LF: Re: 9 Khz Antenna update Putting inductance near the top.

["James Moritz" <james.moritz@btopenworld.com>]

Dear Paul, Rik, LF Group,

Well, I hate to be a pessimist, but I am not convinced that elevating the 
loading coil will be particularly useful at W1VLF.
There seem to be 2 ways an elevated loading coil could be of benefit. One is 
that it effectively reduces the impedance of the top-load section of the 
antenna, so increasing the proportion of the total antenna current flowing 
in the top load "above" the coil. This improves the current distribution and 
so increases the effective height. This could be very useful where the top 
load part of the antenna is fairly small compared to the vertical part. But 
in Paul's case, the top loading wires will already carry quite a large 
proportion of the total antenna current. Also, most of the top loading wires 
will be below the height of the coil, so diverting more current into them 
will not be particularly beneficial from this point of view.
The other possible benefit would be where the antenna downlead runs close to 
a mast, building, tree etc. The elevated coil will reduce the voltage on the 
downlead, reducing the displacement current flowing in the nearby objects, 
and so reducing losses associated with this unwanted capacitance. But from 
Paul's description, it does not seem that the downlead will be particularly 
near anything.
I would reccomend feeding the antenna geometry and likely loading coil 
inductance into some NEC-based antenna simulator software to see if it is 
actually beneficial to the effective height/Rrad of the antenna. But I would 
be surprised if the elevated coil made a significant improvement, and it 
would be a big lump dangling from the top of the mast there...
Cheers, Jim Moritz
73 de M0BMU