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LF: Antenna losses

To: [email protected]
Subject: LF: Antenna losses
From: "Mike Dennison" <mik[email protected]>
Date: Mon, 18 Jun 2001 12:29:56 +0100
Reply-to: [email protected]
Sender: <[email protected]>
My inverted-L has been faulty for several weeks now. It started with one of the top wires coming away from its perspex insulator. Because of the way the three wires are connected, this was not a disaster, but the spacing of the three top wires was reduced, and hence the capacitance was reduced. I compensated for this by removing a series capacitor I have at the feedpoint. The current dropped from 2.2A before the failure to 2A.
Over the next few weeks, the leaves grew on the trees under the far end of the
antenna, and this lowered the resonant frequency so I had to add the series
capacitor again. By this time the current had dropped to 1.8A.
This weekend I lowered the mast and checked why the original failure had
occurred. I had a cable tie connecting a loop of antenna wire and the
insulator. This had become brittle and showed signs of heating. I concluded
that this was due to corona. I now have a short loop of wire instead of the
cable tie.
On raising the repaired antenna, I found it resonated at 134kHz, and because
most of the inductance is at the top of the mast I could not bring it up in
frequency! This was a direct result of the leaves as the antenna was resonant
inband throughout the winter.
Next I dropped the mast at the far end and removed 1m of wire from each of
the top wires - actually I inserted an insulator 1m from the ends.
This small change raised the resonant frequency to 139kHz, so I had to add a
small inductor at the feedpoint to bring it onto the band. I assume this was not
only because the antenna was shorter, but that its capacitance to the tree tops
was much less. This was confirmed by the current going up to 2.2A, the same
as when the trees were bare.
Three lessons here:

1) Don't put so much inductance at the top of the mast that you run out of flexibility.
2) Never use cable ties at high voltage points - even corona destroys them.

3) Leaves are bad news. Even 1m increase in separation reduces the losses.

Now to get back on 73kHz.


Mike, G3XDV (IO91VT)
http://www.lf.thersgb.net



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