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normal mode. Using a regular level ground, and a vertical wire in the model will NOT tell you what you want to know. Try setting up the vertical angled in such a way as to be opposite and equal of the ground slope. then look at the lobes and skew the horizon up in your mind. E.g., slope the model to make the ground level and the wire sloped. And translate back from the result. And you STILL need to pay attention to the ground screen. 73, Guy ----- Original Message ----- From: "Rich Holoch" <rholoch@attbi.com> To: "Guy Olinger, K2AV" <olinger@bellsouth.net> Sent: Thursday, April 24, 2003 2:54 PM Subject: Re: [Force 12 Talk] RE: Sigma 5 Height > In my original posting I described my ground situation, but nobody seemed to > pick up on what I was asking in regards to that. The antenna is on a shelf > on the side of a hill and is 80 - 90' away from the house. The antenna is > about 2/3rds the way up the hill - and is at 600' up and the hill is 900'. > The valley below is only about 100' above sea level max. The Pacific Ocean > is 25 miles to the West. > > I have no neighbors and am on 5 acres. The antenna is in the clear and has > a clear shot to the western horizon, but the hill is quite steep to the > east. The Southern horizon is also visible without anything in the way, but > the house is 80 - 90' to the north of the antenna. > > I have no way to measure this except on the air tests, but I can work into > the Southwest long path with very good signals to South Africa. I can work > into EU on the LP as well, and can work into the Middle during their AM gray > line. These paths are much weaker than the AM LP to South Africa. > > The bottom pipe that holds the antenna is deep enough into the ground so > that it is physically in the water table. The ground has a lot of clay, and > the water table is high because it is in the path of water draining down > from the top of the hill. I'm only mentioning that because I have read the > papers on placing these antennas near salt water - I know the dielectric > quality is different . . .. just wondering if I am getting some reflective > "help"- even though there is no metal ground screen. > > So I agree - the model is only part of the equation. I do believe that I > have something going on that is helping this antenna perform better than I > would have ever expected. If its just propagation, I can accept that. I > guess I am intrigued because there are so many variables here. > > I have very much enjoyed this discussion and appreciate your input. I am > learning a lot - and that's the best part of ham radio for me. > > 73, > > Rich > KY6R > > ex WA2QHN (Newton, NJ) > > > ----- Original Message ----- > From: "Guy Olinger, K2AV" <olinger@bellsouth.net> > To: "Andrew Roos" <andrew@exinet.co.za>; "KEN SILVERMAN" <k2kw@prodigy.net>; > <Force12Talk@qth.com> > Sent: Thursday, April 24, 2003 10:00 > Subject: Re: [Force 12 Talk] RE: Sigma 5 Height > > > > One thing you need to consider in this discussion is that the ground > > 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. > > 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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