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#11
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Reflector mesh surface
On Nov 30, 7:55*pm, wrote:
Art Unwin wrote: On Nov 30, 7:05*pm, wrote: Art Unwin wrote: On Nov 30, 5:55*pm, wrote: Richard Clark wrote: On Sun, 30 Nov 2008 21:45:03 GMT, wrote: Art Unwin wrote: Since we are at the low sun cycle I shoose 1" hexigon poultry wire for the mash on my 3 metre dish At what frequency will there be a noticable effect on aperture starting with top band? Many thanks Art KB9MZ...,XG The rule of thumb is things less that .1 wavelengths in size have negligable effects, so roughly up to about 1 GHz. Hi Jim, You neglect to mention that a 3 meter dish would be wholly (no pun intended) transparent to RF in the top band. *It could be solid metal to no notice by a radiator (or receiver). It would be like blocking the sound 3 feet from the stage at a Led Zepplin concert with a Japanese parasol. * -Huh?- 73's Richard Clark, KB7QHC Yeah, I foolishly forgot the source of the question and assumed "top band" meant highest useable frequency. Unless I hosed the math, a 3 m dish would be a little over 10 db isotropic at 2 m, so the combinatation would "work" from 2 m to 23 cm. -- Jim Pennino Remove .spam.sux to reply. I ask the question because the radiator is a helix end fed, helix being approx 0.3 M diameter. Intuitively, I thought that the 3 M dish would be large enough and thus needed a check on the perforations. Could you show how the "combination" changed things so drastically so I may benefit from the question? I stated "intuitively" since the radiation is axia with respect to the helix Many thanks Art A helix doesn't radiate in the axial mode unless the circumference is between about 0.75 and 1.33 wavelengths. The would put the minimum frequency for a .3 m diameter helix at about 240 Mhz and the maximum frequency at about 420 Mhz. Also, the reflector for a helix is a flat surface, not a dish, and is normally between about 0.8 to 1.1 wavelengths in diameter. If you make it larger, the sidelobe levels increase. -- Jim Pennino Remove .spam.sux to reply. No, No, No. That is not correct. Yes, yes, yes, it is all corrct. Just as Kraus over estimated the gain of helix antennas he also made a mistake Nothing was said about gain. in specifying a particular helix angle Nothing was said about the helix angle. as well as the impression that one needs a large diameter helix with reference to wave length. A circumferance of 0.75 to 1.33 wavelengths is required for radiation in the axial mode. Ofcourse his work is over 50 years old and is being wittled down with futher investigation. Nope; correctness has no expiration date. I modeled it against perfect ground A pointless exercise. as the latest papers state that a wall around the bottom where there is maximum current provides best gain. What papers are these; the ones from the outhouse? A short "wall" around the outer edge of the reflector reduces side lobes. Thus I see the deep and large diameter dish as being an equivalent. Most people would call that delusional. I am aware of that. One person once said the world was round! If one was persuasive enough at the beginninghundred years or so later if one said the world was flat he would be called delusional. If one was very persuasive in the first place he would be rediculed as it was so obvious to all in the first place. I hope you get to live another hundred years so that you can meld into the new crowd of lemmings Regards Art If .1 WL is a rule of thumb then it would seem your first response is correct. Of course my first response was correct; things smaller than 0.1 wavelengths can usually be ignored. -- Jim Pennino Remove .spam.sux to reply. |
#12
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Reflector mesh surface
Art wrote:
"I stated "intuitively" since the radiation is axia with respect to the helix." A 3 M diameter has about 9.42 M circumference so becomes an effective axial radiator instead of a radial radiator at the frequency near where its circumference becomes one wavelength. This frequency is 300/(pi)(diameter) or about 10 MHz. A reflecting surface to make an axial helix unidirectional needs to be at least as large as the projection of the helix on the surface of the reflector. Its diameter should then be about 3 M or more and would be effective only at frequencies exceeding 10 MHz which is where the helix becomes an end-fire device anyway. Best regards, Richard Harrison, KB5WZI |
#13
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Reflector mesh surface
I wrote:
"A 3 M diameter has about 9.42 M circumference so becomes effective as its circumference is about one wavelength or about 11.8 MHz. At lower frequencies radiation from a helix is radial rather than axial." Rounding: 300/10M=30 Mhz. Likewise: 300/1M=300 Mhz. If Art`s helix is only about 1 meter in circumference, not 10 meters, it will only be effective as an end-fire antenna above about 300 MHz, not 30 MHz. The good news is that his reflector can be proportionally smaller. Best regards, Richard Harrison, KB5WZI |
#14
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Reflector mesh surface
On Dec 1, 1:42*am, (Richard Harrison) wrote:
Art wrote: "I stated "intuitively" since the radiation is axia with respect to the helix." A 3 M diameter has about 9.42 M circumference so becomes an effective axial radiator instead of a radial radiator at the frequency near where its circumference becomes one wavelength. This frequency is 300/(pi)(diameter) or about 10 MHz. A reflecting surface to make an axial helix unidirectional needs to be at least as large as the projection of the helix on the surface of the reflector. Its diameter should then be about 3 M or more and would be effective only at frequencies exceeding 10 MHz which is where the helix becomes an end-fire device anyway. Best regards, Richard Harrison, KB5WZI Richard That is good information that has a trend to my way of thinking. A heads up as to where this information comes from would be very helpfull to me in my on going experiments These ratios of diameter to length does have a penalty in wavelengths used but hopefully in less reflector diameter which is not to be viewed as a phase enabling point of view If one viewed a helix as an array of loop radiators then phasing would be important but then prevents the formation of smaller volume radiation designs. For those who rely solely on "off the cuff" thoughts may well deride what I am doing but I have done my homework and am now pursueing implematation since experiments consist of more than the exercises of the brain without the need of personal observations, without which advances would be crippled and science laboratories would become extinct My thinking now leans toward solenoid style with a air bound core for maximum flux density within the core in "rifle shot" pattern. I have had a similar antenna for the 80 metre band where the helix diameter is just 4" which is quite directional even tho as low as 30 feet!. Thus I am very much encouraged in this new design which is light enough for me to carry and install together with improvements added over my past effort tho it will not have the tilting feature at least not in the near future. I am anxious to add the tilting feature some time next year in an effort to extend individual QSO time as propagation changes ,but it is low on program requirements Best regards Art Unwin KB9MZ..........xg |
#15
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Reflector mesh surface
On Dec 1, 10:25*am, (Richard Harrison)
wrote: I wrote: "A 3 M diameter has about 9.42 M circumference so becomes effective as its circumference is about one wavelength or about 11.8 MHz. At lower frequencies radiation from a helix is radial rather than axial." Rounding: 300/10M=30 Mhz. Likewise: 300/1M=300 Mhz. If Art`s helix is only about 1 meter in circumference, not 10 meters, it will only be effective as an end-fire antenna above about 300 MHz, not 30 MHz. The good news is that his reflector can be proportionally smaller. Best regards, Richard Harrison, KB5WZI Richard I would disagree with the above as it has it's roots in a loop array yagi style where phasing is of the essence. What Kraus did do is to destroy the notion that a radiator has to be straight to be effective which is borne out by Gauss. At the time he did his work ( over 50 years ago ) radar and other advances pointed to the use of higher frequencies and away from HF which then opened the advantages of circular polarization Thus his work to my knoweledge has not been extended to HF via known observables, an avenue ideally identified with hams with the experimental bent coupled with a technical educatioin. If others have already experimented in this particular area I would be happy to hear of their experiences and actual scalar obsevables. The fact that my full scale experiments have exposed facts that are contrary to that of 50 years ago is what drives me on this task tho the labour tasking can sometimes be exaspirating but non the less necessary rather than relying solely on just personal thoughts, intuitions and deductions,After all I am a qualified engineer and certainly would not have got very far in life if I thought a testing regimen is not a requirement any more Regards Art Unwin.......xg |
#16
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Reflector mesh surface
On Mon, 1 Dec 2008 08:27:22 -0800 (PST), Art Unwin
wrote: I have had a similar antenna for the 80 metre band where the helix diameter is just 4" which is quite directional even tho as low as 30 feet!. My transistor radio antenna of 50 years ago was smaller than that and could do better at lower frequencies: Triple the wavelength, 4 times smaller, and at one tenth the height. That is a 120 fold advantage over your design. It cost $12, and the antenna came with it for free! 73's Richard Clark, KB7QHC |
#17
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Reflector mesh surface
"Art Unwin" wrote in message ... Since we are at the low sun cycle I shoose 1" hexigon poultry wire for the mash on my 3 metre dish At what frequency will there be a noticable effect on aperture starting with top band? Many thanks Art KB9MZ...,XG he's sucking you all in now... 'TOP BAND'!!! yes, he means a 3m DISH as a reflector for his axial mode helix that is probably only inches in diameter on 160m! this is his dream 'rifle shot' antenna! |
#18
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Reflector mesh surface
Art wrote:
"I would disagree with the above as it has its roots in a large yagi style where phasing is of the essence." Kraus says on page 268 of the 3rd edition of "Antennas": "The natural adjustment of phase velocity so that the fields from each turn add nearly in phase in the axial direction accounts for the persistence of the axial mode of radiation over a nearly 2 to 1 range in frequency." In Kraus` 1950 version of "Antennas" on page 178 he writes: "A first-order transmission mode on the helix, designated T1, becomes permissible when the helix circumference lambda in free space is of the order of 1 wavelength." I don`t argue with Terman or Kraus. Best regards, Richard Harrison, KB5WZI |
#19
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Reflector mesh surface
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#20
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Reflector mesh surface
On Dec 1, 11:38*pm, (Richard Harrison)
wrote: Art wrote: "I would disagree with the above as it has its roots in a large yagi style where phasing is of the essence." Kraus says on page 268 of the 3rd edition of "Antennas": "The natural adjustment of phase velocity so that the fields from each turn add nearly in phase in the axial direction accounts for the persistence of the axial mode of radiation over a nearly 2 to 1 range in frequency." In Kraus` 1950 version of "Antennas" on page 178 he writes: "A first-order transmission mode on the helix, designated T1, becomes permissible when the helix circumference lambda in free space is of the order of 1 wavelength." I don`t argue with Terman or Kraus. Best regards, Richard Harrison, KB5WZI I see no problem with it either, what is the point of the above? They both have said a lot of things that I agree with. Really Richard you needed to start another thread if you had a particular point in mind |
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