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#21
October 19th 04, 11:05 PM
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#22
October 19th 04, 11:25 PM
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On Tue, 19 Oct 2004 14:02:18 -0400, "G&R" wrote:
|Hi Wes, | |Yes I am aware of the difference between the advertising on the site and the |range results. So are you going to fix it or not? | |As you are aware antennas do not have even gain across the entire band. We |have designed the antenna to cover the entire 2m band and as a result do see |a variation in the perfromance across the band, hence max gain. Huh? So are you saying that you have nearly 3 dB gain variation in 2 MHz.? | |And before you cut down my comments, the precision on the results are from |the softare and we are in the process of redesigning the format to more |accurately reflect our actual results both modelled and actual. |Unfortunately our business is antennas and web design. In a paper I wrote for the ARRL Antenna Compendium, I said, "Simplified 'antenna analyzers', especially those with digital readouts can lull the user into unjustified confidence in the accuracy of his measurements." Change the words "antenna analyzers" to "computer software" and it describes this situation to a T. | |The results posted on the CSVHF society are valid as the antenna was tested |with a gain of 11.2dBd at 144.18MHz Horiz Polarization. Design Freq is |146.0MHz. | |We are in the business of designing and selling antennas not developing |antennas and giving them away for free. However, if your serious about the |data for the antenna contact me directly so that we can discuss the details |(email listed below). My slightly distorted email is above. If you choose to send me your parameters, I promise to keep it in confidence. I don't not design, or manufacture antennas (or anything else) for a living. | |As to the DATAQ, well I have no answer for that one, as we don't build it or |sell it. Our software is designed to with run it. Strange though, my |Toshiba does and so does my SONY through a USB to SERIAL converter. I know you don't build or sell it, I'm just offering a caution to those who might be tempted to buy one. It works on my desktop but not my laptop, which is a pity because I had a portable use in mind. | |One final point, the purpose of the posting was that identify that an |antenna does not always need boom length and elements for gain and that |there are other ways to achieve this. Absolutely. One driven element at the focus of a 40' parabola is a decent antenna at 2-meters. Regards, Wes N7WS |
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#23
October 20th 04, 01:21 AM
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"Chuck" wrote in message news:n5gdd.8202$6P5.3752@okepread02... wrote in message news:x_Ycd.281755$3l3.162866@attbi_s03... What brought this to my attention was Moxon who advocated 2 element beams (lightnes translates to greater heights) so I modeled a 20 meter beam on a 7 foot boom and obtained 12dbi max for the band (ala 2:1) Hi Art, Can we assume this gain was calculated at a particular height over ground? But ofcourse a perfect ground and a standard wavelength height Personally, I prefer FS results, in order to eliminate any ambiguity. Field strength (F.S.) introduces ambiguity does it not? but one can get 13dbi if you drop the impedance from 50 to 12 ohms. This paraphrases the spesific example that I gave the question would a transformer cancel the increased gain. Again you paraphased the spoecific sample that I gave which raised the same question I gave regarding transformer losses Optimizing an antenna for a specific Z is not a good idea in my opinion, as the amplitude of an element's current is proportional to the amplitude of its radiation. Ohms law tells us that at a consistant power level, the lower the R (Z), the greater will be the current. Yes there are many laws to remember and I look at critical coupling as something to remember. A zero loss transformer would not reduce the element's current, as the energy being introduced to the element would remain unchanged. Very true but what element would this max energy be applied to? Via critical coupling the driven element may not be the one carrying the maximum current and thus screws up conventional thinking. Energy is one thing but it is current that controls radiation is it not ?. The particular model I was working on used 50 segments per wavelength and used element length and diameters determined by my program input . Thus coupling gains are attained but where it sometimes determined a element diameter is so thin it is incapable of carrying the required current. If I saw a vendor advertise an antenna at 13dbi I would be very suspicious as it not the norm, yet very realiseable when using NEC without being tied down to existing doctrines. Sooooooo ..... I was looking for a datum curve generated by experts from an all encompasing NEC program where the final design reflected the real world design without ambiguities regarding coupling to coils, elements e.t.c. where lumped loads do not reflect the real world appearance. There have been many responses including one suggestion that a suitable datum curve could be formulated from customer claims no less but NONE that responds to my specific request . Thus a conundrum still exists regarding programs based on scientific knoweledge that are held in question as they do not match real world measurements that spawn most of the villifying in this group. Seems like a datum curve could be used as a basis for many discusions where differences do occur instead of just arguing for eight years or more and not resolving the accuracy concerns Seems like I have come full circle and describing the Presidential debates ! Art 73 de Chuck, WA7RAI |
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#24
October 20th 04, 04:11 AM
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On Wed, 20 Oct 2004 00:21:10 GMT, "
wrote: datum curve Hi Art, No such thing. It takes data (plural) to make a curve. Datum (singular) is a point (i.e. NOT a curve). If you don't see the curve you are looking for, you have an NEC engine, what is preventing you from using it to make one, or two, or several? 73's Richard Clark, KB7QHC |
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#26
October 21st 04, 01:53 AM
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Chuck
"Chuck" wrote in message news:XrBdd.8254$6P5.7645@okepread02... snip. Yes there are many laws to remember and I look at critical coupling as something to remember. Critical coupling is simply a means to attain element currents of a higher amplitude than what can be expected when one element is excited parasitically. A zero loss transformer would not reduce the element's current, as the energy being introduced to the element would remain unchanged. Very true but what element would this max energy be applied to? Max Energy? Sorry about that.........I.meant current but my fingers ignored me I was referring to the element the transformer is attached to. Via critical coupling the driven element may not be the one carrying the maximum current and thus screws up conventional thinking. Ummmm... Critical coupling is a state where currents of equal amplitude flow in two inductors that are in close proximity, as a direct result of the coupling. Critical coupling results in a common field as well. This also applies to wires (elements). Energy is one thing but it is current that controls radiation is it not ?. Yes, and that's what I had said previously. The particular model I was working on used 50 segments per wavelength and used element length and diameters determined by my program input . Thus coupling gains are attained but where it sometimes determined a element diameter is so thin it is incapable of carrying the required current. I'm not sure if I follow this correctly... please elaborate further. Certainly To attain maximum gain per unit length the model dimensions were all variables as was the number of elements. Not only was the driven element current often less than another element in the array it was sometimes found that the maximum current element required a diameter of a few thousanths that was not sufficient to carry 1Kw.! Obviously the gain attained was over ruled by the inability of the element to meet operational requirements. I might add that I use fibre fishing rods for my antennas where I can apply the correct wire diameters ( or aluminum foil) to an array without being encumbered by mechanical restrictions. This removes me from the normal restrictions applied to antennas where element diameter is pre-controlled for mechanical reasons which often conflict with scientific requirements The above statement does conflicts with your assesment stated above regarding critical coupling but this is what I found and I will leave it at that Regards Art. snip |
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#27
October 21st 04, 07:14 PM
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#28
October 21st 04, 08:26 PM
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Ian White, G3SEK wrote:
"Drawing gain curves is a fun activity...but don`t try to read too much into them." I searched on "yagi boom". I got many responses. One showed Ian as the custodian of a computer program to design yagis. Another was from the Central States VHF Society and gives results of their 2004 gain comparisons of many different antennas. Gain of these versus boom length looks very ragged. Best regards, Richard Harrison, KB5WZI |
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#29
October 21st 04, 09:05 PM
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"Chuck" wrote in message news:fVSdd.9064$6P5.7971@okepread02... wrote in message news:bMDdd.293802$3l3.275124@attbi_s03... "Chuck" wrote in message news:XrBdd.8254$6P5.7645@okepread02... snip. I'm not sure if I follow this correctly... please elaborate further. Certainly To attain maximum gain per unit length the model dimensions were all variables as was the number of elements. Not only was the driven element current often less than another element in the array it was sometimes found that the maximum current element required a diameter of a few thousanths that was not sufficient to carry 1Kw.! Obviously the gain attained was over ruled by the inability of the element to meet operational requirements. I might add that I use fibre fishing rods for my antennas where I can apply the correct wire diameters ( or aluminum foil) to an array without being encumbered by mechanical restrictions. This removes me from the normal restrictions applied to antennas where element diameter is pre-controlled for mechanical reasons which often conflict with scientific requirements The above statement does conflicts with your assesment stated above regarding critical coupling but this is what I found and I will leave it at that Regards Art. Hi Art, Ok, I'm always open minded to learn something new... Wow,,,...... there are not many people around who could say that !. Since 99.999% of things presented as new are incorrect most experts have determined that the odds favor them if they label EVERYTHING new as in error. If something comes along that is really new they always have the comment ' I knew about that a long while ago" to fall back on. I'd like to establish a few things, though. First of all, what modeling program are you using? Beasely AOP This is the professional version that has more than enough segments and variable dimensions available to lesson the chances of human input errors plus to handle elements that were in close proximetry to each other, together with 'Sommerfield ground' handling capabilities. Is your empirical data consistent with the models? Not measured, my thought were that NEC would always be closer than field measurements generated by an amateur. How are you ascertaining your empirical data? How are you determining the current amplitudes: By model? Or empirically? By model, The program provides % of max current at every segment, phase and all that good stuff And what are the machinations that demand ultra-thin wires to establish the gain? Go for 80 % gain and the rest (20% ) for swr Added half a dozen elements All dimensions variable except boom length and perfect ground height Remove one element at a time until max gain point is obvious Note all dimensions are kept variable at all times. You can email me if you care to not discuss these things in an open forum. Not necessary, This is the very reason I posted in the first place ! The program shows that the normal 2 element is not the optimum in that a polygon of vectors beats a triangle of vectors. At the same time with added elements you get diminishing returns in std and conventional forms. The program showed that 1 to 1.5 dbi was available over the standard 2 element on the same length boom.if one could overcome mechanical restraints. (I was comparing to a Beasely example of what gain could be attained for two elements on a 7 foot boom) Now that is not the end of the experiment as I cannot verify the accuracy of the program, because I did not write it, and I certainly cannot say that my modelling aproach is without error since that is what many 'experts' point to if they don't like the results. It was for that reason I asked if any similar data had been made available for boom length by reputable programmers and antenna 'experts' for comparison purposes ., If these initial results were quoted as accurrate there would be howls from all the resident antenna ' experts" and I would immediately be placed in the six foot hole that they have been trying to put you in for the last eight years Art 73, de Chuck, WA7RAI wa7rai at cox dot net |
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#30
October 21st 04, 09:50 PM
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Another was from the
Central States VHF Society and gives results of their 2004 gain comparisons of many different antennas. Gain of these versus boom length looks very ragged. Best regards, Richard Harrison, KB5WZI Gain vs. boom length makes sense only when comparing or tracking the same antenna design i.e. Yagi with multiple elements. One can design lousy antenna on a long boom. Jim Lawson, W2PV was one who after some modeling showed that gain in the properly designed antenna is roughly proportional to the boom length rather than to number of elements. Some manufacturers "beefed up" their antennas by sticking more elements on the same boom claiming better performance. Yuri, K3BU.us |
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