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#31
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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 |
#32
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Questions on broadband antenna design (e.g. T2FD)
On Wed, 22 Nov 2006 22:35:39 -0800, Richard Clark wrote:
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. Thanks. I have said earlier that I considered a discone (I also considered a cage monopole but didn't mention it here) but those are vertically polarized antennas and I'm pretty sure they're not good for NVIS. Am I wrong? I am also considering an array of inverted vees cut for selected frequencies within the range, Something like that is likely to be much better for NVIS. Sorry if I seemed to fit all those names you called me... you say I'm defeatist, one or two others here say I'm too optimistic. Can't please everybody I guess. For the most part I do very much appreciate all the help everyone has offered here, in a thread that has grown way beyond anything I envisioned when I started it ... you guys are great. :-) |
#33
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Questions on broadband antenna design (e.g. T2FD)
"Dave Platt" wrote in message ... In article , C. J. Clegg wrote: So, my questions are... Take my answers as best-guesstimates, please, rather than as gospel! 1. How do I determine the overall antenna length that will give me the most efficient (which is to say, the least inefficient) performance across the 4 to 9 MHz range? A range greater than 2:1 means that you're almost certainly going to run into at least one operating frequency at which the antenna itself is an extremely difficult load (very low, very high, and/or very reactive) and that most of your power is going to end up in the terminating resistor. Since his range is just slightly more than 2:1, I wonder if he can pick the resonance so that the thing is not full wave at any frequency of interest. Either Mouser or Digikey had 50 W non inductive resistors. Tam/WB2TT |
#34
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Questions on broadband antenna design (e.g. T2FD)
On Thu, 23 Nov 2006 08:22:25 -0500, "C. J. Clegg"
wrote: but those are vertically polarized antennas and I'm pretty sure they're not good for NVIS. Am I wrong? 1. Build the mirror elements; 2. Combine; 3. Turn 90 degrees; 4. Elevate. 73's Richard Clark, KB7QHC |
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