Dear John,
concerning your measurements I would like to know the earth resistance
of each of your three ground wires separately.
When paralleling different earth grounds for LF I have often found that
the resulting ground resistance had NOT decreased, the resistance could
be even higher. This observation has been confirmed to me by another
german OM. He has put the question whether the currents in the ground
wires perhaps might be of different phase angles.
I have no experience so far concerning an LF ground in the form of a
wire mat.
Another question: Is your transmitter chassis also connected to the
neutral mains of the house or not?
To avoid such a connection to the mains ground my antenna feed cable is
using a separation transformer at the antenna feed point (LF currents
in
the house wiring might cause EMC problems; I have had such problems
on 160 m without such a transformer). This transformer is also used to
match the 50 ohms cable to the antenna feed point (around 70 ohms at my
location, the best ground connection here for LF is the water pipe of
the house. By the way, when trying LF in a school in a lecture for qrp
amateurs the connection to the boiler in the physics room resulted in a
ground loss of less than 20 ohms; the antenna has had a length of 70
meters with almost no greenery around) and to correct differences in
matching
between summer and winter season in three steps.
For the time being I am not qrv to transmit on LF but these questions
are based on observations made in the years 1998-2000, for me the most
interesting time on LF so far.
HW?
73 Ha-Jo, DJ1ZB
"John Rabson" <[email protected]> schrieb:
> LF, Robin, Chris,
>
> I am trying to reduce the resistance of the earth connection I use with my LF
> antenna.
>
> The antenna consists of two wires each 16 m long, connected in parallel and
> with a height of about 6 m (it was 8 m but the structure has suffered
> recently in the severe gales).
>
> The earth system consists of three wires radiating from the shack with
> angular spacings of about 60°. They run underground through plastic conduit
> for a distance of about 10 m and the remote ends are terminated in metal rods
> of the type used here to provide safety earth connections for mains
> installations. The three wires are connected together at the transmitter to
> provide the earth.
>
> The local geology is a mixture of granite and limestone with intrusions of
> calcite.
>
> At present the resistance at 137 kHz at the feed point is about 150 ohms
> (plus of course some reactance which it is not difficult to cancel). At a
> frequency of 980 Hz, I get a resistance of about 25 ohms, depending on
> whether there has been recent recent rain.
>
> Reasoning that the much higher resistance at 137 kHz might result from the
> return current flowing through the ground rather than through the earth
> spikes, I tried strapping the remote ends of the earth wires. The additional
> wire ran almost below and effectively parallel to the antenna wires.
>
> This strapping made practically no difference to the 137 kHz resistance, so I
> had further thoughts:
>
> 1) laying down an earth mat or something like chicken netting, or the kind of
> metal mesh which is used for fencing. Unfortunately, such things are
> expensive here (I estimate the cost would run into three figures in Euros), or
>
> 2) making the mat out of hookup wire or something similar. I have plenty of
> such wire. Would I need to join the wires at each crossing, and what spacing
> should I use?
>
> Any suggestions, please?
>
> John F5VLF
>
>
>
>
>
>
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