|
H.F Dish Antenna update
In the past I mentioned that I was modelling an antenna for 20 metres.
This antenna became a single driven element with five reflectors which emulate a dish in it's crudish form not only in position of reflectors but also the elements were not shaped at this time. The following table follows what one would get for a reflective dish used on frequencies in the giga hertz range where the bandwidth is broader than that obtained with the yagi format PLUS relevant atributes stay more constant over a frequency range than that obtained with a super gain type array. The band width is wider than a typical yagi in that the major portion of the radiation is to the front this.This band with is expected to narrow and to increase gain when individual elements are shaped to a parabolic form and it is hoped that forward side lobes will not occur as is normal for a long boom multi element yagi form as used on 440 Mhz. The driven element is at a height of 76 feet over perfect ground. Normally one would say that if results seem out of the ordinary one should question it, so comments are requested regarding my explanations which lead me to think that they really are what one should expect. Freq 14.0 14.1 14.2 14.3 14.35 Gain dbi 13.86 14.05 13.99 13.9 13.87 F/B 30 25.6 27.47 33.17 32.5 Zr 45.9 44.5 43.8 43.2 42.8 Zi -12.87 -6.52 0.6 7.84 11.54 SWR 1.32 1.2 1.14 1.25 1.34 I do not advocate the replacement of the normal yagi with the above because of building difficulties tho it is my intention to build it. Regards Art (Modelled using AO PRO with 80 segment/half wave and set up by a consumate error generator Grin ) |
"art" wrote in message oups.com... In the past I mentioned that I was modelling an antenna for 20 metres. This antenna became a single driven element with five reflectors which emulate a dish in it's crudish form not only in position of reflectors but also the elements were not shaped at this time. The following table follows what one would get for a reflective dish used on frequencies in the giga hertz range where the bandwidth is broader than that obtained with the yagi format PLUS relevant atributes stay more constant over a frequency range than that obtained with a super gain type array. The band width is wider than a typical yagi in that the major portion of the radiation is to the front this.This band with is expected to narrow and to increase gain when individual elements are shaped to a parabolic form and it is hoped that forward side lobes will not occur as is normal for a long boom multi element yagi form as used on 440 Mhz. The driven element is at a height of 76 feet over perfect ground. Normally one would say that if results seem out of the ordinary one should question it, so comments are requested regarding my explanations which lead me to think that they really are what one should expect. Freq 14.0 14.1 14.2 14.3 14.35 Gain dbi 13.86 14.05 13.99 13.9 13.87 F/B 30 25.6 27.47 33.17 32.5 Zr 45.9 44.5 43.8 43.2 42.8 Zi -12.87 -6.52 0.6 7.84 11.54 SWR 1.32 1.2 1.14 1.25 1.34 I do not advocate the replacement of the normal yagi with the above because of building difficulties tho it is my intention to build it. Regards Art (Modelled using AO PRO with 80 segment/half wave and set up by a consumate error generator Grin ) F/B looks too high, you should have some big splinters off the backside. (with a dish it can be hard to get 30) Gain looks to be high for a 1 driven and 5 reflectors. Does model take into account mutual effects of the reflectors? |
"art" wrote in message oups.com... In the past I mentioned that I was modelling an antenna for 20 metres. This antenna became a single driven element with five reflectors which emulate a dish in it's crudish form not only in position of reflectors but also the elements were not shaped at this time. The following table follows what one would get for a reflective dish used on frequencies in the giga hertz range where the bandwidth is broader than that obtained with the yagi format PLUS relevant atributes stay more constant over a frequency range than that obtained with a super gain type array. CORRECTION The BEAM width is wider than a typical yagi in that the major portion of the radiation is to the front .This BEAM width is expected to narrow and to increase gain when individual elements are shaped to a parabolic form It is hoped that forward side lobes will not occur as is normal for a long boom multi element yagi form as used on 440 Mhz. The driven element is at a height of 76 feet over perfect ground. Normally one would say that if results seem out of the ordinary one should question it, so comments are requested regarding my explanations which lead me to think that they really are what one should expect. Freq 14.0 14.1 14.2 14.3 14.35 Gain dbi 13.86 14.05 13.99 13.9 13.87 F/B 30 25.6 27.47 33.17 32.5 Zr 45.9 44.5 43.8 43.2 42.8 Zi -12.87 -6.52 0.6 7.84 11.54 SWR 1.32 1.2 1.14 1.25 1.34 I do not advocate the replacement of the normal yagi with the above because of building difficulties tho it is my intention to build it. Regards Art (Modelled using AO PRO with 80 segment/half wave and set up by a consumate error generator Grin ) |
"hnkie" wrote in message ... "art" wrote in message oups.com... In the past I mentioned that I was modelling an antenna for 20 metres. This antenna became a single driven element with five reflectors which emulate a dish in it's crudish form not only in position of reflectors but also the elements were not shaped at this time. The following table follows what one would get for a reflective dish used on frequencies in the giga hertz range where the bandwidth is broader than that obtained with the yagi format PLUS relevant atributes stay more constant over a frequency range than that obtained with a super gain type array. The band width is wider than a typical yagi in that the major portion of the radiation is to the front this.This band with is expected to narrow and to increase gain when individual elements are shaped to a parabolic form and it is hoped that forward side lobes will not occur as is normal for a long boom multi element yagi form as used on 440 Mhz. The driven element is at a height of 76 feet over perfect ground. Normally one would say that if results seem out of the ordinary one should question it, so comments are requested regarding my explanations which lead me to think that they really are what one should expect. Freq 14.0 14.1 14.2 14.3 14.35 Gain dbi 13.86 14.05 13.99 13.9 13.87 F/B 30 25.6 27.47 33.17 32.5 Zr 45.9 44.5 43.8 43.2 42.8 Zi -12.87 -6.52 0.6 7.84 11.54 SWR 1.32 1.2 1.14 1.25 1.34 I do not advocate the replacement of the normal yagi with the above because of building difficulties tho it is my intention to build it. Regards Art (Modelled using AO PRO with 80 segment/half wave and set up by a consumate error generator Grin ) F/B looks too high, you should have some big splinters off the backside. Why? Do you have splinters in a parabolic dish? (with a dish it can be hard to get 30) Gain looks to be high for a 1 driven and 5 reflectors. If you picture a balloon squashed at one end more than normal would not the gain increase? Does model take into account mutual effects of the reflectors? Yes Art |
A plain dipole 76 feet above perfect ground has a gain of about 8.6 dBi
- surely you've modeled one with AO. So the gain of your antenna over a dipole is about 5.3 - 5.5 dB. (This would be a more meaningful way to state gain.) This is just slightly more than the gain from a three element Yagi, and less than a typical four element beam. The F/B ratio and bandwidth might be better than that of a typical three or four element Yagi (or they might not be -- a Yagi can be adjusted for a wide range of characteristics), but the gain is certainly not exceptional. Roy Lewallen, W7EL art wrote: In the past I mentioned that I was modelling an antenna for 20 metres. This antenna became a single driven element with five reflectors which emulate a dish in it's crudish form not only in position of reflectors but also the elements were not shaped at this time. The following table follows what one would get for a reflective dish used on frequencies in the giga hertz range where the bandwidth is broader than that obtained with the yagi format PLUS relevant atributes stay more constant over a frequency range than that obtained with a super gain type array. The band width is wider than a typical yagi in that the major portion of the radiation is to the front this.This band with is expected to narrow and to increase gain when individual elements are shaped to a parabolic form and it is hoped that forward side lobes will not occur as is normal for a long boom multi element yagi form as used on 440 Mhz. The driven element is at a height of 76 feet over perfect ground. Normally one would say that if results seem out of the ordinary one should question it, so comments are requested regarding my explanations which lead me to think that they really are what one should expect. Freq 14.0 14.1 14.2 14.3 14.35 Gain dbi 13.86 14.05 13.99 13.9 13.87 F/B 30 25.6 27.47 33.17 32.5 Zr 45.9 44.5 43.8 43.2 42.8 Zi -12.87 -6.52 0.6 7.84 11.54 SWR 1.32 1.2 1.14 1.25 1.34 I do not advocate the replacement of the normal yagi with the above because of building difficulties tho it is my intention to build it. Regards Art (Modelled using AO PRO with 80 segment/half wave and set up by a consumate error generator Grin ) |
" wrote in message news:1k%vd.198273$V41.135426@attbi_s52... "hnkie" wrote in message ... "art" wrote in message oups.com... In the past I mentioned that I was modelling an antenna for 20 metres. This antenna became a single driven element with five reflectors which emulate a dish in it's crudish form not only in position of reflectors but also the elements were not shaped at this time. The following table follows what one would get for a reflective dish used on frequencies in the giga hertz range where the bandwidth is broader than that obtained with the yagi format PLUS relevant atributes stay more constant over a frequency range than that obtained with a super gain type array. The band width is wider than a typical yagi in that the major portion of the radiation is to the front this.This band with is expected to narrow and to increase gain when individual elements are shaped to a parabolic form and it is hoped that forward side lobes will not occur as is normal for a long boom multi element yagi form as used on 440 Mhz. The driven element is at a height of 76 feet over perfect ground. Normally one would say that if results seem out of the ordinary one should question it, so comments are requested regarding my explanations which lead me to think that they really are what one should expect. Freq 14.0 14.1 14.2 14.3 14.35 Gain dbi 13.86 14.05 13.99 13.9 13.87 F/B 30 25.6 27.47 33.17 32.5 Zr 45.9 44.5 43.8 43.2 42.8 Zi -12.87 -6.52 0.6 7.84 11.54 SWR 1.32 1.2 1.14 1.25 1.34 I do not advocate the replacement of the normal yagi with the above because of building difficulties tho it is my intention to build it. Regards Art (Modelled using AO PRO with 80 segment/half wave and set up by a consumate error generator Grin ) F/B looks too high, you should have some big splinters off the backside. Why? Do you have splinters in a parabolic dish? You have individual elements, a parabolic has continous surface. Antennas always have splinters, with just 5 elements and no blocking you probably have major lobes on the back side, between elements. There will be places on the back side of your antenna that add in phase with two or more elements, which is a lobe or splinter. Parabolics have a rolloff on the edge to decay the field to minimize the back lobe. (with a dish it can be hard to get 30) Gain looks to be high for a 1 driven and 5 reflectors. If you picture a balloon squashed at one end more than normal would not the gain increase? What is the capture area? Each element captures RF energy and reflects it back to the driven element in phase with the others. Your idea has been tried very sucessfully at 450 to microwave, and they now use flat antennas with each element phase adjusted for delay (same as curverature of a parabolic dish) One antenna has over a thousand elements. Does model take into account mutual effects of the reflectors? Yes Art |
Roy Lewallen wrote:
A plain dipole 76 feet above perfect ground has a gain of about 8.6 dBi Is that true? I've always heard it was slightly over two dB. I re-read your post and I don't think I'm taking what you said out of context. Did I? 73, kz1o http://www.radio-electronics.com/inf...ivity_gain.php http://www.tmeg.com/tutorials/antennas/antennas.htm http://www.starantenna.com/omni_dire...le_antenna.htm http://www.softwright.com/faq/engine...Y%20UNITS.html http://www.ventenna.com/faq.html http://tinyurl.com/5zste |
On Wed, 15 Dec 2004 16:27:40 -0500, Dave Bushong wrote:
A plain dipole 76 feet above perfect ground has a gain of about 8.6 dBi Is that true? I've always heard it was slightly over two dB. I re-read your post and I don't think I'm taking what you said out of context. Did I? Hi Dave, Wrong context. Not many can hoist a dipole that high, and finding perfect ground is even more difficult to find - but once they do, that 2+ dB is undoubtedly measured in dBd (which is to say, compared to a dipole in free space) which at least one of your links uses also. 73's Richard Clark, KB7QHC |
"Roy Lewallen" wrote in message
... A plain dipole 76 feet above perfect ground has a gain of about 8.6 dBi - surely you've modeled one with AO. So the gain of your antenna over a dipole is about 5.3 - 5.5 dB. (This would be a more meaningful way to state gain.) W4RLN ???? said he prefered dbi if I remember correctly. Can't remember the reason why This is just slightly more than the gain from a three element Yagi, and less than a typical four element beam. Difficult to compare Roy as the top reflector position moved beyond the feed point albiet also much higher. The F/B ratio and bandwidth might be better than that of a typical three or four element Yagi (or they might not be -- a Yagi can be adjusted for a wide range of characteristics), Well I thought the F/B was exceptional, especially because it stayed relatively constant over the band width as did the gain. I especially liked the fact that the impedance was in the 50 ohms vicinity where optimum characteristics of an yagi antenna usually drives the impedance to a low level. Force 12 emphasises that to get optimum gain one must accept unrealistic impedance values among other things Can you point to a 4 element yagi where its characteristics are better in every way? ( there must be something where this excels. grin) but the gain is certainly not exceptional I agree, but because it is relatively constant across the band I would consider it very good. On the other hand the elements are all straight from end to end on that model. I may be optimistic but I am anticipating up to 2 db increase in gain when I reduce the beam width to 60 degrees from 70 degrees by shaping the elements into a quasi parabolic shape. All in all it seems it will perform quite well and does put into focus the question of not only having more than one reflecter but having a reflecters that also provides a consistent 50 ohm plus impedance.! This is just an experiment that I am sharing with the antenna group in an area where I have not seen any examples in print and not something that one can say... 'Dah Da the yagi replacement !' But it does show a method where antenna attributes can be made consistent across the band which can be wider than the norm. Comparisons with the Yagi is also difficult to make because where a yagi is at a constant height in this case certain elements have an increased height. Thanks for the comments, tho I must say I do view the model with more enthusiasm than you do, but I do defer to your broader experience and knoweledge. Best Regards Art .. Roy Lewallen, W7EL art wrote: In the past I mentioned that I was modelling an antenna for 20 metres. This antenna became a single driven element with five reflectors which emulate a dish in it's crudish form not only in position of reflectors but also the elements were not shaped at this time. The following table follows what one would get for a reflective dish used on frequencies in the giga hertz range where the bandwidth is broader than that obtained with the yagi format PLUS relevant atributes stay more constant over a frequency range than that obtained with a super gain type array. The band width is wider than a typical yagi in that the major portion of the radiation is to the front this.This band with is expected to narrow and to increase gain when individual elements are shaped to a parabolic form and it is hoped that forward side lobes will not occur as is normal for a long boom multi element yagi form as used on 440 Mhz. The driven element is at a height of 76 feet over perfect ground. Normally one would say that if results seem out of the ordinary one should question it, so comments are requested regarding my explanations which lead me to think that they really are what one should expect. Freq 14.0 14.1 14.2 14.3 14.35 Gain dbi 13.86 14.05 13.99 13.9 13.87 F/B 30 25.6 27.47 33.17 32.5 Zr 45.9 44.5 43.8 43.2 42.8 Zi -12.87 -6.52 0.6 7.84 11.54 SWR 1.32 1.2 1.14 1.25 1.34 I do not advocate the replacement of the normal yagi with the above because of building difficulties tho it is my intention to build it. Regards Art (Modelled using AO PRO with 80 segment/half wave and set up by a consumate error generator Grin ) |
No, you read it correctly.
The gain of a dipole *in free space* is about 2.15 dBi. The gain of a dipole over ground (compared to the standard reference of an isotropic antenna in free space, which is what dBi means) is considerably greater. You get an automatic 3 dB in field intensity when you put a dipole over ground just because the radiated power is concentrated in a hemisphere rather than a whole sphere. Then there can be additional gain due to pattern shaping by interference between the direct and reflected signals -- the effect is exactly the same as you get from a two element array with the elements fed out of phase. This additional directive gain from pattern shaping can be as high as about 5 dB. (See the two element array patterns in Chapter 8 of the ARRL Antenna Book.) So the gain of a horizontal dipole over ground is typically between about 5 and 10 dBi. This is, of course, at the angle at which the pattern is maximum, which is never at the horizon for a horizontally polarized antenna, and in some cases is at a fairly high and perhaps useless angle. Except for very high radiation angles, ordinary ground is nearly as reflective as perfect ground for horizontally polarized fields. So the above conclusions apply also to real ground as well as perfect ground. I've argued many times against the use of "dBd" as a measure of antenna gain, unless the meaning of dBd is carefully spelled out. A good deal of misunderstanding results if one person assumes it means the gain relative to a dipole in free space (2.15 dBi) and another believes it means relative to a dipole at the same height as the antenna being discussed (in which case it means about 5 - 10 dBi). I've seen at least one antenna manufacturer cleverly make use of this misunderstanding to make their antennas sound like they have a lot higher gain than they do. A casual reader might have gotten this same impression from reading Art's data, although I know he's not intentionally trying to mislead anyone. If any of the sources you referenced contradict what I've said, I recommend dropping them a line and suggesting that they correct what they've published. Roy Lewallen, W7EL Dave Bushong wrote: Roy Lewallen wrote: A plain dipole 76 feet above perfect ground has a gain of about 8.6 dBi Is that true? I've always heard it was slightly over two dB. I re-read your post and I don't think I'm taking what you said out of context. Did I? 73, kz1o http://www.radio-electronics.com/inf...ivity_gain.php http://www.tmeg.com/tutorials/antennas/antennas.htm http://www.starantenna.com/omni_dire...le_antenna.htm http://www.softwright.com/faq/engine...Y%20UNITS.html http://www.ventenna.com/faq.html http://tinyurl.com/5zste |
"Sonie" wrote in message ... " wrote in message news:1k%vd.198273$V41.135426@attbi_s52... "hnkie" wrote in message ... "art" wrote in message oups.com... In the past I mentioned that I was modelling an antenna for 20 metres. This antenna became a single driven element with five reflectors which emulate a dish in it's crudish form not only in position of reflectors but also the elements were not shaped at this time. The following table follows what one would get for a reflective dish used on frequencies in the giga hertz range where the bandwidth is broader than that obtained with the yagi format PLUS relevant atributes stay more constant over a frequency range than that obtained with a super gain type array. The band width is wider than a typical yagi in that the major portion of the radiation is to the front this.This band with is expected to narrow and to increase gain when individual elements are shaped to a parabolic form and it is hoped that forward side lobes will not occur as is normal for a long boom multi element yagi form as used on 440 Mhz. The driven element is at a height of 76 feet over perfect ground. Normally one would say that if results seem out of the ordinary one should question it, so comments are requested regarding my explanations which lead me to think that they really are what one should expect. Freq 14.0 14.1 14.2 14.3 14.35 Gain dbi 13.86 14.05 13.99 13.9 13.87 F/B 30 25.6 27.47 33.17 32.5 Zr 45.9 44.5 43.8 43.2 42.8 Zi -12.87 -6.52 0.6 7.84 11.54 SWR 1.32 1.2 1.14 1.25 1.34 I do not advocate the replacement of the normal yagi with the above because of building difficulties tho it is my intention to build it. Regards Art (Modelled using AO PRO with 80 segment/half wave and set up by a consumate error generator Grin ) F/B looks too high, you should have some big splinters off the backside. Why? Do you have splinters in a parabolic dish? You have individual elements, a parabolic has continous surface. Antennas always have splinters, with just 5 elements and no blocking you probably have major lobes on the back side, between elements. Not so! Small lobe yes. When you used the term "splinter" I thought it described a large thin splinter or lobe, which is not the case. Tho the reflectors do have a space between them which would have a terrible effect where "reflection " occurs.In the case where definitive phase angles occur I doubt what you have referred to is correct. There will be places on the back side of your antenna that add in phase with two or more elements, which is a lobe or splinter. see above Parabolics have a rolloff on the edge to decay the field to minimize the back lobe. Yes I am hoping for that effect in conjunction with the narrowing of the main beam. Decay however is not the word I would chose. Redirect yes. (with a dish it can be hard to get 30) Gain looks to be high for a 1 driven and 5 reflectors. If you picture a balloon squashed at one end more than normal would not the gain increase? What is the capture area? Each element captures RF energy and reflects it back to the driven element in phase with the others. Capture area? I don't know ( Now you can see the limits of my expertise) Your idea has been tried very sucessfully at 450 to microwave, and they now use flat antennas with each element phase adjusted for delay (same as curverature of a parabolic dish) . Yes I certainly agree, but for some reason 440 seems to be a cut-off frequency and I do not know why. I did try using the director in a parabolic shape but it was unrewarding. The delay thing you referred to may well be why the reflectors deviated from a true parabolic shape. One antenna has over a thousand elements 1000 elements could well be connected to what you referred to above with respect to "blocking" where the spacing between elements must be commensurate with the frequency used. Regards Art Does model take into account mutual effects of the reflectors? Yes Art |
Roy
After I mused a while on your post I then accepted it for the unbiased analysis that it was rather than a desired reflection of my own thoughts. It then drove me to consider chosing gain as the main criteria, after all my interests are talking to my mates in the U.K., but at the same time carrying on with my main thought of moving away from the long boom analogy and its accompanying gain aproach. By adding a mast of 30 feet to accommodate additional elements rather than extending boom length I was able to narrow the beam width to some where in the 57 degree region but my excellent F/B that I obtained earlier went way down. This was done by constructing elements in "V" form with no restriction on alignment with respect to the driven element other than the center of the elements must be close to the mast i.e to take advantage of low torque requirements. With six elements on the 30 foot mast on the tower not only did the gain increase to 16 dbi ( others please take note of prior posts on this thread) but the 'take off angle' dropped to pretty close to !0 degrees ! This aproach for somebody who wants to work DX seems a good way to go. I now intend to pursue the aproach of letting the elements deviate from the "V" shape to what I expect to be a parabolic shape, to determine whether the extra work is worth while, as well as the "ommission" of the most upper most element for when heavy static occurrs. What I really do find interesting is that one CAN get away from the boom length aproach when seeking gain as well as obtaining a reasonable impedance. I also intend to attempt a tri band aproach as I move along, which may take a while as my program has provision for a limited number variable dimensions to optimize at one time and the number of dimensions in the next model will certainly exceed 40 at a minimum, thus modelling cannot be achieved in a single shot. My present model, which moves me towards gain, seems to exceed what performance I can expect compared to high F/B and is certainly opening fresh avenues as to what type of antenna I will construct after the new year. Comments solicited from all........ and happy hollidays Art |
Cecil Moore wrote:
Dave, Your 2+ dBi gain is for a dipole in free space Vs an isotropic in free space. Roy chose to compare the dipole over perfect ground to the isotropic in free space. The ~3dB of reflections from the perfect ground cause the additional gain due to constructive interference. Art chose to compare his antenna to an isotropic antenna in free space -- he reported the gain in dBi. So I compared a dipole at the same height to an isotropic antenna in free space. This combination allows comparison of Art's antenna in its position over ground to a dipole at the same height above ground. Roy Lewallen, W7EL |
The elevation angle at which radiation is maximum (the "takeoff angle")
can be influenced by narrowing the free-space elevation pattern of the antenna. A Yagi provides considerable directivity in the horizontal direction, but not very much in the vertical direction. The elevation pattern of the forward lobe of a Yagi doesn't look much different from the elevation pattern of a dipole. Therefore, the elevation patterns of a Yagi and dipole are very similar (except for very long Yagis, which do begin to have some noticeable directivity in the elevation pattern). One consequence of this similarity is that the gain of a Yagi over a dipole in free space is about the same as the gain of a Yagi over a dipole at any elevation angle when the two are mounted at the same height -- a single number is adequate to describe the gain of a Yagi over a dipole, no matter where the two are mounted (as long as they're at the same height). There are some antennas that do compress the vertical pattern. One notable antenna in this category is the W8JK. The result is that the "takeoff angle" is lower for the W8JK than for a dipole at the same height, and the gain of a W8JK compared to a dipole is different at different elevation angles. I haven't looked at one in a long time, but believe the "Lazy H" antenna also compresses the vertical pattern somewhat, resulting in a lower "takeoff angle" than a dipole when mounted over ground. It's not too surprising that Art's antenna has the ability to compress the vertical pattern somewhat, for the same general effect as the W8JK, due to its physical geometry. This general characteristic of a lower "takeoff angle" can be an advantage for working DX, so it might make the constructional difficulties worthwhile for some people. The reduction in F/B when maximizing gain also isn't surprising. I don't believe there's any fundamental reason for this to always happen, but it's all too often the case. Most arrays, both parasitic and driven, have relatively poor F/B when adjusted for maximum gain. It becomes up to the user to decide on the optimum tradeoff for his particular application. Roy Lewallen, W7EL art wrote: Roy After I mused a while on your post I then accepted it for the unbiased analysis that it was rather than a desired reflection of my own thoughts. It then drove me to consider chosing gain as the main criteria, after all my interests are talking to my mates in the U.K., but at the same time carrying on with my main thought of moving away from the long boom analogy and its accompanying gain aproach. By adding a mast of 30 feet to accommodate additional elements rather than extending boom length I was able to narrow the beam width to some where in the 57 degree region but my excellent F/B that I obtained earlier went way down. This was done by constructing elements in "V" form with no restriction on alignment with respect to the driven element other than the center of the elements must be close to the mast i.e to take advantage of low torque requirements. With six elements on the 30 foot mast on the tower not only did the gain increase to 16 dbi ( others please take note of prior posts on this thread) but the 'take off angle' dropped to pretty close to !0 degrees ! This aproach for somebody who wants to work DX seems a good way to go. I now intend to pursue the aproach of letting the elements deviate from the "V" shape to what I expect to be a parabolic shape, to determine whether the extra work is worth while, as well as the "ommission" of the most upper most element for when heavy static occurrs. What I really do find interesting is that one CAN get away from the boom length aproach when seeking gain as well as obtaining a reasonable impedance. I also intend to attempt a tri band aproach as I move along, which may take a while as my program has provision for a limited number variable dimensions to optimize at one time and the number of dimensions in the next model will certainly exceed 40 at a minimum, thus modelling cannot be achieved in a single shot. My present model, which moves me towards gain, seems to exceed what performance I can expect compared to high F/B and is certainly opening fresh avenues as to what type of antenna I will construct after the new year. Comments solicited from all........ and happy hollidays Art |
Roy,
Thanks for the pertinent points raised in your post. I might add that since the element centers are close to the mast I have avoided the installation of a boom at the top of the mast as one would do to lower the take off angle by the use of a stacked beam. This 'top beam' provides major instability when using a single mast. With me using my normal fishing pole elements the arrangement will help quite a bit in mast mechanical terms. Since it is element height over the feed element that, in the main, is creating a compressed lobe (as I believe your post infers) I will take a different tac with respect to the driven element, where it will be the only element shaped in parabolic form and all others will remain in a "V" configuration( straight portion for clamping at the center). The parabolic shaped driver would then hopefully allow the addition of staggered elements for other bands. The mast used will be a 2 inch diameter fibre glass to prevent vertical radiation mutations. Regards Art |
| All times are GMT +1. The time now is 07:48 AM. |
Powered by vBulletin® Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
RadioBanter.com