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Your question about the ground screen is a good one and is relevant to any vertical antenna application. There have been posted recently other questions about any potential benefit from elevating the Sigma antennas. This question is also relevant to any vertical antenna application. The Sigma vertical antennas happen to an efficient and rugged design that permits hams to experiment with a number of different vertical antenna installations. Guy's detailed response to the ground screen question is a "modern" and "correct" answer based on engineering principles and data, not "ham mythology" and wishful thinking. Guy's comment's also speak to the question of elevating vertical antennas above ground. I agree 100% with Guy's comments. Guy's initial brief comment on "clearing local obstructions" is so very important and is all too often under appreciated by many hams, as judged from how they install their vertical antennas. Many hams have noted material improvement in vertical antenna performance when they are able to elevate the vertical antenna above the ground or even place the vertical on the roofs of buildings. No ground screens here. The improvement is real and does not come from any substantial "gain" from low angle propagation. Instead the apparent "gain" comes from improvement in the propagation efficiency of the radiated EM wave propagating from the antenna. In other words, the improvements are from reduced near field losses, not gain. The improved propagation efficiency comes largely from removing the antenna (and it's near field) well away from local obstructions. I would look for more potential benefit from locating my Sigma vertical in the clear than I would ever get from a ground screen system. Regards, John Petrich, W7FU ----- Original Message ----- From: "Guy Olinger, K2AV" <k2av@contesting.com> To: "D C Macdonald" <k2gkk@hotmail.com>; <lloydbartlett@earthlink.net>; <Force12Talk@qth.com> Sent: Sunday, April 28, 2002 7:10 PM Subject: Re: [Force 12 Talk] Sigma-5 > 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 |
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