A few weeks ago there was a discussion about whether to earth
masts, or leave them 'floating'.
In the continuous search for more dBs I tried earthing my mast on
Saturday.
I have a 14m (45ft) mast attached to the brick wall of the house by
'T' and 'K' brackets, standing off about 300mm (1ft). Its base is
600mm (2ft) above ground level. The vertical part of the antenna is
about 1m (3ft) away from the mast.
I connected the mast to the wire which runs from the Tx to the earth
stakes. I expected the capacitance between the mast and the
vertical to change the tuning. The change was in fact very small, but
the first thing I noticed was the variometer tuning became much
more critical. The second thing was a 10 per cent increase in
antenna current. Conclusion: the Q of the antenna had increased.
I was still prepared to believe that I had simply added a capacitor
from the antenna to earth, but when I checked the receive
performance the signals improved by 1-2dB (tested by monitoring
DCF and detuning until one of the 'S-meter' LCD blobs flickered,
then noting that the blob stayed on with the earth connected), and
the local noise level reduced at the same time.
Another effect, in the short time since I made this change, has been
the antenna current has been stable. Before, it would vary by
between 10 and 20 percent from one day to the next, depending on
the weather, and I assumed this was absorption by trees.
It seems that the mast/wall was acting as a resistor to earth,
absorbing a different amount of RF from day to day.
I also earthed the mast at the far end of my inverted-L and
connected the two with a wire, but this had much less effect. This
mast is already stuck in the mud at its base so was probably already
earthy.
What to do next?
Mike, G3XDV (IO91VT)
http://www.lf.thersgb.net
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