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#11
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Antenna Ratings
On Fri, 02 Jan 2004 13:00:34 GMT, Shortwave Sportfishing
"Changing the height of the antenna above the ground will change the current flow assuming that the power to the antenna is constant." Again, it would appear that the "ground plane/wave" is not insignificant. Now, as I understand it, at VHF frequencies, the methodology of providing energy to the antenna (loading/feed) is not as important to the generation/reception of the signal as is height. In fact, if I read the pattern charts correctly, the height of the antenna has more to do with the lobe pattern (the donut you were discussing) than the method of feeding the antenna. Yes/No? Later, Tom S. Woodstock, CT The method of feeding the antenna has nothing to do with the take off angle of an antenna. The stuff you are reading in the handbook about ground effect on vertical angle are discussing HF antennas where due to the low height compared to wavelength and the propagation method at hf the ground has a great effect. An antenna at VHF is usually several wavelengths above ground and ground has little effect on take off angle. Given a 1/4 wave ground plane antenna with radials at 90 degrees, the ground plane of that antenna will be the controlling factor. It does tilt the angle up a little from what a 1/2 wave dipole would be. If you take the ground radials and bend them down and folded over the coax feed line so that they are vertical you then have a half wave center fed vertical dipole antenna. This particular version would have the coax going up through the center of the folded down ground radials. A tube or pipe could be substituted for the folded over ground radials. In most cases that is exactly what is done. A metal tube is used in place of the radials. This is commonly referred to as a "coaxial sleeve antenna". Just about all of the fiberglass type VHF antennas have some form of this type of ground plane in them. Another type of 1/2 wave vertical antenna that does not need a ground plane is one that is fed at the end of the 1/2 wave length rather than at the center as above. The METZ type half wave is one such type. It uses a coil at the feed end to transform the low impedance of the coax to the high impedance feed end of the antenna. At a high impedance feed point the current is much less than it is at a low impedance feed point (center of 1/2 wave) so not much of a ground plane is needed. In this case the coax shield acts as the ground plane for the 1/2 wave antenna. It is not the best ground plane but again not much is needed in this case. The high current point is in the center (quarter wave point) of the antenna. Once you get a few wavelengths above ground additional height does not much effect the radiation lobes (pattern) of the vhf antenna. But height does effect the line of sight or in this case radio line of sight, (which is slightly greater than visual). What Larry is talking about with the donut shaped vertical pattern is a result of gain in the antenna. The higher the gain the flatter (sharper) the vertical pattern of the antenna. Any tilt of the high gain antenna will raise the pattern above the horizon or tilt it into the sea. It will not be at the horizon where it does most good. A lower gain antenna, like the 1/2 wave, has a much fatter pattern. Like a fat doughnut or a ball. Tilting it one way or the other still maintains about the same amount of radiation at the horizon. This is because it normally radiates in a wide vertical pattern. With a wide vertical pattern a great portion of the signal is wasted as it gets radiated at high and low angles that are not useful. But the advantage is that it can be moved (tilted) a long way and still maintain about the same amount of useful signal. To get gain, a gain antenna narrows the wide vertical pattern. It robs some of the power normally radiated at high and low angles and places more of it at the horizon. Thus the narrower vertical pattern. The advantage is a stronger signal. The disadvantage is that if it is tilted very far the signal drops off sharply because the vertical angle is very narrow. Regards Gary |
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#13
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Antenna Ratings
In article ,
Gary Schafer wrote: Now, as I understand it, at VHF frequencies, the methodology of providing energy to the antenna (loading/feed) is not as important to the generation/reception of the signal as is height. In fact, if I read the pattern charts correctly, the height of the antenna has more to do with the lobe pattern (the donut you were discussing) than the method of feeding the antenna. Yes/No? Later, Tom S. Woodstock, CT snipped Regards Gary As always a very excelent explanation, Gary. Bruce in alaska -- add a 2 before @ |
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#14
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Antenna Ratings
On Fri, 02 Jan 2004 18:06:06 GMT, Gary Schafer
wrote: On Fri, 02 Jan 2004 13:00:34 GMT, Shortwave Sportfishing "Changing the height of the antenna above the ground will change the current flow assuming that the power to the antenna is constant." Again, it would appear that the "ground plane/wave" is not insignificant. Now, as I understand it, at VHF frequencies, the methodology of providing energy to the antenna (loading/feed) is not as important to the generation/reception of the signal as is height. In fact, if I read the pattern charts correctly, the height of the antenna has more to do with the lobe pattern (the donut you were discussing) than the method of feeding the antenna. Yes/No? Later, Tom S. Woodstock, CT The method of feeding the antenna has nothing to do with the take off angle of an antenna. The stuff you are reading in the handbook about ground effect on vertical angle are discussing HF antennas where due to the low height compared to wavelength and the propagation method at hf the ground has a great effect. An antenna at VHF is usually several wavelengths above ground and ground has little effect on take off angle. Given a 1/4 wave ground plane antenna with radials at 90 degrees, the ground plane of that antenna will be the controlling factor. It does tilt the angle up a little from what a 1/2 wave dipole would be. If you take the ground radials and bend them down and folded over the coax feed line so that they are vertical you then have a half wave center fed vertical dipole antenna. This particular version would have the coax going up through the center of the folded down ground radials. A tube or pipe could be substituted for the folded over ground radials. In most cases that is exactly what is done. A metal tube is used in place of the radials. This is commonly referred to as a "coaxial sleeve antenna". Just about all of the fiberglass type VHF antennas have some form of this type of ground plane in them. Another type of 1/2 wave vertical antenna that does not need a ground plane is one that is fed at the end of the 1/2 wave length rather than at the center as above. The METZ type half wave is one such type. It uses a coil at the feed end to transform the low impedance of the coax to the high impedance feed end of the antenna. At a high impedance feed point the current is much less than it is at a low impedance feed point (center of 1/2 wave) so not much of a ground plane is needed. In this case the coax shield acts as the ground plane for the 1/2 wave antenna. It is not the best ground plane but again not much is needed in this case. The high current point is in the center (quarter wave point) of the antenna. Once you get a few wavelengths above ground additional height does not much effect the radiation lobes (pattern) of the vhf antenna. But height does effect the line of sight or in this case radio line of sight, (which is slightly greater than visual). What Larry is talking about with the donut shaped vertical pattern is a result of gain in the antenna. The higher the gain the flatter (sharper) the vertical pattern of the antenna. Any tilt of the high gain antenna will raise the pattern above the horizon or tilt it into the sea. It will not be at the horizon where it does most good. A lower gain antenna, like the 1/2 wave, has a much fatter pattern. Like a fat doughnut or a ball. Tilting it one way or the other still maintains about the same amount of radiation at the horizon. This is because it normally radiates in a wide vertical pattern. With a wide vertical pattern a great portion of the signal is wasted as it gets radiated at high and low angles that are not useful. But the advantage is that it can be moved (tilted) a long way and still maintain about the same amount of useful signal. To get gain, a gain antenna narrows the wide vertical pattern. It robs some of the power normally radiated at high and low angles and places more of it at the horizon. Thus the narrower vertical pattern. The advantage is a stronger signal. The disadvantage is that if it is tilted very far the signal drops off sharply because the vertical angle is very narrow. Excellant Gary - that's what I was trying to get at by taking some leading questions and seeing where the discussion went. There were some details that I think were confusing and I just wanted to clear some of it up without it sounding like I was a newbie big shot trying to take over the subject. Nice job. Regards, Tom, NM1Q S. Woodstock, CT ---------- "I object to fishing tournaments less for what they do to fish than what they do to fishermen." Ted Williams - 1964 |
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#15
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Antenna Ratings
On Fri, 02 Jan 2004 13:00:34 GMT, Shortwave Sportfishing
wrote: Well, you kind of danced around the answer, but I'd still like to know how the ground plane effects the radiation angle which logically would also have an effect on reception of a signal. On page 3-9 of the ARRL Antenna Handbook (16th addition - sorry, it's the latest I have at hand at the moment) states: On a vertical antenna that USES a ground plane, the radiation angle increases towards straight up as the ground plane becomes "smaller", electrically less efficient. On a half-wave, end-fed vertically polarized antenna, where no ground plane is used as part of the antenna design, I doubt you could measure any difference. "The total current in the antenna consists of two components. The amplitude of the first is determined by the power supplied by the transmitter and the free-space radiation resistance of the antenna. The second component is induced in the antenna by the wave reflected by the ground. This second component, while considerably smaller than the first at most usefull antenna heights, is by no means insignificant." Ham antennas, dipoles, beams, etc., used for HF communications are HORIZONTALLY polarized antennas. This is a whole new ball game when they are close to "ground" be it a sheet metal roof or the ground, itself. Unlike the radiation pattern of the vertical halfwave in question, the radiation pattern of a horizontal dipole, which is still perpendicular to the dipole wire, INTERSECTS the ground plane below it and the RF re-radiates or reflects off the ground plane. The radiation pattern of a horizontal dipole very near ground is straight up and has a hot-air-balloon shape straight up. As the antenna moves away from ground, a dimple forms in the "balloon pattern" which forms a null at zenith with the radiation now two "lobes", perpendicular to the dipole whos angle of radiation drops from zenith out towards the horizon as the dipole becomes 1/2 wavelength off ground. Beyond 1 wavelength off ground, the pattern becomes the familiar donut perpendicular to the horizontal wire radiating upward and outward, even down towards the ground plane, whos reflections and re-radiation phase angles caused the odd pattern in the first place. Vertically-polarized signals point the NULL in the radiation pattern off the ends of the dipole. One of these nulls is towards the ground plane so little re-radiation takes place. A 1/4 wavelength "ground plane antenna" has a radiation pattern elevated only slightly towards zenith, which isn't much of a problem at all. So it would seem that the "ground plane/wave" is not an umimportant consideration when considering antennas. Ground effect is VERY important in a horizontally polarized dipole or beam antenna. That's why we put the beams way up on towers so they radiate towards the horizon, not at high radiation angles. The phase shifted re-radiated patterns of the slightly longer reflector and slightly shorter directors (lagging and leading, respectively) "pull" the donut towards the directors and away from the reflector, pointing the beam's radiation pattern in the desired, narrow direction....and giving great gain.....if it's not too close to the ground, that is! Further on that same page, is the following: "Changing the height of the antenna above the ground will change the current flow assuming that the power to the antenna is constant." Again, this is for "ham antennas" which are generally horizontally polarized. The reflected wave from the ground back to the horizontal dipole GREATLY changes its impedance characteristics because that reflected wave causes a phase shifted current in the radiating element, itself. Close to ground, this creates a large REACTIVE component, which shows up as reflected power at the transmitter output, not good at all. Again, it would appear that the "ground plane/wave" is not insignificant. Now, as I understand it, at VHF frequencies, the methodology of providing energy to the antenna (loading/feed) is not as important to the generation/reception of the signal as is height. In fact, if I read the pattern charts correctly, the height of the antenna has more to do with the lobe pattern (the donut you were discussing) than the method of feeding the antenna. Yes/No? No, not on VERTICALLY polarized antennas. On VHF there is no replacement for POWER and ALTITUDE. VHF is line-of-sight communications. The higher the transmitter and receiver antennas are, the longer your range and better your signal at the longer range. It's why WCSC runs hundreds of kilowatts from a 2000' tower.....RANGE. They pay very dearly for both. You should see the WEEKLY electric bills and tower maintenance bills. On a skywave system, like AM radio at night or ham radio HF, altitude is not very important other than to make the antenna's radiation pattern and impedance what we want because it's horizontal polarization. |
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#16
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Antenna Ratings
All antennas are two terminal devices.
There is no such thing as a single point feed antenna. Changing a horizontal antenna to a vertical antenna at the same height does not improve the radiation angle. If it did everyone would have their horizontal antennas in the vertical position. AM radio stations depend on ground wave signals not sky wave. A horizontal antenna theoretically does not have a ground wave so verticals are used for AM stations as they produce a ground wave signal. For sky wave signals height is important in order to produce better lower angle of radiation. Lower angle radiation provides longer hops. Adjusting the height to adjust the impedance of the antenna is not to be worried about. That's what matching systems are for. With multielement antennas (beams) the feed point impedance can be very low even if the antenna is high in the air. Some other means of matching the antenna to the line is required. Regards Gary On Sat, 03 Jan 2004 04:13:27 GMT, (Larry W4CSC) wrote: On Fri, 02 Jan 2004 13:00:34 GMT, Shortwave Sportfishing wrote: Well, you kind of danced around the answer, but I'd still like to know how the ground plane effects the radiation angle which logically would also have an effect on reception of a signal. On page 3-9 of the ARRL Antenna Handbook (16th addition - sorry, it's the latest I have at hand at the moment) states: On a vertical antenna that USES a ground plane, the radiation angle increases towards straight up as the ground plane becomes "smaller", electrically less efficient. On a half-wave, end-fed vertically polarized antenna, where no ground plane is used as part of the antenna design, I doubt you could measure any difference. "The total current in the antenna consists of two components. The amplitude of the first is determined by the power supplied by the transmitter and the free-space radiation resistance of the antenna. The second component is induced in the antenna by the wave reflected by the ground. This second component, while considerably smaller than the first at most usefull antenna heights, is by no means insignificant." Ham antennas, dipoles, beams, etc., used for HF communications are HORIZONTALLY polarized antennas. This is a whole new ball game when they are close to "ground" be it a sheet metal roof or the ground, itself. Unlike the radiation pattern of the vertical halfwave in question, the radiation pattern of a horizontal dipole, which is still perpendicular to the dipole wire, INTERSECTS the ground plane below it and the RF re-radiates or reflects off the ground plane. The radiation pattern of a horizontal dipole very near ground is straight up and has a hot-air-balloon shape straight up. As the antenna moves away from ground, a dimple forms in the "balloon pattern" which forms a null at zenith with the radiation now two "lobes", perpendicular to the dipole whos angle of radiation drops from zenith out towards the horizon as the dipole becomes 1/2 wavelength off ground. Beyond 1 wavelength off ground, the pattern becomes the familiar donut perpendicular to the horizontal wire radiating upward and outward, even down towards the ground plane, whos reflections and re-radiation phase angles caused the odd pattern in the first place. Vertically-polarized signals point the NULL in the radiation pattern off the ends of the dipole. One of these nulls is towards the ground plane so little re-radiation takes place. A 1/4 wavelength "ground plane antenna" has a radiation pattern elevated only slightly towards zenith, which isn't much of a problem at all. So it would seem that the "ground plane/wave" is not an umimportant consideration when considering antennas. Ground effect is VERY important in a horizontally polarized dipole or beam antenna. That's why we put the beams way up on towers so they radiate towards the horizon, not at high radiation angles. The phase shifted re-radiated patterns of the slightly longer reflector and slightly shorter directors (lagging and leading, respectively) "pull" the donut towards the directors and away from the reflector, pointing the beam's radiation pattern in the desired, narrow direction....and giving great gain.....if it's not too close to the ground, that is! Further on that same page, is the following: "Changing the height of the antenna above the ground will change the current flow assuming that the power to the antenna is constant." Again, this is for "ham antennas" which are generally horizontally polarized. The reflected wave from the ground back to the horizontal dipole GREATLY changes its impedance characteristics because that reflected wave causes a phase shifted current in the radiating element, itself. Close to ground, this creates a large REACTIVE component, which shows up as reflected power at the transmitter output, not good at all. Again, it would appear that the "ground plane/wave" is not insignificant. Now, as I understand it, at VHF frequencies, the methodology of providing energy to the antenna (loading/feed) is not as important to the generation/reception of the signal as is height. In fact, if I read the pattern charts correctly, the height of the antenna has more to do with the lobe pattern (the donut you were discussing) than the method of feeding the antenna. Yes/No? No, not on VERTICALLY polarized antennas. On VHF there is no replacement for POWER and ALTITUDE. VHF is line-of-sight communications. The higher the transmitter and receiver antennas are, the longer your range and better your signal at the longer range. It's why WCSC runs hundreds of kilowatts from a 2000' tower.....RANGE. They pay very dearly for both. You should see the WEEKLY electric bills and tower maintenance bills. On a skywave system, like AM radio at night or ham radio HF, altitude is not very important other than to make the antenna's radiation pattern and impedance what we want because it's horizontal polarization. |
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#17
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Antenna Ratings
On Mon, 05 Jan 2004 02:02:17 GMT, Gary Schafer
wrote: All antennas are two terminal devices. There is no such thing as a single point feed antenna. Changing a horizontal antenna to a vertical antenna at the same height does not improve the radiation angle. If it did everyone would have their horizontal antennas in the vertical position. AM radio stations depend on ground wave signals not sky wave. A horizontal antenna theoretically does not have a ground wave so verticals are used for AM stations as they produce a ground wave signal. Hmm....I've been using horizontal antennas to transmit ground wave communications since I was 11 years old in 1957! All VHF and UHF TV stations use ground wave only signals and every one of them in the USA are HORIZONTALLY POLARIZED. Kinda blows that theory all to hell, doesn't it? Until very recently, all FM radio stations were all horizontally polarized, too, but that was changed because cars have vertically polarized antennas......or did when they changed the rules. Embedded FM antennas in windshields are horizontally polarized, a dipole. For sky wave signals height is important in order to produce better lower angle of radiation. Lower angle radiation provides longer hops. Depends on how far you wish to talk. I use very high angles of radiation on 3915 Khz to talk to my buddies around SC, NC and Georgia on 75 meters. This is easily possible because my inverted-V dipole is only up about 30', not much % of a wavelength when the wavelength is 240 feet long. Listen from 3.5 to 4.0 Mhz nights and 7.0-7.3 Mhz days and hear lots of us "high-angle-radiators" shooting the breeze, ad nauseum. Works great since 1957....(c; Adjusting the height to adjust the impedance of the antenna is not to be worried about. That's what matching systems are for. With multielement antennas (beams) the feed point impedance can be very low even if the antenna is high in the air. Some other means of matching the antenna to the line is required. Absolutely nothing radiates like a TUNED antenna, without the lossy tuner between feedline and antenna. Damned Navy has tuners so inefficient trying to load a flagpole whip they have to have BLOWERS in them to keep from melting them. All that power ISN'T radiated, obviously. I used to operate WB4THE/MM2 when I was a young sailor on USS Everglades (AD-24) back in the 60's. It was my call at that time. My captain never got over I could make phone patches to his wife back in Charleston through a friend of mine on James Island (K4OKD) with my $249 Heathkit HW-100 kit transceiver, even barefoot, when the Navy's million-dollar radio installation wouldn't. He used to tease the comm officer by saying to him, "I want to talk to Charleston. Can we do that?" Of course, the answer was no. "That's BS. I just talked to my wife on ET1 Butler's ham radio not 10 minutes ago! What's wrong with our radios?" The comm officer hated my guts.....and said so...(c; They used to listen to my homebrew quad 813s linear amp up in radio when the old man was on the air. The chief radioman used it, too.... Regards Gary 73 DE W4CSC/MM2 Watch out for those ground waves.....(c; |
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#18
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Antenna Ratings
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#19
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Antenna Ratings
On Tue, 06 Jan 2004 18:17:04 GMT, Gary Schafer
wrote: TV and FM stations do not depend on ground wave propagation. It is line of sight. Nothing to do with polarization or ground wave. Gary, maybe you can help me out with something here, because either I've forgotten or I was taught incorrectly. Over short distances (Up to five to eight miles), the ground wave component of any VHF signal is actually the primary method by which VHF signals are received and not by Line Of Sight. Not true? Later, Tom S. Woodstock, CT ---------- "My rod and my reel - they comfort me." St. Pete, 12 Lb. Test |
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#20
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Antenna Ratings
On Tue, 06 Jan 2004 22:11:54 GMT, Shortwave Sportfishing
wrote: On Tue, 06 Jan 2004 18:17:04 GMT, Gary Schafer wrote: TV and FM stations do not depend on ground wave propagation. It is line of sight. Nothing to do with polarization or ground wave. Gary, maybe you can help me out with something here, because either I've forgotten or I was taught incorrectly. Over short distances (Up to five to eight miles), the ground wave component of any VHF signal is actually the primary method by which VHF signals are received and not by Line Of Sight. Not true? Later, Tom S. Woodstock, CT I don't think so. There is some bending, refraction of the signal that makes the VHF radio horizon slightly longer than line of sight but it is due to the atmosphere and not ground wave propagation. If you have a VHF station on a high mountain peak or a very high tower there could be no ground effect at all. Regards Gary |
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