[Fwd: LF: antenna measurements]

Andre Kesteloot akestelo@bellatlantic.net
Fri, 10 Apr 1998 07:39:32 -0400

Roger Kendall wrote:

> Further to the e-mail from Peter Martinez re antenna measurements I
> have also been trying to make measurements on my system.  I have
> not found that connecting the house earth to the anntenna earth
> increased the resistance.
> I have used a model for my system consisting of a series circuit made
> up of the following:-
>         Lc   the loading coil inductance,
>         Rc  the resistance of the loading coil,
>         Rw  the resistance of the feeders and antenna wire,
>         Rr    the radiation resistance,
>         Ca   the capacitance of the antenna to ground,
>         Rs   the resistance between Ca and ground due to
>                 the imperfect conductivity of the ground,
>         Rg   the resistance of the ground conductors to the
>                 main body of the ground.
> At resonance Lc and Ca will cancel out leaving just the resistive
> terms.  Rc and Rw can be calculated or measured and both
> methods give me about 4 ohms for my system.  Rr can be calculated
> and is about 15m ohms for my system although I have yet to find any
> accurate equations which allow for top capacity loading.  Rs is a
> rather difficult factor to determine directly but Rg can be measured.
> I did this at 73kHz using the three probe method i.e. the earth
> system under test, a second 'probe' carrying the current and a third
> probe to measure the voltage.  This gave me a value of about 10
> ohms for Rg.  The Q of my system gave me a total resistance of
> about 30 ohms so this leaves about 16 ohms for Rs!
> Thinking on this recently I have come up with the idea that Rs and
> Rg could both be eliminated if a large enough conductive mat was
> placed under the antenna so that Ca returned directly to this.  I
> believe that this is what the large commercial LF stations do. It
> might be expensive though.  If time and the weather allow I shall
> try a length of galvanised fence wire underneath my antenna to see
> if it makes any difference.   At the moment I am more interested in
> building an ark!
>         73 to all,
> Roger Kendall, G0UPU.