I found iteresting replies about this subject and for this reason I am sending a summary back to the reflector. Thank you all, it was vy helpful! Carlos - PY1CAS > > Dear friends, > > We all know that the height of a dipole over the ground deeply > influences the take-off angle of a horizontaly polarized antenna. With a > higher antenna we should expect a lower radiation angle. Right? > > Well, we also know that for 80m we need at least 1/2 wavelenght in order > to obtain a 30 degrees angle. For that we need a very high tower! But if > we install a dipole for 80 meters on the top of a very tall building? > What happens? Can we expect a low angle or not, or there are more > parameters involved on it? > > Thanks in advance, > > Carlos - PY1CAS > E-mail: py1cas@ax.apc.org > =========================================================================== Hi Carlos,,,,,well the answer to your high dipole on a building is what does it see for ground....and I bet it sees all that steel flooring. Might suggest you try installing it on the very edge of the building and see what happens, but pick which side carefully...it's going to be very directional toward the perpendicular of that wall. 73 Ed ------------------------------------------------------------------------------- The higher the better for long haul DX work..... Bryan W5KFT ------------------------------------------------------------------------------- >we install a dipole for 80 meters on the top of a very tall building? >What happens? Can we expect a low angle or not, YES & 73 ------------------------------------------------------------------------------- Carlos: There would be a few more parameters, but I don't think they will have a bunch of influence. First, you should get a lobe that is about vertical because of the reflection from the top of the building. The low-angle lobes should be as predicted for an antenna of that height provided that the antenna is high enough off the roof that the antenna can "see" the ground at a depression angle that is equal to the radiation angle. The other factor has to do with the surrounding terrain. The lobing caused by ground effects are due to reflections from a flat earth. Chances are that if you are on a building top, there will be other buildings in the area. If the building density is really high like in the downtown part of a major city, I don't think you can make the flat earth assumption to predict the patterns. Of course this doesn't mean that the antenna won't work well, you just won't be able to predict the performance with the certainty that you would with a flat earth. John KC4ZXX ------------------------------------------------------------------------------- Hello Carlos: Saw your message. Our ham club station here in New York City is located in a bldg that is about 350 meters high. On certain bands the dipoles work like a house on fire. Big signals. They are about 30 feet above the tower bldgs which puts them about 100 feet above the main roof. We have had many beams and they have worked very well. Now we have a log as the other beams could not survice the weather. A Hy-Gain DX-88 vertical has worked quite well - out doing many stations with beams in pileups to Europe and elsewhere. We have a 1/2 wave and 5/8 wave(verticals) on 10 meters. At the very tip of Long Island we get reports of 20/30 over 9 with 125 watts. More with the amp on of course. On 160 we load the bldg as a shunt fed vertical and have the radial wires up top. We do run max power with outboard audio processing. We get, at times, 20-30 over 9 in Europe when condx are good. We have no trouble working DX at all. Our station is way down in the bldg and our 5 - 743 foot feeders to the roof are 1 5/8" hardline so we don't really have any loss re Andrew/Cellwave data sheets. Our bldg is right on NY Harbor in lower Manhattan in NYC. I have asked the same question but if you are high up and the dipole is high above the main roof - they seem to play like wild fire. If you have a tin roof, antenna not so high above the roof it could be a different story however. 73, John ***************************************** Name: John Ferebee Radio: Talk Radio on 160 meters - W2CYA Email: w2cya@broadcast.net Company: Bell Atlantic, New York, NY Voice: (212) 766-0411, (212) 429-8882 Fax: (212) 577-7426 - 24 hours Date: 09/12/97 Time: 16:02:06 ***************************************** ------------------------------------------------------------------------------- A dipole mounted on the roof of a tall building presents a complex situation. The feedpoint impedance will likely be determined by the height of the dipole above the roof (and conducting steel support beams etc). The ground reflection will depend on how high and close the the edge of the roof the antenna is placed. If the antenna is mounted close to the edge, the ground reflection in that direction will be from the earth below. If the roof is wide, the reflection across the roof will be from the roof (high angle). de Tom N4KG ------------------------------------------------------------------------------- Hello Carlos, The biggest other parameter is metal in the building resonating with the antenna. I built antennas on the electrical engineering building of the university near here. The 40m antenna worked very well, but the 80m antenna was "insane" with respect to its feedpoint impedance due to interaction with the building. Another parameter is the presence of objects around the building at various heights. If you are at the top of a building "by itself" with the surrounding ground at normal height, fine. If you are at the top of a building surrounded by other buildings, I don't think you'll see the advantage you might wish for. Regards, Pete KS4XG ------------------------------------------------------------------------------- Carlos, If the 80-meter antenna is installed on top of a tall building, there will be some effect on the elevation angle of max radiation, but it is likely to be small. Most of yur radiation going downward will miss the building and hence the ground and other structures will be the major influences. Using supports as high as possible on top of the building will help by allowing more radiation to miss the roof. Many buildings do not have metalic roof structures, and some of the radiation downward may also pass through the roof to something lower before reflecting. Hence, I suggest that it is certainly worth a trial antenna to check results. Installing an antenna on the building top is easier than erecting two high towers. Good luck on the project. -73- LB, W4RNL ------------------------------------------------------------------------------- Hi Carlos -- I believe the right answer may be somewhere in between. At the U.S. Department of State Radio Club, we used to have dipoles for 40 and 80 installed on top of a relatively small building, 40 m. above ground but only 3-4 m. above a rooftop no larger than the length of the longest dipole. They certainly did not perform like dipoles 3-4 m. above ground, but they weren't as good as one might have hoped from their height above ground. I had a similar experience at the 4U1WB (World Bank)club station during the 1992 CW WPX contest. My suspicion is that the antenna is really seeing two ground planes, probably dominated by the one relatively far from the antenna, because the Fresnel reflection zone at 80m. is thousands of meters from the antenna. Still there are losses and reflections from conducting structures close to the antenna, and its pattern reflects that too. You can model an approximation with Mininec, which allows you to have several ground planes of different heights as long as you make them concentric circles. I hope this is helpful. 73, Pete Smith N4ZR n4zr@contesting.com ------------------------------------------------------------------------------- Hi Carlos - you're right a lot of other factors come into play 1. Assume the building is 500 ft above sea level and the dipole is 40 ft above the roof you are now 540 ft above sea level so far so good. 2. The next question is the composition of the roof i.e. tin or concrete with LOTS of rebar? What I'm getting At is that the roof may well act like the soil in your area for it's electrical characteristics and the net result Being that of a similar dipole mounted on the ground resulting in the same launch angle but in any case My preference would be the tall building all other things being equal. Gud luck es 73 Dick KH2G ------------------------------------------------------------------------------- Good afternoon, Carlos. The 80 dipole will be affected by the roof and how close the dipole is to the edge of the roof. I have had rotatable 80 mtr dipoles on two roof tops on Dxpeditions. The last one was XZ1N. The best location for the dipole is right at the edge of the roof. The dipole is parallel to the side of the building. If you can get the dipole out, over the edge of the building, it will be best. Having a rotatable 80 was good for testing. When the dipole was rotated so part was over the roof, the VSWR changed very much. This indicates the dipole is coupling to the roof - not good! The rotatable dipoles were about 16' above the roof (80 mtrs). The next best is to have the dipole very high above the roof. This will minimize the coupling to the roof. A height of 30' might be all right. The roof is a lot of steel. It also has wiring, etc. This means it can act as ground. Yes, the ground reflection zone is beyond the roof; however, the roof is in the near field. Have a good day and 73, Tom, N6BT Force 12 Antennas and Systems (Home Page http://www.QTH.com/force12 ) -------------------------------------------------------------------------------