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characteristics immediately underneath the antenna, in the near field and out to a thousand feet, will cause the eventual result to fluctuate far more than the height. Also, the modeling programs assume level and homogenous ground, free of clutter, which almost never occurs. Utility poles, with their ground wires and cables going down to buried distributions, house wiring, metal rain gutters, buried power wiring, etc, etc, ad nauseum. In general, in any direction, a dwelling will effectively nullify any ground reflection reinforcement. A null due to out-of-phase ground reflection calculated to hundredths of a db is overwhelmed several orders of magnitude by the loss of ground reflection going through clutter. Tactical decisions on vertical antenna placement should be based on getting sight to the horizon above clutter, placing the antenna as far away from clutter as possible. Has nothing to do with personally being anti-modeling. I own the expensive commercial programs. I run models on everything I do. In this case you're worrying about drips when you stand to lose the bucket. In the case that you are one of the lucky ones to be operating at a clutter-less site, you have not even mentioned a dense ground screen to assist the antenna, or doing anything to get yourself out of the minus gain column. 73, Guy. ----- Original Message ----- From: "Andrew Roos" <andrew@exinet.co.za> To: "KEN SILVERMAN" <k2kw@prodigy.net>; <Force12Talk@qth.com> Sent: Thursday, April 24, 2003 12:12 PM Subject: RE: [Force 12 Talk] RE: Sigma 5 Height > Hi Kenny > > 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 > -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. 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