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Thanks for your comments. I read the null depths from the EZNEC 2-d (vertical slice) plot as follows: Base height (m) Null Depth (dB) 10 2.87 11 2.94 12 2.96 13 2.89 14 2.76 15 2.59 16 2.40 17 2.20 18 1.99 19 1.77 20 1.57 In each case this is the null between the bottom-most lobe and the next lobe up. At the higher base heights a third lobe starts to develop at around 60 degrees elevation and there is a deeper null (about 10 dB) between it and the second lobe at 28 degrees or so. However for the heights above ground that I considered (up to 20m), the elevation angle of this deeper lobe - about 48 degrees - seemed to make it inconsequential as far as DX performance is concerned. I think the reason for the smaller than expected nulls may be as follows: when thinking in general terms about antenna systems it is a useful approximation to assume that all radiation occurs from a point source located at the feedpoint. Such a source would have well defined nulls. However in a vertical, there is some radiation from the whole of the vertical element (and some horizontally polarized radiation from the T bars as well due to incomplete field cancellation). Since nulls are formed from the different path lengths between the direct wave and the ground reflection, this "vertical smearing" of the radiation source means that althouth there may be a 180 degree phase difference for signals from the feedpoint, there won't be exactly 180 degree difference for signals from other points on the antenna, so the null is less deep. Also remember that there is greater ground absorption on verticals than with horizontally polarized antennas, which means the ground reflected signal is weaker, so even with 180 degree phase difference there still isn't exact cancellation. That's my take on it any way. Your milage may differ :) 73, Andrew > -----Original Message----- > From: KEN SILVERMAN [mailto:k2kw@prodigy.net] > Sent: Thursday, April 24, 2003 3:48 PM > To: Force12Talk@qth.com > Subject: Re: [Force 12 Talk] RE: Sigma 5 Height > > > > My conclusion is that if you want a single lobe (on 20m) then a base > height > > of 5m is about right. > > At least someone is keeping me honest! Yes, the feedpoint should be up > around 1/4 wavelength to maximize one big lobe. TOTAL height for a half > wave vertical dipole (which is all I really work with these days) > should be > no more than 5/8 at the tip. Sorry for the goof. > > > Although there is a dip in the radiation pattern between the high-angle > and > > low-angle lobes, I would not go so far as to call it a null as it is > > generally less that 3 dB. > > The null should be much bigger than that. Nulls are formed based on > feedpoint height for both vertical and horizontal arrays - the > mechanism is > the same. > > Kenny K2KW > > > > > -------------------------------------------- > Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. > Force12 Web Site: http://www.force12inc.com > > To Submit Message to the List: Force12Talk@qth.com > To unsubscribe and view the Message Archive: see http://qth.com/force12/list For problems with the list: contact n4zr@qth.com -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force12 Web Site: http://www.force12inc.com To Submit Message to the List: Force12Talk@qth.com To unsubscribe and view the Message Archive: see http://qth.com/force12/list For problems with the list: contact n4zr@qth.com |
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