| Index | Thread | Archive | Apr-2002 | Archive | Send |
 |
  |
|
   |
|
  |
|
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. 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 |
| This Thread |
This Author (Apr-2002)
|