Dear Chris, Laurie, Richard, LF Group,
Thanks for the replies. Re the comments:
G4OKW wrote:
...The mesh might not be as good on a rocky hilltop. There is probably
conductive soil under only 10cm or so of concrete drive, whereas a mountain
may be insulating rock all the way down to Old Nick....
I agree - I don't think the wire mesh would neccessarily improve the ground
losses, but compared to the radial wires it gives similar performance in a
quite compact form, which could well be an advantage in itself.
... Have you tried arrays of tent pegs?
No, I limited myself to one per radial. My thought was that for portable
operation with a vertical antenna, it would probably be easier just to add
some more radials, while for fixed station use full-length ground rods work
better, and are probably less affected by the surface soil drying out. For
the ground-loop type of antenna used for cave radio, I can see the array of
tent pegs would be a good idea.
G3AQC wrote:
...> does seem to support my findings that the Mains/water pipes take most
of the current,
making all the work on radials earth rods etc a bit of a waste of time...
The ground systems I tried are more-or-less insulated from the ground, so
are coupled to the ground mainly by their capacitance, which has a
relatively high reactance compared to their resistance. A ground rod on the
end of a considerable length of wire probably has a substantial inductive
reactance compared to its resistance, due to the inductance of the wire.
Either way, even though they may have low resistance, the overall impedance
of these grounds will be relatively high compared to a ground with a short
connection to the return terminal of the TX or ATU, and so little current
will flow in them.
...I would need to series resonate with a capacitor, value depending of
course on my particular system..... but did you say that the reactance was
capacitive ?
The insulated radials, etc. have a capacitive reactance - the individual
wires on the ground were roughly equivalent to 28pF/m. So these would
require a tuning inductance to neutralise their reactance. For ground rods,
or buried bare-wire radials, I expect the reactance would be more inductive,
so a tuning capacitance would be required. Since we are talking about
nanofarads of capacitance, or perhaps 10s of uH of inductance for the ground
systems, the tuning components required would be fairly small
inductance/large capacitance.
G3CWI wrote:
...I am intrigued about loaded radials in that the loading is effectively
supplied by the inductor in the ATU circuit. There was no sign that such an
approach worked on my last trip but possibly the inductance was
insufficient...
The reactance of the radials adds to the total capacitive reactance of the
antenna, so more inductance is needed to resonate the antenna. According to
your previous mail, you had somewhere around 60m of radials, giving an
additional reactance of roughly 200ohms on top of perhaps 900 ohms for the
200ft long antenna. So the increase in overall tuning inductance required
would have been quite large.
Cheers, Jim Moritz
73 de M0BMU
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