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Questions on broadband antenna design (e.g. T2FD)
I have a need to design and erect a single antenna that covers a number of government frequencies from around 4 MHz to around 9 MHz. It needs to cover a radius of around 300 miles, give or take, and so an NVIS installation seems indicated. It cannot use any sort of antenna tuner and must be fed with a single coax of 50 or 75 ohms. The frequencies to be used are scattered here and there within the 4 to 9 MHz range and so I can't just put up a trap dipole or multiple-legged dipole with legs cut for different frequencies (well, I probably could, but it would be impractical). I've been looking at the T2FD (tilted terminated folded dipole) design or some variation thereof. I realize that these antennas are a compromise at best, and suffer from varying degrees of inefficiency at various frequencies within their design range. However, it seems they are the only antennas I can find (so far, at least) that claim to cover the desired frequency range with a single coax feedline and no tuner. I don't need 3 to 30 and so I'm not inclined to spend $200+ on the B&W variant. I think I have ample space and trees located at reasonable endpoints to put up a model that's at least 150 feet long, perhaps longer. On the other hand, if shorter is just as good given the desired frequency range, that will be easier to do. Every indication I've found on the web seems to say that 75-ohm coax, a 4:1 balun, and a 390-ohm noninductive terminating resistor will give a reasonable match across the design range of the antenna. So, my questions are... 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? 2. How do I determine the minimum power rating for the terminating resistor for an antenna that will be driven by 100 watts maximum? 3. Where can I buy a few noninductive terminating resistors that meet the power rating determined in (2)? 4. I would rather feed it with 50 ohm than 75 ohm, since I think the transmitter would be happier with that, and I'm told that for best results I should use as much as a 10:1 balun for that, and a 470-ohm terminating resistor. Would I be better off doing that? Where can I buy a 10:1 (or other oddball ratio) balun? 5. Are there other antenna types I should consider? A discone might work over the frequency range but it's vertically polarized with a relatively low radiation angle so I suspect it isn't much good for NVIS. Anything else? Thanks... |
Questions on broadband antenna design (e.g. T2FD)
"C. J. Clegg" wrote in message ... I have a need to design and erect a single antenna that covers a number of government frequencies from around 4 MHz to around 9 MHz. It needs to cover a radius of around 300 miles, give or take, and so an NVIS installation seems indicated. It cannot use any sort of antenna tuner and must be fed with a single coax of 50 or 75 ohms. The frequencies to be used are scattered here and there within the 4 to 9 MHz range and so I can't just put up a trap dipole or multiple-legged dipole with legs cut for different frequencies (well, I probably could, but it would be impractical). I've been looking at the T2FD (tilted terminated folded dipole) design or some variation thereof. I realize that these antennas are a compromise at best, and suffer from varying degrees of inefficiency at various frequencies within their design range. However, it seems they are the only antennas I can find (so far, at least) that claim to cover the desired frequency range with a single coax feedline and no tuner. I don't need 3 to 30 and so I'm not inclined to spend $200+ on the B&W variant. I think I have ample space and trees located at reasonable endpoints to put up a model that's at least 150 feet long, perhaps longer. On the other hand, if shorter is just as good given the desired frequency range, that will be easier to do. Every indication I've found on the web seems to say that 75-ohm coax, a 4:1 balun, and a 390-ohm noninductive terminating resistor will give a reasonable match across the design range of the antenna. So, my questions are... 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? 2. How do I determine the minimum power rating for the terminating resistor for an antenna that will be driven by 100 watts maximum? 3. Where can I buy a few noninductive terminating resistors that meet the power rating determined in (2)? 4. I would rather feed it with 50 ohm than 75 ohm, since I think the transmitter would be happier with that, and I'm told that for best results I should use as much as a 10:1 balun for that, and a 470-ohm terminating resistor. Would I be better off doing that? Where can I buy a 10:1 (or other oddball ratio) balun? 5. Are there other antenna types I should consider? A discone might work over the frequency range but it's vertically polarized with a relatively low radiation angle so I suspect it isn't much good for NVIS. Anything else? Thanks... I don't have the technical data you ask about, but I do have experience in installing systems such as you describe. We simply used the commercially available B&W broadband loaded dipole. Our customer typically ran 1KW into the antenna, and was very happy with the price/performance. In spite of the negatives often cited about the antenna, it is a very effective compromise for some situations. The other type of antenna we used was an open wire fed dipole with a rather hefty remote antenna tuner. It's performance was better than the B&W, but our customer preferred the B&W for reasons of cost and ease of installation. |
Questions on broadband antenna design (e.g. T2FD)
A wire based Log Periodic?
Wayne wrote: "C. J. Clegg" wrote in message ... I have a need to design and erect a single antenna that covers a number of government frequencies from around 4 MHz to around 9 MHz. It needs to cover a radius of around 300 miles, give or take, and so an NVIS installation seems indicated. It cannot use any sort of antenna tuner and must be fed with a single coax of 50 or 75 ohms. The frequencies to be used are scattered here and there within the 4 to 9 MHz range and so I can't just put up a trap dipole or multiple-legged dipole with legs cut for different frequencies (well, I probably could, but it would be impractical). I've been looking at the T2FD (tilted terminated folded dipole) design or some variation thereof. I realize that these antennas are a compromise at best, and suffer from varying degrees of inefficiency at various frequencies within their design range. However, it seems they are the only antennas I can find (so far, at least) that claim to cover the desired frequency range with a single coax feedline and no tuner. I don't need 3 to 30 and so I'm not inclined to spend $200+ on the B&W variant. I think I have ample space and trees located at reasonable endpoints to put up a model that's at least 150 feet long, perhaps longer. On the other hand, if shorter is just as good given the desired frequency range, that will be easier to do. Every indication I've found on the web seems to say that 75-ohm coax, a 4:1 balun, and a 390-ohm noninductive terminating resistor will give a reasonable match across the design range of the antenna. So, my questions are... 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? 2. How do I determine the minimum power rating for the terminating resistor for an antenna that will be driven by 100 watts maximum? 3. Where can I buy a few noninductive terminating resistors that meet the power rating determined in (2)? 4. I would rather feed it with 50 ohm than 75 ohm, since I think the transmitter would be happier with that, and I'm told that for best results I should use as much as a 10:1 balun for that, and a 470-ohm terminating resistor. Would I be better off doing that? Where can I buy a 10:1 (or other oddball ratio) balun? 5. Are there other antenna types I should consider? A discone might work over the frequency range but it's vertically polarized with a relatively low radiation angle so I suspect it isn't much good for NVIS. Anything else? Thanks... I don't have the technical data you ask about, but I do have experience in installing systems such as you describe. We simply used the commercially available B&W broadband loaded dipole. Our customer typically ran 1KW into the antenna, and was very happy with the price/performance. In spite of the negatives often cited about the antenna, it is a very effective compromise for some situations. The other type of antenna we used was an open wire fed dipole with a rather hefty remote antenna tuner. It's performance was better than the B&W, but our customer preferred the B&W for reasons of cost and ease of installation. |
Questions on broadband antenna design (e.g. T2FD)
On Tue, 21 Nov 2006 22:19:44 +0000, Wayne wrote:
I don't have the technical data you ask about, but I do have experience in installing systems such as you describe. We simply used the commercially available B&W broadband loaded dipole. Good afternoon, Wayne. That's certainly an option, but as I said I would rather not spend $200+ unnecessarily, especially since I already have all of the necessary materials except for the terminating resistor. Anyway, the BWD-65's lower end is 4 MHz, and some of what I've read about T2FD antennas seems to indicate that near the low end is where efficiency is worst (inside of the design range ... outside of the design range it is MUCH worse). I suppose I could use the BWD-90 but I have a hard time understanding how they get all the way from 1.8 to 30 ... that seems very, very wide even for a T2FD antenna (I suppose it's a moot point, though... I'm sure it will get to my upper limit of 9 MHz nicely). As an aside, I see from the latest HRO catalog that the BWD-20, BWD-45, and BWD-65 are all $220 and the larger / longer BWD-90 is only $200. Why do you all suppose the bigger one is cheaper than all of the smaller ones? |
Questions on broadband antenna design (e.g. T2FD)
On Tue, 21 Nov 2006 22:19:44 +0000, Wayne wrote:
We simply used the commercially available B&W broadband loaded dipole. Our customer typically ran 1KW into the antenna, and was very happy with the price/performance. In spite of the negatives often cited about the antenna, it is a very effective compromise for some situations. The other reason I don't necessarily want to use the B&W is that with a little freedom of design parameters and my limited frequency range of interest (don't need 1.8 to 30 or even 4 to 30, just need 4 to 9), I was hoping I could adjust the design characteristics to minimize the inefficiency and mitigate the negatives. |
Questions on broadband antenna design (e.g. T2FD)
On Tue, 21 Nov 2006 17:23:02 -0500, Dave wrote:
A wire based Log Periodic? Good evening, Dave. I guess I forgot to mention ... this antenna needs to be omnidirectional or nearly so. Anyway it needs to be NVIS and I suspect a log periodic wouldn't work well in that configuration. |
Questions on broadband antenna design (e.g. T2FD)
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. One way to evaluate different lengths would be to model out such an antenna using NEC2/NEC4 or the like. Vary the antenna-length-to- operating-frequency-wavelength ratio, and evaluate the amount of power radiated vs. the amount of power dissipated in the termination resistance at each. Then, given the specific frequencies at which you actually want to spend most of your time operating, figure out which length gives you the best overall efficiency for your own operating conditions. At a guess - and it's just a guess - I think you might get reasonably satisfactory results by choosing a length which would give you a folded dipole that's resonant right in the middle of your 4-to-9 range, or perhaps a bit longer than that. You'd avoid the "too short to load up efficiently" problem at 4 MHz, as well as the "a full wavelength long, and thus presenting a high feedpoint Z" problem at 9 MHz. 2. How do I determine the minimum power rating for the terminating resistor for an antenna that will be driven by 100 watts maximum? Ummm... I'd say that you'd need a resistor capable of dissipating 100 watts, continuous, when used under ambient-free-air conditions in the highest operating temperature you'll encounter. I'd probably de-rate it by at least 50% (200 watt resistor) just to be sure, especially if you're going to be operating RTTY or any other high-duty-cycle mode. One of the unfortunate things about a T2FD is that there are going to be frequencies where most of your power warms up the feet of the birds perching on the termination resistor :-( 3. Where can I buy a few noninductive terminating resistors that meet the power rating determined in (2)? You could probably stick a bunch of Caddock MP9100 power film resistors, wired in series, on a chunk of aluminum heatsink and make it work OK. Caddock sells direct, I believe, and if I recall correctly Mouser carries many of their parts. You might ask B&W if they'd be willing to sell a "replacement" for their termination resistor assembly. -- 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! |
Questions on broadband antenna design (e.g. T2FD)
C. J. Clegg wrote:
It cannot use any sort of antenna tuner and must be fed with a single coax of 50 or 75 ohms. Why not an autotuner at the antenna feedpoint? -- 73, Cecil http://www.w5dxp.com |
Questions on broadband antenna design (e.g. T2FD)
On Wed, 22 Nov 2006 00:13:48 +0000, Cecil Moore wrote:
Why not an autotuner at the antenna feedpoint? Too much money. Anyway, eventually (not right away) this antenna is going to be used in an ALE network and I rather doubt that autotuners work very well in that kind of an environment. |
Questions on broadband antenna design (e.g. T2FD)
Dave wrote in
: A wire based Log Periodic? Not too useful unless you need the directivity as well. The loaded folded dipole idea is actually quite efficient if you stay above the "knee" frequency. This is about .5 wave at the lowest frequency. -- Dave Oldridge+ ICQ 1800667 |
Questions on broadband antenna design (e.g. T2FD)
"C. J. Clegg" wrote in
: On Wed, 22 Nov 2006 00:13:48 +0000, Cecil Moore wrote: Why not an autotuner at the antenna feedpoint? Too much money. Anyway, eventually (not right away) this antenna is going to be used in an ALE network and I rather doubt that autotuners work very well in that kind of an environment. Anything over about 90 feet should work. 125 would be just about perfect. And no, they DON'T have bad spots....usually they stay under 2 to 1 across a fairly wide band and efficiency stays good above the knee frequency. Model it, if you want exact performance figures. -- Dave Oldridge+ ICQ 1800667 |
Questions on broadband antenna design (e.g. T2FD)
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Questions on broadband antenna design (e.g. T2FD)
"C. J. Clegg" wrote in message ... On Tue, 21 Nov 2006 22:19:44 +0000, Wayne wrote: I don't have the technical data you ask about, but I do have experience in installing systems such as you describe. We simply used the commercially available B&W broadband loaded dipole. Good afternoon, Wayne. That's certainly an option, but as I said I would rather not spend $200+ unnecessarily, especially since I already have all of the necessary materials except for the terminating resistor. Anyway, the BWD-65's lower end is 4 MHz, and some of what I've read about T2FD antennas seems to indicate that near the low end is where efficiency is worst (inside of the design range ... outside of the design range it is MUCH worse). I suppose I could use the BWD-90 but I have a hard time understanding how they get all the way from 1.8 to 30 ... that seems very, very wide even for a T2FD antenna (I suppose it's a moot point, though... I'm sure it will get to my upper limit of 9 MHz nicely). As an aside, I see from the latest HRO catalog that the BWD-20, BWD-45, and BWD-65 are all $220 and the larger / longer BWD-90 is only $200. Why do you all suppose the bigger one is cheaper than all of the smaller ones? Yes, as I said, I don't have the technical data. I just wanted to cite experience that would indicate that you can most likely be successful in using an antenna such as a T2FD for your application. Good luck. |
Questions on broadband antenna design (e.g. T2FD)
On Wed, 22 Nov 2006 01:01:56 +0000, Dave Oldridge wrote:
The loaded folded dipole idea is actually quite efficient if you stay above the "knee" frequency. This is about .5 wave at the lowest frequency. Good evening, Dave. But, isn't there another frequency, around 1 full wavelength at the operating frequency, above which efficiency starts to go to pot again? 125 ft (which you suggested in another message) is a full wave at something a shade under 8 MHz, well within my upper limit of 9 MHz. So, how will a 125-foot T2FD operate in the range of, say, 7 to 9? |
Questions on broadband antenna design (e.g. T2FD)
On Tue, 21 Nov 2006 23:24:59 +0000, Dave Platt wrote:
2. How do I determine the minimum power rating for the terminating resistor for an antenna that will be driven by 100 watts maximum? Ummm... I'd say that you'd need a resistor capable of dissipating 100 watts, continuous, when used under ambient-free-air conditions in the highest operating temperature you'll encounter. I'd probably de-rate it by at least 50% (200 watt resistor) just to be sure, especially if you're going to be operating RTTY or any other high-duty-cycle mode. One of the unfortunate things about a T2FD is that there are going to be frequencies where most of your power warms up the feet of the birds perching on the termination resistor :-( 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. If I can't keep the efficiency above 50 percent across the range, then it probably isn't going to be worth doing. |
Questions on broadband antenna design (e.g. T2FD)
On Tue, 21 Nov 2006 23:10:13 -0500, J. Mc Laughlin wrote:
I will be pleased to provide a quotation. Thanks. Unfortunately there isn't any money to pay someone to do this. I'm doing it for free, and in fact I'm paying for the materials out of my pocket, for whatever design I decide on. |
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 |
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. |
Questions on broadband antenna design (e.g. T2FD)
An LP near ground also functions as a NVIS antenna, a broadband NVIS antenna.
You will need more space than the B&W for example; but, if space is available a six or seven element design should work just fine. Also, any TERMINATED TRAVELING WAVE designs should work. A 1/2 wavelength at 3 MHz terminated in 600 ohms and fed with a good 9:1 balun would do as well. /s/ DD, W1MCE BTW: it is proximity to ground and horizontal polarization that causes NVIS # # # C. J. Clegg wrote: On Tue, 21 Nov 2006 17:23:02 -0500, Dave wrote: A wire based Log Periodic? Good evening, Dave. I guess I forgot to mention ... this antenna needs to be omnidirectional or nearly so. Anyway it needs to be NVIS and I suspect a log periodic wouldn't work well in that configuration. |
Questions on broadband antenna design (e.g. T2FD)
C. J. Clegg wrote:
On Tue, 21 Nov 2006 23:24:59 +0000, Dave Platt wrote: 2. How do I determine the minimum power rating for the terminating resistor for an antenna that will be driven by 100 watts maximum? Ummm... I'd say that you'd need a resistor capable of dissipating 100 watts, continuous, when used under ambient-free-air conditions in the highest operating temperature you'll encounter. I'd probably de-rate it by at least 50% (200 watt resistor) just to be sure, especially if you're going to be operating RTTY or any other high-duty-cycle mode. One of the unfortunate things about a T2FD is that there are going to be frequencies where most of your power warms up the feet of the birds perching on the termination resistor :-( 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. If I can't keep the efficiency above 50 percent across the range, then it probably isn't going to be worth doing. Narrowing the frequency range isn't going to help much with the design. I'm no expert, but if I read my signs correctly, the T2FD has the same ups and downs in SWR as does most other multiband antennas. IOW, it relies on frequency harmonic relationships. Cebik has a good web page at: http://www.cebik.com/wire/t2fd.html |
Questions on broadband antenna design (e.g. T2FD)
Michael Coslo wrote:
C. J. Clegg wrote: On Tue, 21 Nov 2006 23:24:59 +0000, Dave Platt wrote: 2. How do I determine the minimum power rating for the terminating resistor for an antenna that will be driven by 100 watts maximum? Ummm... I'd say that you'd need a resistor capable of dissipating 100 watts, continuous, when used under ambient-free-air conditions in the highest operating temperature you'll encounter. I'd probably de-rate it by at least 50% (200 watt resistor) just to be sure, especially if you're going to be operating RTTY or any other high-duty-cycle mode. One of the unfortunate things about a T2FD is that there are going to be frequencies where most of your power warms up the feet of the birds perching on the termination resistor :-( 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. If I can't keep the efficiency above 50 percent across the range, then it probably isn't going to be worth doing. Narrowing the frequency range isn't going to help much with the design. I'm no expert, but if I read my signs correctly, the T2FD has the same ups and downs in SWR as does most other multiband antennas. IOW, it relies on frequency harmonic relationships. Pah! I should have said wavelengths, not harmonics!! - 73 de Mike KB3EIA - |
Questions on broadband antenna design (e.g. T2FD)
One of the unfortunate things about a T2FD is that there are going to
be frequencies where most of your power warms up the feet of the birds perching on the termination resistor :-( 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. If I can't keep the efficiency above 50 percent across the range, then it probably isn't going to be worth doing. Narrowing the frequency range isn't going to help much with the design. I'm no expert, but if I read my signs correctly, the T2FD has the same ups and downs in SWR as does most other multiband antennas. IOW, it relies on frequency harmonic relationships. Cebik has a good web page at: http://www.cebik.com/wire/t2fd.html Looks like a very useful page. And, the section down at the bottom labelled "Terminator Resistor Losses" tells the sad tale. With both of the designs Cebik modelled, the power loss into the termination resistor is only rarely less than 50%. For the longer of the two models (165', wide spacing) the losses between 4 MHz and 9 MHz range from 4 dB to over 6 dB, with an average loss of around 5 dB. It looks as if it might be possible to model/design a T2FD with an intermediate length and spacing, which might result in average losses in the 4 - 9 MHz range which are lower than either the 100' or 165' models. However, from the shape of these curves I doubt that you'll get the average loss down much below 5 dB. Even getting the average loss down below 3 dB looks as if it won't happen, and I don't think you're going to ge the worst-case loss down below 3 dB at all. You might want to go back and reconsider the possibility of using a feedpoint-mounted autotuner - in particular, one which has a frequency-based tuning memory capability and fast-retune feature, and which does not require a large amount of RF power to initiate a tune. Some such tuners require only a couple of watts of RF in order to tune, and can re-tune to a previously-memorized frequency and match-set within a couple of hundred milliseconds. I believe that an autotuner of this sort would be compatible with low-power ALE, as long as you had "pre-programmed" it by doing a full tuning cycle at each frequency in the range you're interested in, and given it a chance to search for and "memorize" a suitable match. The lady who has demo'ed ALE at a number of hamclubs in my area (regrettably I don't recall her callsign) uses an ALE-equipped military radio, and a Motorola (or Harris?) longwire autocoupler hooked to a random length of wire and a counterpoise. -- 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! |
Questions on broadband antenna design (e.g. T2FD)
On Wed, 22 Nov 2006 19:29:39 +0000, Dave Platt wrote:
the section down at the bottom labelled "Terminator Resistor Losses" tells the sad tale. Yes, it does. My heartfelt thanks to all of you for all of your assistance and advice. I think I have enough info now to guide my experimentation and come to a conclusion. Doesn't look promising... |
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 |
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. |
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! |
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 |
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... |
Questions on broadband antenna design (e.g. T2FD)
"C. J. Clegg" wrote in
: On Wed, 22 Nov 2006 01:01:56 +0000, Dave Oldridge wrote: The loaded folded dipole idea is actually quite efficient if you stay above the "knee" frequency. This is about .5 wave at the lowest frequency. Good evening, Dave. But, isn't there another frequency, around 1 full wavelength at the operating frequency, above which efficiency starts to go to pot again? I didn't see this. But as the size increases, the pattern tends to start to come apart. 125 ft (which you suggested in another message) is a full wave at something a shade under 8 MHz, well within my upper limit of 9 MHz. So, how will a 125-foot T2FD operate in the range of, say, 7 to 9? Actually, I just modelled it. The 125-foot version is very good between 7 and 9, but the SWR (with a 650 ohm terminator) jumps to 4 to 1 at 5.25 mhz. Like I suggested, you need to model your actual situation, then build to the model. -- Dave Oldridge+ ICQ 1800667 |
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 |
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. :-) |
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 |
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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