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| To: | <[email protected]> |
|---|---|
| Subject: | LF: Re: 9 Khz Antenna update Putting inductance near the top. |
| From: | "James Moritz" <[email protected]> |
| Date: | Tue, 6 Apr 2010 20:38:44 +0100 |
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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 Moritz73 de M0BMU |
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