[Fwd: LF: antenna measurements]
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.