[Lf] mesuring Ground Losses

[Andre' Kesteloot]

Dick Rollema wrote:

> Larry and others,
>
> I measure the loss resistance  Rtotal in the antenna system by inserting an
> impedance bridge between the bottom end of the loading coil and the earth
> connection. I have both a professional British Wayne Kerr admittance bridge
> and a homemade RX-bridge with built-in noise source. Both produce identical
> results: 30 ohms. But the latter is easier to use and therefore preferred.
>
> The loss resistance of the loading coil was determined by measuring its
> unloaded Q. The coil is resonated to 137 kHz with a good quality variable
> capacitor in parallel; a signal generator excites the coil via a one turn
> link that is kept at some distance from the coil; the probe of a sensitive
> selective level meter is held in the vicinity of the coil (both measures to
> avoid loading the coil) and the bandwidth B in kHz between the
> -3dB frequencies measured. Than Q = 137/B.
>
> The inductance L of the coil must be known and can be measured in several
> ways.
> Then Rcoil = (2 *  pi * f * L) /Q; in my case 9 ohms
>
> Must be old hat to you ...
>
> As the loss resistance in the antenna wire is negligible (a few tenths of
> ohm) I assume that the
> earth resistance = Rtotal - Rcoil = 30 - 9 = 21 ohms.
>
> However part of it may be due to losses in the surrounding of the antenna
> (not in trees, there are none near my antenna). Unfortunately it is
> impossible to separate these from the real earth resistance.
>
> But I  certainly would not classify my measurement of earth resistance as
> "absolute".
>
> There is a classical way of measuring earth resistance using two auxiliary
> electrodes. But it requires special test equipment, suitable for measuring
> at 137 kHz  to obtain a meaningful result. Unfortunately I lack the
> equipment to perform such an "absolute" measurement.
>
> Even then the result might be misleading  as Laurie has shown that  the
> "footprint" of the
> antenna plays a role and that is not taken into account using  the
> three-electrode test.
>
> 73, Dick, PA0SE
>
> ----- Oorspronkelijk bericht -----
> Van: Larry Kayser <kayser at sympatico.ca>
> Aan: <rsgb_lf_group at blacksheep.org>
> Verzonden: woensdag 19 juli 2000 1:08
> Onderwerp: LF: Re: Re: Ground Losses
>
> > Dick:
> >
> > >The earth resistance at PA0SE varies somewhat with time of the
> > >year.
> > >
> > >Today I measured the total resistance of the antenna system as 30 ohms.
> > >
> > >The loss resistance of the loading coil is 9 ohms.
> > >
> > >So for the earth resistance I find 21 ohms.
> > >
> > >The only earth here is the gas mains.
> >
> >
> > Sometime when your feeling up to it I would sure like to know how you make
> > absolute measurements of your antenna system.  I have a great interest in
> > this and would like to follow up further, again it is not an immediate
> > interest - I doubt I could make any more significant changes to the
> > processes to be used here in November in VO1 at this late date, but I sure
> > would like to make absolute measurements of the antenna here as I get
> ready
> > to do BPSK trials over the North Atlantic next winter from here in Eastern
> > Ontario.
> >
> > Larry
> > VA3LK