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Wideband VHF Yagi - Do I have to use a folded dipole configuration?
I'm just about getting around to making my 5 or 6 element marine band yagi,
covering 156 - 162 Mhz RECEIVE ONLY. Do I have to employ a folded dipole or can I use just a simple hertz dipole as the "driven" element? I'm trying to get away from using a folded dipole if I can. Any wideband designs out there that use a simple hertz dipole that I can scale? I'm looking, but if you have some links aready. TIA. |
On Mon, 31 May 2004 15:59:31 +0100, "Richard"
wrote: |I'm just about getting around to making my 5 or 6 element marine band yagi, |covering 156 - 162 Mhz RECEIVE ONLY. Do I have to employ a folded dipole No. |or |can I use just a simple hertz dipole as the "driven" element? I'm trying to |get away from using a folded dipole if I can. Any wideband designs out there |that use a simple hertz dipole that I can scale? I'm looking, but if you |have some links aready. TIA. All dimensions in inches X Y Z X Y Z Dia 0.000 -18.553 0.000 0.000 18.553 0.000 0.187 9.275 -18.368 0.000 9.275 18.368 0.000 0.312 12.250 -17.077 0.000 12.250 17.077 0.000 0.187 23.800 -16.672 0.000 23.800 16.672 0.000 0.187 34.213 -16.672 0.000 34.213 16.672 0.000 0.187 47.211 -16.229 0.000 47.211 16.229 0.000 0.187 |
I'm just about getting around to making my 5 or 6 element marine band yagi,
covering 156 - 162 Mhz RECEIVE ONLY. Do I have to employ a folded dipole or can I use just a simple hertz dipole as the "driven" element? I'm trying to get away from using a folded dipole if I can. Any wideband designs out there that use a simple hertz dipole that I can scale? I'm looking, but if you have some links aready. TIA. A folded dipole as the driven element will have the best match over your frequency range.. What you do have to worry about is the feedpoint resistance. Some Yagi-Uda designs are expressly designed for (a driven) folded dipole, which in isolation have a much higher feed resistance than 50 ohms. When you put parasitics in the near field, in certain gain-optimized Yagi-Uda designs, that feed resistance falls closer to 50 ohms. Typically, a non-matched regular dipole as driven has a feedpoint resistance of much less than 50 ohms in a high gain Yagi-Uda app. Thus you may be seeing 1-2 dB or mismatch from this lower feedpoint resistance. Or you can make a T-match (for example). If you have s specific design that uses a folded dipole, I suggest you stick with it. Unless you want to eat a dB or 2 in possible mismatch, or put a matching system in on a conventional dipole as a driven. 73, Chip N1IR |
"Richard" wrote in message
... I'm just about getting around to making my 5 or 6 element marine band yagi, covering 156 - 162 Mhz RECEIVE ONLY. Do I have to employ a folded dipole or can I use just a simple hertz dipole as the "driven" element? I'm trying to get away from using a folded dipole if I can. Any wideband designs out there that use a simple hertz dipole that I can scale? I'm looking, but if you have some links aready. TIA. Richard, I'm sure you have a specific reason for a direcitonal marine antenna, but just for others who like the sound of that idea... unless all your marine band traffic is DX from a relatively narrow range of direction, the beam antennas for marine are more trouble than they are worth in my experience. I wasted a lot of time spinning the rotator around 359 degrees with various directional antennas and finally scrapped them all. Even considering that most beams can hear front and back, more than 10 degrees to the side(s) and they are deaf by design. A great design popular with amateur radio and a superb performer on marine-band is the copper pipe J-pole. Tuned right on center 156.800 Mhz, I have yet to see a vertical hi-gain whip outperform this setup (probably because factory whips are so far off-center). I have over 100 miles coverage of USCG high-sites and approximately 15 miles coverage from on the water small craft with a J-pole. My antenna is mast-mounted on a 2d story chimney, and in spite of some erroneous construction advice, it is grounded (well!). The blunt-tip copper pipe cap on a J-pole is the ultimate lightning rod, and #4 or heavier conductor to at least an 8' ground rod is highly recommended. 73, Jack Painter Virginia Beach, Va |
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Lucky for those of us when it does, Wes.
Nice model. Seems to meet a certain need. It is possible to get better F/B and F/S though. Not with your approach, however. 73, Chip N1IR |
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"Wes Stewart" wrote in message ... On Mon, 31 May 2004 15:59:31 +0100, "Richard" wrote: |I'm just about getting around to making my 5 or 6 element marine band yagi, |covering 156 - 162 Mhz RECEIVE ONLY. Do I have to employ a folded dipole No. |or |can I use just a simple hertz dipole as the "driven" element? I'm trying to |get away from using a folded dipole if I can. Any wideband designs out there |that use a simple hertz dipole that I can scale? I'm looking, but if you |have some links aready. TIA. All dimensions in inches X Y Z X Y Z Dia 0.000 -18.553 0.000 0.000 18.553 0.000 0.187 9.275 -18.368 0.000 9.275 18.368 0.000 0.312 12.250 -17.077 0.000 12.250 17.077 0.000 0.187 23.800 -16.672 0.000 23.800 16.672 0.000 0.187 34.213 -16.672 0.000 34.213 16.672 0.000 0.187 47.211 -16.229 0.000 47.211 16.229 0.000 0.187 Sorry I'm a bit uncertain of what you wrote. Obviously an efficient way to provide data for yagi's, but how do you read it? Is there a webpage associated with the design, so I can look at the general construction? Rich. |
I'm not taking credit. This is a minor adjustment of a Cebik design.
Seems to fit the OP's rqmts, however: no folded-dopole feed, 50 ohm matched BW of 156-162 MHz and 5-6 elements. Regards, Wes This looks like a modofied approach to an NBS Yagi-Uda. Modern Y-U design allows for a higher feedpoint SWR. Matching loss is offset by higher gain and better consistency of F/B. If no matching, then a shaped, folded dipole is used. Typically a F/B better than 22 dB (sim) is acheived with a 10% bandwidth. The design you discuss sounds more than adequate for this need though, in fact most needs. 73, Chip N1IR |
Fractenna wrote:
I'm not taking credit. This is a minor adjustment of a Cebik design. Seems to fit the OP's rqmts, however: no folded-dopole feed, 50 ohm matched BW of 156-162 MHz and 5-6 elements. Regards, Wes This looks like a modofied approach to an NBS Yagi-Uda. The only special feature of the NBS designs (other than the prestigious mailing address) was a self-imposed restriction to equal spacings between directors. That turned out to be an evolutionary dead-end, and you won't find it in any modern optimized yagi design. The design proposed by Wes doesn't have that feature - fortunately - and is actually one of the OWA (Optimized Wideband Array) family. One of the special features of that family of yagis is a very close spacing between the driven element and the first director, which raises the feedpoint impedance to 50 ohms and allows a simple split-dipole driven element. Even this feature is not original to the OWA family. I don't know when (or indeed how often) it might have been invented, but it was first systematically used by DL6WU some 25 years ago. With additional help from computer analysis and optimization, the OWA family continued the development. Modern Y-U design allows for a higher feedpoint SWR. Matching loss is offset by higher gain and better consistency of F/B. If no matching, then a shaped, folded dipole is used. Typically a F/B better than 22 dB (sim) is acheived with a 10% bandwidth. We're now in the excellent position of having several alternative ways to design a yagi to meet each user's specific combination of requirements. That approach is certainly one of them. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
On Tue, 1 Jun 2004 10:11:14 +0100, "Richard"
wrote: | |"Wes Stewart" wrote in message .. . | On Mon, 31 May 2004 15:59:31 +0100, "Richard" | wrote: | | |I'm just about getting around to making my 5 or 6 element marine band |yagi, | |covering 156 - 162 Mhz RECEIVE ONLY. Do I have to employ a folded dipole | | No. | | |or | |can I use just a simple hertz dipole as the "driven" element? I'm trying |to | |get away from using a folded dipole if I can. Any wideband designs out |there | |that use a simple hertz dipole that I can scale? I'm looking, but if you | |have some links aready. TIA. | | All dimensions in inches | | X Y Z X Y Z Dia | | 0.000 -18.553 0.000 0.000 18.553 0.000 0.187 | 9.275 -18.368 0.000 9.275 18.368 0.000 0.312 | 12.250 -17.077 0.000 12.250 17.077 0.000 0.187 | 23.800 -16.672 0.000 23.800 16.672 0.000 0.187 | 34.213 -16.672 0.000 34.213 16.672 0.000 0.187 | 47.211 -16.229 0.000 47.211 16.229 0.000 0.187 | | |Sorry I'm a bit uncertain of what you wrote. | |Obviously an efficient way to provide data for yagi's, but how do you read |it? | |Is there a webpage associated with the design, so I can look at the general |construction? | |Rich. This is the format for almost any of the common antenna modeling programs. See for example: Free: http://www.qsl.net/wb6tpu/swindex.html (look for files beginning 4nec2) Almost free: http://www.qsl.net/ac6la/multinec.html Free demo: www.eznec.com etc. With a fixed width font, the columns would line up and each line would represent one element, starting from the rear (reflector end) of the boom. The data are the X,Y,Z coordinates of the ends of the elements and the diameter of that element. Z is this case is zero since the model assumes free space. It also assumes no effects from the supporting boom; the usual compensations must be made for this, depending on the element mounting method. See: http://www.antennspecialisten.se/en/ham/tech/bc.html I assumed that you had some experience with Yagi design and construction and were simply looking for a design that gave both the required bandwidth and a fifty-ohm feedpoint; something this design delivers. |
I can easily be mistaken, but I don't believe the NBS Yagi designs were
ever promoted as being optimal in any way. Their purpose, I believe, was to provide a set of easy-to-duplicate designs whose gains were well documented (although I understand an error was made in measuring the originals) and which anyone could construct as reference antennas with known gain. Roy Lewallen, W7EL Ian White, G3SEK wrote: The only special feature of the NBS designs (other than the prestigious mailing address) was a self-imposed restriction to equal spacings between directors. That turned out to be an evolutionary dead-end, and you won't find it in any modern optimized yagi design. The design proposed by Wes doesn't have that feature - fortunately - and is actually one of the OWA (Optimized Wideband Array) family. One of the special features of that family of yagis is a very close spacing between the driven element and the first director, which raises the feedpoint impedance to 50 ohms and allows a simple split-dipole driven element. Even this feature is not original to the OWA family. I don't know when (or indeed how often) it might have been invented, but it was first systematically used by DL6WU some 25 years ago. With additional help from computer analysis and optimization, the OWA family continued the development. Modern Y-U design allows for a higher feedpoint SWR. Matching loss is offset by higher gain and better consistency of F/B. If no matching, then a shaped, folded dipole is used. Typically a F/B better than 22 dB (sim) is acheived with a 10% bandwidth. We're now in the excellent position of having several alternative ways to design a yagi to meet each user's specific combination of requirements. That approach is certainly one of them. |
Roy Lewallen wrote:
I can easily be mistaken, but I don't believe the NBS Yagi designs were ever promoted as being optimal in any way. Their purpose, I believe, was to provide a set of easy-to-duplicate designs whose gains were well documented (although I understand an error was made in measuring the originals) and which anyone could construct as reference antennas with known gain. They were in fact described as "optimum designs" in the original paper, but it's clear that claim was made subject to a number of reservations. Most of the hype came from antenna manufacturers who subsequently picked up the designs from the published reports, and misused the "NBS" name to boost the credibility of their own products. With hindsight, the NBS designs were really quite good, and their forward gain is still competitive with more modern designs of the same boom length. But modern yagi designs are generally better, because there have been a further 35-40 years of development and optimization. In particular, the last 10 years have benefited from the availability of computer modeling techniques - you can now do more good work in a few evenings than the NBS program achieved in as many years. As a result of this development, you can now have a better combination of features - for example, cleaner patterns, wider bandwidth and/or greater tolerance to dimensional errors, and easier impedance matching - and keep the good forward gain as well. Many modern designs have been developed as complete 'families', with simple design rules that let you add or remove elements (changing the boom length accordingly) to create new yagis, each of which will be close to optimum for its boom length. The NBS yagis don't have that 'family' feature - each one is an individual design, and attempts to adapt them have generally not been successful. This lack of adaptability is a direct consequence of the original decision to use the same spacing between all elements; that's why I described it as an "evolutionary dead-end". It costs exactly the same to build a good, modern yagi as it does to build an older, inferior design - the only differences are in *where* you apply the hacksaw and the drill. Therefore there's not much point in building an inferior design... which is what the NBS yagis have now become. For many years, the greatest value of the NBS yagis was that they had accurate gain measurements, so they could be used as reliable benchmarks in antenna gain shootouts. (The known error in the gain measurements applies to the 2-element yagi only.) Even that use has now been overtaken by computer modeling. In summary, the NBS yagis deserve respect for their major contribution to the art and science of yagi design, but they are now mainly of historical interest. For examples of modern yagis, and tips on construction, see the 'VHF/UHF Long Yagi Workshop' on my website. -- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
"Wes Stewart" wrote in message ... On Mon, 31 May 2004 15:59:31 +0100, "Richard" wrote: |I'm just about getting around to making my 5 or 6 element marine band yagi, |covering 156 - 162 Mhz RECEIVE ONLY. Do I have to employ a folded dipole No. |or |can I use just a simple hertz dipole as the "driven" element? I'm trying to |get away from using a folded dipole if I can. Any wideband designs out there |that use a simple hertz dipole that I can scale? I'm looking, but if you |have some links aready. TIA. All dimensions in inches X Y Z X Y Z Dia 0.000 -18.553 0.000 0.000 18.553 0.000 0.187 9.275 -18.368 0.000 9.275 18.368 0.000 0.312 12.250 -17.077 0.000 12.250 17.077 0.000 0.187 23.800 -16.672 0.000 23.800 16.672 0.000 0.187 34.213 -16.672 0.000 34.213 16.672 0.000 0.187 47.211 -16.229 0.000 47.211 16.229 0.000 0.187 IOW: X Y Z Dia REF End 1 0.000 -18.553 0.000 0.187 End2 0.000 18.553 0.000 DE End1 9.275 -18.368 0.000 0.312 End2 9.275 18.368 0.000 DIR1 End1 12.250 -17.077 0.000 0.187 End2 12.250 17.077 0.000 DIR2 End1 23.800 -16.672 0.000 0.187 End2 23.800 16.672 0.000 DIR4 End1 34.213 -16.672 0.000 0.187 End2 34.213 16.672 0.000 DIR5 End1 47.211 -16.229 0.000 0.187 End2 47.211 16.229 0.000 I'll see how I get along with the 4Nec2 program. What is the DE,just a regular hertz dipole? Is DE specified as to what it is (hertz, folded) somewhere in 4nec2? In the Geometry Edit I see "Radius" for wire diameter I suppose. Do I calculate the radius from the dia given in the table above? TIA. |
Ian,
Thanks very much for the correction and additional information. Like a lot of other folks, I learn a lot from this newsgroup. Roy Lewallen, W7EL |
On Wed, 2 Jun 2004 18:44:28 +0100, "Richard"
wrote: |IOW: yes |I'll see how I get along with the 4Nec2 program. | |What is the DE,just a regular hertz dipole? Is DE specified as to what it is |(hertz, folded) somewhere in 4nec2? No, it's not specified as such. Modeling programs assume straight "wires" so to model a FD you have to "construct" one from straight wires. This is not trivial however, so don't try it until you understand the pitfalls of "closely-spaced wires." I strongly recommend that you download Roy's EZNEC demo, if for no other reason than to get the excellent "Help" section. |In the Geometry Edit I see "Radius" for wire diameter I suppose. Do I |calculate the radius from the dia given in the table above? TIA. Yes. Also, when you get to building the real antenna, you have the problem of how to feed the DE. The design in question offers a nice 50 ohm impedance, but just opening the center and feeding directly with coax is going to be problematic. In the Arizona desert where I live I've gotten away with pigtailing the coax and using ferrite sleeve baluns. Since this is a marine band antenna, I assume there's water around someplace and that may be unsatisfactory for you. What did you have planned? |
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