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Subject: Re: [Force 12 Talk] Sigma-5 Guy & Mac: Thank you for your input...I've learned a lot. Everything I do in this unfriendly ham radio environment will be a compromise, but I will do my best to get a good signal on the bands. 73, Lloyd, K7CIX Tucson, AZ > Unless what you are really doing is clearing local obstructions, e.g. > a neighborhood full of buildings, no, actually not. > > E-plane ground penetration loss is spread across the entire radiation > pattern, as it is due to closely coupling the antenna to the lossy > ground. > > The low angle reinforcement comes from eliminating "first bounce" loss > at low angles. If the first bounce of radiation otherwise headed BELOW > the horizon, is off plain dirt or worse, the bounce is very lossy. It > does not survive to reinforce the radiation originally headed upward > at the angle equal to the bounce angle. > > If the "dirt" also contains sufficiently dense wire, it will begin to > appear like a copper sheet, and the downward radiation will bounce > efficiently, and live to reinforce the upward. > > Take a situation where you have a 25 foot radius ground screen > underneath the antenna. Put the antenna as low as you can. Note the > lowest angle where the first bounce is still off the ground screen. As > you begin to raise the antenna, the lowest bounce angle still off the > screen goes up as well. > > The higher the antenna, the larger the ground screen needed to keep a > given first bounce off the ground screen. This is a 3 db reinforcement > you are talking about keeping or losing, at angles that are the most > important for DX. > > And though F12 correctly states that the ANTENNA efficiency is not > effected by the lack of a ground screen, the antenna SYSTEM efficiency > (includes the ground underneath, since it is strongly coupled) IS > affected. > > These phenomena will be the same if it is a Sigma, Cushcraft or any of > the vertical dipole designs. It's not an F12 issue. > > In a word, sales folk don't think they can sell vertical antenna's if > they harp on the typical need for a ground screen for that last 3 db, > and the manufacturer can't guarantee the quality or performance of a > customer antenna taking the screen into account. > > If you need an "authoritative" printed treatise on this, ON4UN's Low > Band Dxing book lays it out pretty well. He brings it up specifically > for low bands because in some places verticals are just about all > that's possible for low bands. But the principles apply up the line. > > On 2 meters the ground screen is the roof of my Aerostar. EXCELLENT > ground screen. Same set of principles, including the low angle > reinforcement. > > 73, Guy > > ---- Original Message ----- > From: "D C Macdonald" <k2gkk@hotmail.com> > To: <lloydbartlett@earthlink.net>; <Force12Talk@qth.com> > Sent: Sunday, April 28, 2002 9:07 PM > Subject: Re: [Force 12 Talk] Sigma-5 > > > > > > I would expect that another way to gain better low-angle > > would be to elevate the antenna more than just a minimum > > above the ground. This, if I have read the theory right, > > would lessen the ground loss in the near field, thus > > providing better radiation at angles closer to the ground. > > > > It will be interesting. I have one on order. > > > > 73 --- Mac, K2GKK/5 > > Oklahoma City, OK > > > > ----Original Message Follows---- > > From: "Lloyd A. Bartlett" <lloydbartlett@earthlink.net> > > To: <Force12Talk@qth.com> > > Subject: [Force 12 Talk] Sigma-5 > > Date: Sun, 28 Apr 2002 09:01:39 -0700 > > > > Force 12 states : "SIGMA-5 efficiency is independent of > > the ground. To enhance the low angle energy, one can add > > an extensive ground screen (dense for 20-30') under the > > antenna (then more screen or wires out to perhaps 5 > > wavelengths)." > > > > I would like to hear from those who do, and those who > > don't use ground systems with Sigma-5. > > > > 73, > > > > Lloyd, K7CIX > > > > > > > > > > -------------------------------------------- > > 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 > > > > > > > > > > _________________________________________________________________ > > Send and receive Hotmail on your mobile device: > http://mobile.msn.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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