ 360 pF).
> To avoid digging in the garden, my ground systems now consists of 28 insulated
> wires lying on the ground (mostly lawn), the lengths differing from 10 mtrs to
> 30 mtrs depending on the available space in that direction. When the aerial is
> tuned I measure a total series resistance of about 95 ohms. The series loss
> resistance of the aerial variometer is abt 20 ohms, so ground loss resistance is
> 75 ohms. Earlier, with just 18 insulated radials, I had measured a total
> resistance of 100 ohms. Therefore I think that more radials will give no further
> improvement in the available space (may be some longer radials will).
> With 28 radials I can ground the whole system with an earth stake, with a just
> noticeable increase in ground resistance. But when I connect the system to the
> house safety ground via 12 mtrs of cable, the total resistance increases to 140
> ohms (ground resistance to 120 ohms)! Attempts to ground the outer ends of the
> insulated radials (as recommended elsewhere) did not improve but made the system
> worse. This I have reported earlier, and also the need to match the aerial to
> the 50 ohms feed cable by a ferrite ring transformer with two different coils to
> separate the electrical house ground from the aerial ground system (a necessity
> also seen by Peter, G3PLX). Measuring the capacity of the insulated ground
> system towards the earth stake using a Metex M-3650 multimeter resulted in a
> value of 34 nF (the AADE LC Meter 2B did not respond).
> Recently I have tried out the behaviour of this ground system when being
> elevated at least 30 cm or one foot above ground. The centre of the system had
> been placed on a (wooden/plastic) garden table, and the ends of the wires had
> been tied by ribbons to the garden fence or to trees or bushes or what offered
> itself for this purpose (a rather laborious work, but I wanted to see the
> difference). Capacitance to the earth stake had been reduced to abt 5,3 nF.
> Total series resistance of the tuned aerial system had remained the same (!!!!)
> at abt 95 ohms, therefore the (ground loss I think is not the correct word)
> total loss had not changed. It must be noted, however, that the aerial
> variometer had to be sightly tuned towards higher inductivity (obviously to tune
> out the difference in capacity of the elevated system to ground).
> But the sensitivity of the elevated ground system against grounding by an earth
> stake has increased considerably. When connecting my simple earth stake of just
> 6 inch in length to the centre of the system, total loss again increased to 140
> ohms. Imaginating that a lead resistance should be present for safety grounding
> of such an elevated ground system to discharge any static electricity, this
> resistance had to be at least 6 to 10 kiloohms in order not to increase the
> total loss resistance. In the final installation a spark gap should be
> connected in parallel to this resistor, and a second sparc gap directly from the
> aerial downlead to ground. For clarity I may add that my measuring bridge is
> battery powered, so there is no danger of an unwanted contact to a mains ground.
> I think Dave's barbed wire fence and his 100 mtrs insulated radial or
> counterpoise can also be regarded as an elevated ground system, and obviously
> these are sensitive to separate grounding! When realizing this, I think we must
> carefully sort out what we really need from a ground system. We do need an
> aerial system with a total loss as low as possible to achieve high aerial
> currents. Whether this can be achieved by using a really grounded or an elevated
> ground system is of secondary importance.
> But from reading the lf_group correspondance I know that there are advocates
> claiming that a good ground system has to have intimate contact to the soil,
> preferably a soil of good conductivity, of course. But this statement may
> discourage those who are living on rocky grounds that they would never have the
> chance for a good ground system and have to try out loops (this is also very
> interesting, no question).
> Insulated (or elevated) ground systems may have obvious merits when operating
> portable or on rocky soil. I think we should try them out pragmatically and
> learn how they can be operated effectively and safely, without introducing
> avoidable losses.
> 73 Ha-Jo, DJ1ZB