From: "Mike Dennison" <mike.dennison@rsgb.org.uk>
To: rsgb_lf_group@blacksheep.org
Date: Fri, 23 Feb 2001 13:48:36 -0000
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII; format=flowed
Content-Transfer-Encoding: 8bit
Subject: Re: LF: Antennas/ERP
Message-ID: <3A966A34.20886.B598D6@localhost>
In-reply-to: <7583.200102231227@gemini>
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

On 23 Feb 2001, at 12:32, James Moritz wrote:

> A decrease in current from 2.7 to 2.2A would result in FS 
> decreasing by 1.8dB if the antenna remained the same. So overall, 
> there was an improvement of 2.7dB from the radiating point of 
> view, although offset by increased losses, apparently. If ways 
> could be found of reducing these added losses in a practical way, 
> this would be a significant improvement in LF antenna technique.

My next plan is to take my 'old' low-loss Litz wound 3mH top loading 
coil and insert some ferrite into it so that the L is increased to the 
7mH required for self-resonance. The tricky bit will be getting the 
right type of ferrite, so that it has low loss and light weight. Any 
ideas?

The coil I have now at the top of the mast was increased to 11mH by 
adding quite a small amount of ferrite made up from bits of pot cores 
used for audio filters, so I know it can be done. These bits of ferrite 
were used when operating on 73kHz and got very slightly warm with 
200W RF.

As for using increased RF power as an alternative to improving the 
antenna, this is a matter of choice, but I prefer to do both as it will 
take a huge amount of power to get me close to 1W with my 
relatively small antenna (700W RF to get 140mW ERP = a gain of 
about minus 37dB). Even with my present top loading I still need to 
run 5kW to exceed 1W ERP.


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