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Message-ID: <F5197315CD8F42189E79E62E491A0E52@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <3F10330D89BC46B2B0071A38A6E5EC09@PCFausto> <5961FA4401BA460E89A59A764DF00469@JimPC> <0F390647FE98400C9E61BC6187E28763@PCFausto>
In-Reply-To: <0F390647FE98400C9E61BC6187E28763@PCFausto>
Date: Thu, 6 Jan 2011 19:31:04 -0000
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Subject: LF: Re: Vertical antenna and connection line
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Dear Fausto, LF Group,

Having the top loading wires as high up as possible is always best - greater 
height means higher radiation resistance and usually lower loss resistance, 
so that will boost your antenna efficiency. A rough guess would be that you 
could get 15m effective antenna height or perhaps a little more; with Rrad 
of 73 milliohms you would only need about 2.7A antenna current to reach the 
1W ERP "legal limit". There do not seem to be many buildings and trees 
around your QTH, so loss resistance should be fairly low - probably <50ohms. 
So a few hundred watts TX power should be enough. A single downlead going 
directly to the antenna tuner is a much better idea than a transmission 
line, due to the fairly low antenna capacitance.

The voltage with 300pF antenna capacitance and Iant around 3A would be about 
12kV. This requires care, but is quite workable. I used an antenna with 
about 340pF capacitance (although only about 10m high) with antenna currents 
up to 6A, which was quite reliable eventually. I did initially have several 
problems with voltage brealdown. Initially, the loading coil was indoors, 
with the antenna connected via a marine-type feed-through porcelain 
insulator. The insulator was mounted on a plastic panel, and passed through 
an 80mm diameter hole in some wooden cladding. This worked OK when the 
weather was dry, but when the wood got damp in the rain, it caught fire!... 
I also burned a number of plastic insulators supporting the antenna wire. 
Eventually, the loading coil moved outside into its own shelter, and the 
porcelain insulator was mounted through a hole bored in a glass cooking dish 
about 250mm diameter, and this has proved a reliable insulator when exposed 
to the weather. I stopped the plastic insulators melting by fitting "corona 
rings" to the ends of the antenna wire to reduce the potential gradient.

Cheers, Jim Moritz
73 de M0BMU