From: "Mike Dennison" <mike.dennison@rsgb.org.uk>
To: rsgb_lf_group@blacksheep.org
Date: Mon, 18 Jun 2001 12:29:56 +0100
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Subject: LF: Antenna losses
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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