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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 |
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