[Lf] Re: Ground Losses]

[Andre' Kesteloot]

Peter Dodd wrote:

>  I measured the impedance and loss resistance of my antenna system
> (as described by  PA0SE)  by inserting the 3-M
> impedance bridge between the bottom end of the loading coil and the earth
> connection.  The results were as follows:
>
>                     Results        +/-Errors
> Freq MHz   Res      jX       Res    jX
> ----------------------------------------
>  .1364       37.9   - 29.4      2      2.1
>  .1366       37.4   - 22.8      4.6    5.3
>  .1368       33.5   - 22.2     5.1    5.7
>  .1370       33.4   - 17.3      5.8    7.4
>  .1372       35.6   - 8.8       2.8    3.5
>  .1374       36.4   -0 1        1.7    2
>  .1376       36.9   +0 5       1.1    1.3
>  .1378       36.5   + 13.3    1      1.3
>  .1380       38.1   + 21       1.1    1.8
>
> The total resistance component is around 37.5ohms. The 3-M system
> indicates probable errors on some readings and the 33ohm readings are ignored.
>
> I tried to measure coil Q, as described by Dick,
>
> > 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.
>
> My coil is made up of two coils, one commercial 300uH variometer and
> a 2100uH fixed coil. The best Q obtainable was 142, which gives a
> resistance of around 13ohms. I had some problems connecting the
> signal generator and the level meter, as described above. If the
> coupling was too loose there was direct pick up and I had to increase
> the coupling to the coil.
>
>  This implies a ground resistance at 137kHz of  around 25ohms, which
> makes this a good LF QTH, something I have known all along.
>
> The ground comprises a 2.5m layer of clay on sand and shingle. The
> station is located 400m from the sea high water line. The ground
> connection comprises a 2m long copper pipe in the ground close to the
> loading coil. I also have a 10m square wire netting ground on the
> surface. The maximum current is 3.5amps regardless of which earth is
> used. Connecting both grounds together does not increase the current.
> The next experiment is to try the Decca loading coil.
>
> Is it possible to get some indication of Q of the antenna system from
> changes in reactance relative to frequency given above?
>
>
>
> --
> Regards, Peter, G3LDO
>
> <g3ldo at zetnet.co.uk>