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Rhombics
Howdy Rhombic fans,
OK on Ian's rhombic experiences on V/UHF, but that I believe would be different situation, I.e. rhombic not working in conjunction with ground reflections/effects. With "normal" HF rhombic we have a situation where antenna is spread out over few wavelengths and interacting with ground. Spacing two antennas few wavelengths can give us diversity effect by the virtue of propagation and waves hitting them differently. Rhombic is kind of antenna/feeders spread out over few wavelengths over ground. Perhaps there is also some of traveling wave mode going on, like in a Beverage. You can't simulate that or use VHF analogy being many wavelength away from the ground effects. Don't forget that skywave polarization is all over and rolling around. I see big differences when trying to model vertical arrays destined to work on ground and modeled in "free space", different pattern and just plain ridiculous to do that. I know from my hardware experiments with Razors (quad - yagi) that if I changed height of the boom and I reoptimized the spacing/dimensions I would get different configuration. So my conclusion was that antenna has to be designed for the height it is going to be used at. Closer to ground, the more pronounced effect. What I am trying to say and not sell anything, is that perhaps the ground effect and rhombic's spread over it over few wavelengths might have something to do with it's good performance. I think that software modeling is a great tool, but I also know that it can not capture all the variables and effects that are happening around antennas, the ground and the sky. All I am pointing out that based on mine and other's experience, rhombic is a great antenna, performing perhaps better than modeling shows (W8JI web site downplays it). If I get the chance, I will try to do some real life tests and comparisons on HF. We have some 30 rhombics, some phased side by side and will try to model and compare them with other antennas. BTW our rhombics have a load resistor made of open (resistive?) wire stub, folded few times back and forth. They were used with 50 kW transmitters. see http://www.teslaradio.org/site_survey.htm I am just trying to bring attention to possible discrepancy that perhaps is worth exploring. If some believe in gospel of modeling and paper antennas, than enjoy it. I prefer reality. When I wrote my observations about propagation happening by ducting and refraction some 25 years ago, I was ridiculed. Now the propagation experts are accepting it and playing discoverers (only ON4UN gives me some credit :-). Yuri, K3BU Tesla RC N2EE "Ian White GM3SEK" wrote in message ... Alan Peake wrote: Dave Oldridge wrote: Properly designed, they have a good reputation for doing what the theory says they will do. Just remember, though, that you're going to have to sewer almost half your transmitted power into the terminating resistor. But that's the half that would be going the wrong way, basically. But it would get there - eventually Long or short path. But I don't know if both paths are ever open at the same time to the same extent. If not, then it may not be a problem. What about running a transmission line from where the terminating R would be, back to the feed point? Assuming you can match it all that is. No problem. Pipe the signal back from the far end into the shack, feed it into a circulator, and add it to the outgoing signal. Cecil will explain what happens to the power :-) Replying to Yuri's point: from personal experience of using a rhombic 100 wavelengths long for 2m moonbounce, it had only about the same maximum gain as a box of 4 mid-size yagis - and that is only while the moon is passing through the very narrow main beam, which only happens for a magic 20 minutes on certain days of the month. In other words, the rhombic did work, but the performance was nowhere near as spectacular as we had expected from its huge electrical length. What is undeniably true is that it *looked* spectacular! I've used many kinds of antennas since then, up to an 85ft dish, but not one of them has given me the same buzz as that rhombic. And there is the trap: buzz isn't the same thing as performance. We need to be very careful about applying dual standards. An unavoidable feature of all very long rhombics is that the main beam is very narrow, because the edges of the main lobe are sliced away by large numbers of sidelobes that are not many dB down. If we saw that kind of E-plane pattern in a yagi, we wouldn't hesitate to call it a "bad design"... so what's "good" about the same feature in a rhombic? 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek |
#2
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Rhombics
Yuri Blanarovich wrote:
[snip] I am just trying to bring attention to possible discrepancy that perhaps is worth exploring. If some believe in gospel of modeling and paper antennas, than enjoy it. I prefer reality. When I wrote my observations about propagation happening by ducting and refraction some 25 years ago, I was ridiculed. Now the propagation experts are accepting it and playing discoverers (only ON4UN gives me some credit :-). Yuri, K3BU Yuri, I am a bit puzzled by your comment. Ducting and refraction received a lot of technical and mathematical study back at least to the mid-1930's. Terman's books show similar figures to those in your CQ article, and they give numerous technical references from the 1940's. You may have been ridiculed by the amateur radio community, but the pros accepted that sort of propagation explanation many decades earlier. 73, Gene W4SZ |
#3
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Rhombics
Yuri Blanarovich wrote:
OK on Ian's rhombic experiences on V/UHF, but that I believe would be different situation, I.e. rhombic not working in conjunction with ground reflections/effects. With "normal" HF rhombic we have a situation where antenna is spread out over few wavelengths and interacting with ground. The same is true at VHF. All rhombics interact with ground, because their length is many times greater than their height above ground. The number of wavelengths above ground will change the details, but not the basic fact. Spacing two antennas few wavelengths can give us diversity effect by the virtue of propagation and waves hitting them differently. Rhombic is kind of antenna/feeders spread out over few wavelengths over ground. Perhaps there is also some of traveling wave mode going on, like in a Beverage. You can't simulate that Are you quite sure of that? In other words, do you have firm evidence and reasons why? or use VHF analogy being many wavelength away from the ground effects. That analogy was your assumption, never mine :-) Don't forget that skywave polarization is all over and rolling around. I see big differences when trying to model vertical arrays destined to work on ground and modeled in "free space", different pattern and just plain ridiculous to do that. I know from my hardware experiments with Razors (quad - yagi) that if I changed height of the boom and I reoptimized the spacing/dimensions I would get different configuration. So my conclusion was that antenna has to be designed for the height it is going to be used at. Closer to ground, the more pronounced effect. What I am trying to say and not sell anything, is that perhaps the ground effect and rhombic's spread over it over few wavelengths might have something to do with it's good performance. Maybe it does... but I tend not to believe such things unless they come with good solid reasons. I think that software modeling is a great tool, but I also know that it can not capture all the variables and effects that are happening around antennas, the ground and the sky. All I am pointing out that based on mine and other's experience, rhombic is a great antenna, performing perhaps better than modeling shows (W8JI web site downplays it). If I get the chance, I will try to do some real life tests and comparisons on HF. We have some 30 rhombics, some phased side by side and will try to model and compare them with other antennas. BTW our rhombics have a load resistor made of open (resistive?) wire stub, folded few times back and forth. They were used with 50 kW transmitters. see http://www.teslaradio.org/site_survey.htm I am just trying to bring attention to possible discrepancy that perhaps is worth exploring. If some believe in gospel of modeling and paper antennas, than enjoy it. I prefer reality. I don't ever see that as an "either-or" choice - I am not comfortable until both viewpoints agree. If modeling and 'reality' observations do not seem to agree, it means we're still missing some pieces of the puzzle. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#4
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Rhombics
Yuri, K3BU wrote:
"All I am pointing out that based on mine and other`s experience, rhombic is a great antenna,---." Agreed. I worked for an oil company on Tierra del Fuego. We frequently had no contact with our office in Buenos Aires. I put up a rhombic driven by a 20-watt Hallicrafters transmitter surplus from our Bolivian operations. They had upgraded to Collins 30-K5 transmtters there. The Argentine Post Office used a surplus 300-watt BC-610 into a dipole near our camp. When I called the same station in Buenos Aires used by the Post Office, they at first would not believe that I was as far away as Tierra del Fuego. Best regards, Richard Harrison, KB5WZI |
#5
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Rhombics
Yuri Blanarovich wrote:
. . . Spacing two antennas few wavelengths can give us diversity effect by the virtue of propagation and waves hitting them differently. Rhombic is kind of antenna/feeders spread out over few wavelengths over ground. Perhaps there is also some of traveling wave mode going on, like in a Beverage. You can't simulate that . . . NEC-based modeling programs don't have any problem simulating a Beverage and its traveling wave operation -- they do quite well at it. MININEC-based programs, no longer in wide use, weren't able to because of the use of perfect ground during impedance and current calculations. Roy Lewallen, W7EL |
#6
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Rhombics
"Ian White GM3SEK" wrote in message ... Alan Peake wrote: Dave Oldridge wrote: Properly designed, they have a good reputation for doing what the theory says they will do. Just remember, though, that you're going to have to sewer almost half your transmitted power into the terminating resistor. But that's the half that would be going the wrong way, basically. But it would get there - eventually Long or short path. But I don't know if both paths are ever open at the same time to the same extent. If not, then it may not be a problem. What about running a transmission line from where the terminating R would be, back to the feed point? Assuming you can match it all that is. No problem. Pipe the signal back from the far end into the shack, feed it into a circulator, and add it to the outgoing signal. Cecil will explain what happens to the power :-) Replying to Yuri's point: from personal experience of using a rhombic 100 wavelengths long for 2m moonbounce, it had only about the same maximum gain as a box of 4 mid-size yagis - and that is only while the moon is passing through the very narrow main beam, which only happens for a magic 20 minutes on certain days of the month. In other words, the rhombic did work, but the performance was nowhere near as spectacular as we had expected from its huge electrical length. What is undeniably true is that it *looked* spectacular! I've used many kinds of antennas since then, up to an 85ft dish, but not one of them has given me the same buzz as that rhombic. And there is the trap: buzz isn't the same thing as performance. We need to be very careful about applying dual standards. An unavoidable feature of all very long rhombics is that the main beam is very narrow, because the edges of the main lobe are sliced away by large numbers of sidelobes that are not many dB down. If we saw that kind of E-plane pattern in a yagi, we wouldn't hesitate to call it a "bad design"... so what's "good" about the same feature in a rhombic? -- 73 from Ian GM3SEK http://www.ifwtech.co.uk/g3sek |
#7
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Rhombics
Ian, GM3SEK commented thoughtfully on many points regarding the rhombic.
It is an excellent antenna for fixed point communications. Unfortunately, the earth and moon are not stationary with respect to each other. The rhombic may not be so re-directable as antennas like the Yagi or corner reflector. For transmitting, the rhombic has a lower maximum voltage than a resonant standing-wave antenna. For high power, it`s a consideration. The power remaining at the far end of the rhombic has been recycled by some to eliminaate waste. Cecil does an excellent job of explaining where the power in a transmission line goes. Unfortunately, re-entrant rhombics tend to lose some of their advantages of simplicity and bandwidth in the elaboration process. On the subject of diversity, it works by switching to a better receiving system. Without switching, spaced antennas only provide a phased array in which destructive interference happens during some signal conditions. Where I worked, we used triple diversity receivers with 3 rhombic antennas spaced about 10 wavelengths apart at the low frequency end of the spectrum. A Crosby or Pioneer combiner selected the best of the 3 receiver outputs to feed a program line. Several receiving systems shared the same 3 rhombics for simultaneous program relay. Sidelobes are only a small problem when using triple diversity and selectable sideband in reception. Best regards, Richard Harrison, KB5WZI |
#8
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Rhombics
Sorry I mis-spoke in my story about Tierra del Fuego. The power into the
rhombic and dipole was about the same. Despite the size difference between a Hallicrafters HT-20, and the military surplus BC-610, both put out about 100 watts on AM. Best regards, Richard Harrison, KB5WZI |
#9
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Rhombics
Ian White GM3SEK wrote:
. . . What is undeniably true is that it *looked* spectacular! I've used many kinds of antennas since then, up to an 85ft dish, but not one of them has given me the same buzz as that rhombic. And there is the trap: buzz isn't the same thing as performance. . . . As always, Ian put his finger directly on a fundamental principle. The problem with modeling programs is that they don't model the buzz. Roy Lewallen, W7EL |
#10
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Rhombics
Ian White GM3SEK wrote in news:bVv
: Alan Peake wrote: Dave Oldridge wrote: Properly designed, they have a good reputation for doing what the theory says they will do. Just remember, though, that you're going to have to sewer almost half your transmitted power into the terminating resistor. But that's the half that would be going the wrong way, basically. But it would get there - eventually Long or short path. But I don't know if both paths are ever open at the same time to the same extent. If not, then it may not be a problem. What about running a transmission line from where the terminating R would be, back to the feed point? Assuming you can match it all that is. No problem. Pipe the signal back from the far end into the shack, feed it into a circulator, and add it to the outgoing signal. Cecil will explain what happens to the power :-) Replying to Yuri's point: from personal experience of using a rhombic 100 wavelengths long for 2m moonbounce, it had only about the same maximum gain as a box of 4 mid-size yagis - and that is only while the moon is passing through the very narrow main beam, which only happens for a magic 20 minutes on certain days of the month. In other words, the rhombic did work, but the performance was nowhere near as spectacular as we had expected from its huge electrical length. What is undeniably true is that it *looked* spectacular! I've used many kinds of antennas since then, up to an 85ft dish, but not one of them has given me the same buzz as that rhombic. And there is the trap: buzz isn't the same thing as performance. We need to be very careful about applying dual standards. An unavoidable feature of all very long rhombics is that the main beam is very narrow, because the edges of the main lobe are sliced away by large numbers of sidelobes that are not many dB down. If we saw that kind of E-plane pattern in a yagi, we wouldn't hesitate to call it a "bad design"... so what's "good" about the same feature in a rhombic? Yes, they are much better as HF point-to-point antennas than they are as general purpose. For that kind of service, you can pick your height and rhombic size to specifically service the one path. -- Dave Oldridge+ ICQ 1800667 |
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