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

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Shoot me down if this is a rubbish idea, but I'll share it anyway....

If Paul has space and 4 tall trees, how about creating a large spiral top to the vertical, much as I did on 500kHz? Not sure what you'd use as insulating "string" (nylon cord?) to hang in a cross between the trees, but you could imagine a spiral of, say, 50 turns of wire progressively spiralling in to the top of the vertical section.

In my case on 500kHz this made about 5-6dB improvement in ERP over the previous 10m band wire halo as a top capacity hat. My spiral top was supported with a wooden frame, but for 9kHz I was thinking a nylon rope "cross" as high up as possible, supported by 4 trees, would work.

Perhaps one of the experts could advise if such an arrangement would have any advantages over a top horizontal wire?

73s
Roger G3XBM

On 6 April 2010 20:38, James Moritz <james.moritz@btopenworld.com> wrote:

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

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