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Questions on broadband antenna design (e.g. T2FD)
On Tue, 21 Nov 2006 23:15:44 -0500, "C. J. Clegg"
wrote: Yeah, this is exactly what I'm trying to avoid, by careful choice of design parameters like length, resistor value, balun type. Feeding the antenna with 100 watts and having all 100 of those watts dissipated in the resistor, at any frequency within my range of 4 to 9, isn't going to work. Hi OM, By turns, you've painted yourself into a corner when we add up this wish list. Worse yet is the complaint you anticipate with: If I can't keep the efficiency above 50 percent across the range, then it probably isn't going to be worth doing. 3dB is hardly they abyss of performance, and, in fact, you would probably be hard pressed to notice it. If you could, your dream antenna (for the price you are willing to pay) would automatically qualify for the dung-heap. When you lead with your chin with: It cannot use any sort of antenna tuner this simply breaks the camel's back. Also, come to terms with there is also no such thing as a miracle BalUn. In traditional engineering, there is the adage that a well defined problem contains its own answer. You can reconcile facing abject failure by falling back and building a cost/benefit analysis of all the characteristics of your desired antenna. It should take more than half an hour to come up with the price tag. You may also discover that you can live without some of those restrictions and you might be able to live with a tuner - generations have survived and flourished under similar circumstances. 73's Richard Clark, KB7QHC |
#2
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Questions on broadband antenna design (e.g. T2FD)
On Tue, 21 Nov 2006 22:10:19 -0800, Richard Clark wrote:
By turns, you've painted yourself into a corner when we add up this wish list. Worse yet is the complaint you anticipate with: If I can't keep the efficiency above 50 percent across the range, then it probably isn't going to be worth doing. When you lead with your chin with: It cannot use any sort of antenna tuner this simply breaks the camel's back. Also, come to terms with there is also no such thing as a miracle BalUn. Good morning, Richard. I was hoping to mitigate all of that by restricting the frequency range. I'm not asking for 1.8-30 or even 4-30, but 4-9. Can a T2FD antenna not be made 50 percent efficient across that restricted frequency range? The 50 percent efficiency floor is somewhat arbitrary but is driven by the fact that some of the users of this antenna are going to be using power levels as low as 5 watts. That's hard enough to do with a cut NVIS dipole, without throwing an inefficient antenna into the mix. The inability to use a tuner is driven by the fact that eventually, these antennas are going to need to be usable with ALE radios. I don't know very much about ALE (yet) but I have a hard time imagining an autotuner that is consistently fast enough to use with ALE. Also, the users of these antennas are, like me, going to be paying for them out of their pockets. Few if any of the potential users that I know of today can afford to buy an autotuner. I know I can't. If it can't be done, then it can't be done. I'm realistic enough to accept that and move on to something else, or drop the idea. It just seems to me that within the limited frequency range, something like that should be possible with the right choice of design parameters. |
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Questions on broadband antenna design (e.g. T2FD)
On Wed, 22 Nov 2006 07:10:55 -0500, "C. J. Clegg"
wrote: I was hoping to mitigate all of that by restricting the frequency range. I'm not asking for 1.8-30 or even 4-30, but 4-9. Hi OM, That has been evident from the beginning. Still and all: The 50 percent efficiency floor is somewhat arbitrary Exactly. It is just such arbitrariness along with competing, conflicting restrictions that leads to self-defined failure tacitly accepted with: If it can't be done, then it can't be done. However, it has been done, and without the unnecessary complications of forcing ANY design to span an octave without a tuner. Hoping that a resistor will solve this is dope-slapped with the expectation of efficiency. some of the users of this antenna are going to be using power levels as low as 5 watts So what is all this angst about efficiency? Pour 100W into any hank of wire and you will achieve at least that. Or do you mean that the design you are looking for will be used as a model for others? If that is the case, more the pity that the requirement of having a $25 tuner is an unreasonable expectation of Amateur radio operators. I cannot imagine they will spend less on those resistors.... 73's Richard Clark, KB7QHC |
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Questions on broadband antenna design (e.g. T2FD)
On Wed, 22 Nov 2006 12:37:20 -0800, Richard Clark wrote:
So what is all this angst about efficiency? Pour 100W into any hank of wire and you will achieve at least that. Or do you mean that the design you are looking for will be used as a model for others? Good afternoon, Richard. Actually, what I mean is that some, including me, will be using 5-watt radios (Yaesu FT-817). If the option of pouring 100W into any hank of wire was available to all, the issue wouldn't be critical. It's not, so it is. :-) more the pity that the requirement of having a $25 tuner is an unreasonable expectation Remember the ALE requirement. I am only just now becoming convinced that some autotuners might work for ALE. I'm quite certain manual tuners won't. |
#5
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Questions on broadband antenna design (e.g. T2FD)
In article ,
C. J. Clegg wrote: Actually, what I mean is that some, including me, will be using 5-watt radios (Yaesu FT-817). If the option of pouring 100W into any hank of wire was available to all, the issue wouldn't be critical. It's not, so it is. :-) There's another approach to the antenna that you might want to consider, _if_ the ALE ranges are limited to a few (say, no more than four or five) and aren't too wide (say, no more than perhaps 5% of the center frequency). If your usage pattern fits this model, then you could construct a multi-wire "fan" dipole array... simply a set of individual wire half-wavelength dipoles, each cut for the center frequency of a given ALE range, spread apart physically, and fed from a single coax at a single feedpoint. They can be spread vertically (e.g. hang the longest one as a flat-top from a convenient set of trees, suspend the next-shortest beneath it on 6" spreaders, hang the next-shortest on another set of spreaders, etc.) or horizontally (run one north/south, another east/west, etc.). There will be some amount of coupling/loading between them, so you'll need to trim them for lowest SWR once they're installed, and you may find the SWR bandwidth less than you'd get from individual dipoles. Coupling/loading is greater for the vertically-stacked case, and less for a star-like horizontal pattern (which takes more space and more trees or etc., of course). The nice thing about this approach, if it's suitable for your needs, is that the losses are quite low. The wire which is resonant on the band in question loads up and radiates, and the other wires (which will typically have high, reactive impedances) accept very little current. No lossy termination resistor is needed. I've been using a three-wire fan dipole (5" vertical spacing) cut for 40/20/10 for several years, with very satisfactory results. This approach won't give you continuous coverage of your 4:9 range... unfortunately I can't suggest any which would, which don't require an agile tuner/transmatch and which don't introduce high losses in some portions of the band. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#6
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Questions on broadband antenna design (e.g. T2FD)
On Wed, 22 Nov 2006 15:56:45 -0500, "C. J. Clegg"
wrote: more the pity that the requirement of having a $25 tuner is an unreasonable expectation Remember the ALE requirement. I am only just now becoming convinced that some autotuners might work for ALE. I seriously doubt that. What you are describing is a sacrifice in Link Quality for the sake of not using something like a fan dipole which would immediately satisfy every requirement and fulfill mission. How this is justified with multiple-legged dipole with legs cut for different frequencies (well, I probably could, but it would be impractical). forces a new meaning to the word impractical. Perhaps you would care to elaborate how the simplicity of two extra wires has been trumped. 73's Richard Clark, KB7QHC |
#7
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Questions on broadband antenna design (e.g. T2FD)
On Wed, 22 Nov 2006 15:15:35 -0800, Richard Clark wrote:
Perhaps you would care to elaborate how the simplicity of two extra wires has been trumped. I'll try, though from the tone of your message it sounds like your mind is made up. :-) We will be operating on many different frequencies across the range of 4 to 9. I don't even know (yet) how many different frequencies will be in use (they won't tell me). So I can envision many pairs of dipole elements, each cut for a certain frequency in the range, and laid out like the spokes of a wheel. I have plenty of land here but I don't have ready supports for that kind of an array. That's what I mean by "impractical". I've learned a lot from you guys the last few days, though, and maybe something like that isn't so impractical ... I'll have to experiment. Perhaps an array of inverted vees on a single feedline... |
#8
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Questions on broadband antenna design (e.g. T2FD)
On Wed, 22 Nov 2006 18:59:31 -0500, "C. J. Clegg"
wrote: On Wed, 22 Nov 2006 15:15:35 -0800, Richard Clark wrote: Perhaps you would care to elaborate how the simplicity of two extra wires has been trumped. I'll try, though from the tone of your message it sounds like your mind is made up. :-) My mind is made up? I've repeatedly wondered why you have approached this with a defeatist attitude. We will be operating on many different frequencies across the range of 4 to 9. Again, this has been apparent from the beginning. I don't even know (yet) how many different frequencies will be in use (they won't tell me). That doesn't matter all that much, except to anticipate failure. So I can envision many pairs of dipole elements, each cut for a certain frequency in the range, and laid out like the spokes of a wheel. If you re-read my posting, I've done nothing more complex than to add TWO more wires. ALE may easily jump between 200 frequencies, but there is absolutely nothing about that which demands a resonant frequency for each of them. I have plenty of land here but I don't have ready supports for that kind of an array. That's what I mean by "impractical". Then the solution is not impractical by any definition, you are simply over embroidering the problem with a slavish interpretation of necessity. One pair of wires cut to a low end, one pair of wires to a high end, both pairs fed at the same point. It may take as many as four pairs (I doubt it), but to abstract this wildly to 200 goes beyond the pale when a skeleton biconical could easily accomplish this with flat response (over a much larger bandwidth) with only 16 pairs of equal sized wires. This cage monopole: http://home.comcast.net/~kb7qhc/ante.../Cage/cage.htm is flat over 5 Ham bands. This discone: http://www.qsl.net/kb7qhc/antenna/Discone/discone.htm operates flat over more than two octaves of bandwidth. With scaling, I can count at least 55 discrete frequencies that would fall into the 2:1 mismatch region - and this say nothing of those frequencies between them, nor of their end points which could be made to span 4 to 9 MHz. Yes, a lot of wire, but use less wire for a rougher approximation. If you are looking at an arbitrary 50% efficiency, a 5:1 circle encompasses a lot more points for less wire. 73's Richard Clark, KB7QHC |
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