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Antenna Ratings
On Sun, 28 Dec 2003 21:49:17 -0800, "Chuck Tribolet"
wrote: Larry: please define "very high gain"? 9db? 6db? The higher the "gain" of the antenna, the flatter and less "thick" its radiation pattern. The 9db antenna has a much flatter radiation pattern than the 6db. FWiW, I have have a 6db 8' Shake Galaxy on my 17' whaler. Works fine even when it's rock and roll. "In range" you'll really not see the difference on FM, which signal strength has less effect on until it's near the fringe. I just don't like the big, long fiberglass whips on small boats. Neither does USCG. They got fed up with them breaking off and all use the Metz, now. Metz brags about it on their webpage. If jumping waves with the SeaRayder jetboat can't break it, your boat won't, either. Mine is even mounted way up in the bow and has survived even submarining the bow at speed.....drowning us all in the process...(c; A fiberglass tube filled with hookup wire antenna elements would never make it....hee hee. |
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Antenna Ratings
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Antenna Ratings
On Wed, 31 Dec 2003 11:30:07 GMT, Jack Burton
wrote: On Tue, 30 Dec 2003 04:01:15 GMT, (Larry W4CSC) wrote: On Sun, 28 Dec 2003 21:49:17 -0800, "Chuck Tribolet" wrote: Larry: please define "very high gain"? 9db? 6db? The higher the "gain" of the antenna, the flatter and less "thick" its radiation pattern. The 9db antenna has a much flatter radiation pattern than the 6db. So, how does the horizon work into the gain equation? Picture the vertical antenna standing vertically. It's a high-gain fiberglass type with many phased elements imbedded into it. Its radiation pattern is perpendicular to the antenna in a flattened donut shape. When you step on a donut, its outer edge moves outward as its thickness is squeezed. That's exactly what's happening to get more radiation to the horizon where the receiving station is located. The phased array squeezes the fat round donut of the 1/2 wave, which bulges out in the middle making a bigger signal perpendicular to the antenna. This pattern moves with the antenna, always perpendicular to it. Now, tilt the antenna towards the receiving station. (Boat pitched?) The flattened donut is now pointing into the water on the side towards the receiving station. The signal level at the receiving station drops, drastically, because this pattern is so flattened. On an FM receiver, you hear no difference in signal "loudness" as signal varies, UNTIL that signal drops below the receiver's noise floor, usually a few hundredths of a microvolt, at which point the receiver "hiss" of its FM detector gets louder and louder. As the signal increases, again, the FM receiver "quiets", the hissing drops. This is the only time the effect of the high gain antenna's flattened pattern will cause communications problems. If the antenna's received signal drops from 800 uV to 40 uV, you won't notice it. 40 uV will keep the receiver hiss quiet. But, if we are talking about adding this pattern tilting effect to the boat riding down into the trough of those 30' rollers in a "worst case scenario" where it really counts, THEN we are talking about disrupted comms. The best antenna for this situation is NOT the 9dB fiberglass beast mounted on the side of the helm console.....but the 1/2 wave Metz stainless whip mounted as high up as you can get it.....with its fat pattern less effected by tilting that's still sticking up above the waves on top of the sailboat mast....like a beacon from the lighthouse. FWiW, I have have a 6db 8' Shake Galaxy on my 17' whaler. Works fine even when it's rock and roll. "In range" you'll really not see the difference on FM, which signal strength has less effect on until it's near the fringe. I just don't like the big, long fiberglass whips on small boats. Neither does USCG. They got fed up with them breaking off and all use the Metz, now. Metz brags about it on their webpage. Interesting - all the USCG small boats around these parts use the Shakespeare 396-1 which is a center-fed 1/2 wave. I use one and it's been great antenna. That's the best antenna mounted as high as you can get it. It's end-fed by a transformer in that cylinder base. It requires no groundplane. I just don't like the way I can't replace the broken cable permanently mounted to it up inside.....or the screwed-on whip because I can't slip a 34" piece of coat hanger into the end when the whip gets busted off by that little pitchpole we did sideways to that Perfect Strom roller in the night. The Metz whip is mounted in a gripping ferrule. Coat hanger wire slips right in where the broken whip comes out. |
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Antenna Ratings
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Antenna Ratings
On Thu, 01 Jan 2004 18:06:55 GMT, Shortwave Sportfishing
wrote: How can an antenna work without a ground plane? At the frequencies we're discussing, the ground effect in FM is about the same as it is in AM if I understood your discussion points (deleted from this post) correctly. Modulation has no effect on antenna physics. The object is to fit at least 1/2 wavelength onto a conductor. Where it is fed is of no consequence to radiation, but does effect the feedpoint impedance. As to the "ground plane"..... In an HF antenna, we are always dealing with an antenna where the practical length is far shorter than the ideal length. At 7 Mhz, a 1/2 wave antenna is about 65' long. Any 1/2 wave dipole antenna is complete and there is no ground plane. A ground plane is only required if your antenna design includes an image antenna. Case in point: Look at 90% of the AM broadcast antennas in your area. Some AM stations actually DO have 1/2 wavelength antennas, but a 1000 Khz this is 468' long and is expensive to erect and keep erected, so they only do it when they have no other choice. A 1/2 wavelength AM tower requires no extensive ground radial system, either, so it would be located in a dense city where you cannot lay out long radials to bury. Most AM transmit antennas are near 1/4 wavelength in length (a 108" CB stainless whip on Bubba's pickup is a 1/4 wavelength antenna on 27 Mhz). To get this 1/4 wavelength to "tune" (resonate), we have to bend the other half wavelength and lay it out sideways in an L pattern. However, erecting just an L 1/2 wave antenna fed at the corner of the L creates a radiation pattern in the direction of the horizontal of the L. To counter this effect, more horizontal elements are laid out around the base of the 1/4 wave vertical part to make the pattern omnidirectional, like the 1/2 wave resonant antenna. The end result is like a CB "ground plane" antenna for 27 Mhz. A 1/4 wave vertical over a set of "ground plane" radials, which don't have to be buried to work. On VHF marine at 160 Mhz, a whole half-wave antenna needs only be 34" long, so there is no need to resort to shortened whips working against "ground plane". The Metz antenna is a 34" stainless whip with an autotransformer on the end of it that raises the feedpoint from 52 ohms, the cable impedance to the transmitter, up to several hundred ohms, which excites the already-resonant element. No ground of any kind is required, or desired. Like you say, to each his own. |
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Antenna Ratings
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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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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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