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I have about 40 feet of garden to erect an antenna....Could anyone tell me if I took 2 20 feet length of twin ladder and shorted the ends fed one side of the dipole with twin ladder and connected a 52 ohm resistor across the other side so the tx would see a match would be better than feeding a 40 foot endfed through an atu... My thinking is that most of the small antenna on the market are just dummy loads with a small amount of wire attached and appear to work so would my 40 feet work better.. ?? -- Regards Ted Wager Using Debian Linux |
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Ted wrote:
I have about 40 feet of garden to erect an antenna....Could anyone tell me if I took 2 20 feet length of twin ladder and shorted the ends fed one side of the dipole with twin ladder and connected a 52 ohm resistor across the other side ... Where is the ground for that 52 ohm resistor? -- 73, Cecil, http://www.qsl.net/w5dxp |
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Where is the ground for that 52 ohm resistor?
-- 73, Cecil ========================================= Really Cecil, you shouldn't ask awkward questions like that. You might gain the reputation of being a wicked troller. On the other hand, it cannot be denied, your question is apt and could be considered to be educational. Please continue! Due to a certain person who ought to know better, I suffer from the same misinterpretation of motives myself. smiley ---- Reg. |
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I'm a bit confused by your description. Is this a folded dipole, 40
feet long, with an added resistor in the conductor which would normally be unbroken, inserted directly opposite the feedpoint? Or is it something else? Why not just a 40-foot-long center-fed doublet, with a tuner at the transmitter end? What bands do you want to use your antenna on? What do you want to accomplish with it? (DX, local, intermediate range, ...) Cheers, Tom |
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Cecil Moore wrote:
Ted wrote: I have about 40 feet of garden to erect an antenna....Could anyone tell me if I took 2 20 feet length of twin ladder and shorted the ends fed one side of the dipole with twin ladder and connected a 52 ohm resistor across the other side ... Where is the ground for that 52 ohm resistor? The feed goes to the centre..The ends are strapped together and the resistor connects between the free ends opposite the feed connection... -- Regards Ted Wager Using Debian Linux |
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Ted wrote:
Cecil Moore wrote: Ted wrote: I have about 40 feet of garden to erect an antenna....Could anyone tell me if I took 2 20 feet length of twin ladder and shorted the ends fed one side of the dipole with twin ladder and connected a 52 ohm resistor across the other side ... Where is the ground for that 52 ohm resistor? The feed goes to the centre..The ends are strapped together and the resistor connects between the free ends opposite the feed connection... I'm afraid we need a schematic. Is this correct? 20ft 20ft +-------------------FP------------------+ | 52 ohm resistor +-------------------FP------------------+ -- 73, Cecil, http://www.qsl.net/w5dxp |
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Cecil Moore wrote:
Ted wrote: Cecil Moore wrote: Ted wrote: I have about 40 feet of garden to erect an antenna....Could anyone tell me if I took 2 20 feet length of twin ladder and shorted the ends fed one side of the dipole with twin ladder and connected a 52 ohm resistor across the other side ... Where is the ground for that 52 ohm resistor? The feed goes to the centre..The ends are strapped together and the resistor connects between the free ends opposite the feed connection... I'm afraid we need a schematic. Is this correct? 20ft 20ft +-------------------resistor------------+ | connected connected +-------------------FP------------------+ Like so...Ends strapped and r at centre opposite feed... -- Regards Ted Wager Using Debian Linux |
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Ted wrote:
I'm afraid we need a schematic. Is this correct? 20ft 20ft +-------------------resistor------------+ | connected connected +-------------------FP------------------+ OK, close to a T2FD. Here are the formulas for such an antenna. http://www.hard-core-dx.com/nordicdx.../t2design.html Looks like yours would be resonant around 8.2 MHz. -- 73, Cecil, http://www.qsl.net/w5dxp |
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Cecil Moore wrote:
Ted wrote: I'm afraid we need a schematic. Is this correct? 20ft 20ft +-------------------resistor------------+ | connected connected +-------------------FP------------------+ OK, close to a T2FD. Here are the formulas for such an antenna. http://www.hard-core-dx.com/nordicdx.../t2design.html Looks like yours would be resonant around 8.2 MHz. Thanks very much for the info and the address.. -- Regards Ted Wager Using Debian Linux |
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Ted wrote in news:43bd1032$0$82676
: I have about 40 feet of garden to erect an antenna....Could anyone tell me if I took 2 20 feet length of twin ladder and shorted the ends fed one side of the dipole with twin ladder and connected a 52 ohm resistor across the other side so the tx would see a match would be better than feeding a 40 foot endfed through an atu... My thinking is that most of the small antenna on the market are just dummy loads with a small amount of wire attached and appear to work so would my 40 feet work better.. ?? If you're looking for transmit efficiency, then a resistor is not going to help you except to keep the standing waves down. You don't, in practice want such a resistor to be dissipating more than about half your power. Even that much is a three decibel loss (about half an S unit). If it was my space, I'd start as high as I could get at one end and run a sloping dipole with 300-ohm or 450-ohm feeder to the atu. Make sure your atu has a 4:1 balun in it or get a balun that can give you at least this ratio. And just feed the nice sloper on as many bands as you can tune it! It won't be earth-shaking on 80m and will probably wimp out on 160, but I bet it would be tolerably good on 40 and really decent on 30 and up. Dress the feedline away from the main wire at right angles and twist it a bit. Make sure to follow proper line-dressing procedures where you bring it into the house (or use a 4-1 balun and run coax the last bit). And fiddle with the length of the lower arm of the dipole until you get no common-mode antenna currents on the feeder, say on 30m. That should do it. -- Dave Oldridge+ ICQ 1800667 |
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Ted wrote:
I have about 40 feet of garden to erect an antenna....Could anyone tell me if I took 2 20 feet length of twin ladder and shorted the ends fed one side of the dipole with twin ladder and connected a 52 ohm resistor across the other side so the tx would see a match would be better than feeding a 40 foot endfed through an atu... My thinking is that most of the small antenna on the market are just dummy loads with a small amount of wire attached and appear to work so would my 40 feet work better.. ?? Many thanks to all who replied to my query... -- Regards Ted Wager Using PCLinuxos |
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I have about 40 feet of garden to erect an antenna....Could anyone tell
me if I took 2 20 feet length of twin ladder and shorted the ends fed one side of the dipole with twin ladder and connected a 52 ohm resistor across the other side so the tx would see a match would be better than feeding a 40 foot endfed through an atu... My thinking is that most of the small antenna on the market are just dummy loads with a small amount of wire attached and appear to work so would my 40 feet work better.. ?? ...................................... If you are trying to avoid a dummy load, a resister is the last thing you want. It should be illegal to add resisters to otherwise perfectly good antennas. It's hard to comment about the antenna, as I don't know what bands, how much height, etc... Not enuff info... MK |
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Cecil Moore wrote:
wrote: It should be illegal to add resisters to otherwise perfectly good antennas. Except Rhombics, of course. :-) And Beverages ... |
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"Cecil Moore" wrote - Except Rhombics, of course. :-) ==================================== Cecil, get yourself up to date. A terminated rhombic is only 50 percent (or even less) efficient. It transmitts to, and receives from, only half of the available directions. A wicked waste of power. I can't imagine why anybody bothers to use one which is not rotateable. Have you ever erected a 160-meter or 80-meter or even a 40-meter rotateable rhombic? smiley ---- Reg, G4FGQ. |
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On Mon, 9 Jan 2006 20:55:39 +0000 (UTC), "Reg Edwards"
wrote: "Cecil Moore" wrote - Except Rhombics, of course. :-) ==================================== Cecil, get yourself up to date. A terminated rhombic is only 50 percent (or even less) efficient. Actually, that is not true. A big enough rhombic is nearly self-terminating. The energy radiated is not available to the terminating resistor and thus is not -lost-. It transmitts to, and receives from, only half of the available directions. A wicked waste of power. I can't imagine why anybody bothers to use one which is not rotateable. Have you ever erected a 160-meter or 80-meter or even a 40-meter rotateable rhombic? smiley I completed my 2-meter Worked All Continents award by working VK5MC, who was using stacked rhombics (350 feet/leg), partially steerable by a rope and pulley arrangement. Worked for me [g]. |
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"Wes Stewart" wrote in message ... On Mon, 9 Jan 2006 20:55:39 +0000 (UTC), "Reg Edwards" wrote: "Cecil Moore" wrote - Except Rhombics, of course. :-) ==================================== Cecil, get yourself up to date. A terminated rhombic is only 50 percent (or even less) efficient. Actually, that is not true. A big enough rhombic is nearly self-terminating. The energy radiated is not available to the terminating resistor and thus is not -lost-. It transmitts to, and receives from, only half of the available directions. A wicked waste of power. I can't imagine why anybody bothers to use one which is not rotateable. Have you ever erected a 160-meter or 80-meter or even a 40-meter rotateable rhombic? smiley I completed my 2-meter Worked All Continents award by working VK5MC, who was using stacked rhombics (350 feet/leg), partially steerable by a rope and pulley arrangement. Worked for me [g]. =========================================== Wes, of course it worked. You would have done even better had it not been terminated. =========================================== |
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Reg Edwards wrote:
"Cecil Moore" wrote - Except Rhombics, of course. :-) ==================================== Cecil, get yourself up to date. A terminated rhombic is only 50 percent (or even less) efficient. It transmitts to, and receives from, only half of the available directions. A wicked waste of power. I can't imagine why anybody bothers to use one which is not rotateable. Have you ever erected a 160-meter or 80-meter or even a 40-meter rotateable rhombic? smiley ---- Reg, G4FGQ. Reg, during the Vietnam War we used a terminated rhombic from the State of Utah [USA desert area] to SEA [AKA Vietnam]as part of the antenna farm at AGA5HI [USAF MARS Gateway Station] for phone patch traffic. It would outperform the Yagi and the LP antenna. I was AFF1C TDY at AGA5HI [Hill Air Force Base, Utah] I don't believe there is a net loss of gain in the preferred direction. If one loses -3 dB in the resistor, does one not waste -3 dB in the non-preferred direction [180 degrees from the desired direction]? |
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Don't get too hung up on efficiency. What counts is signal strength.
Suppose you have a bidirectional antenna. Unless you're talking to two people in opposite directions at the same time, it doesn't matter if the antenna is 100% efficient and half the power goes into an unused reverse lobe or whether it goes into a resistor which makes the antenna 50% efficient. The result is exactly the same as far as the other station is concerned. So to the extent that the rhombic isn't optimal, it's because it's inherently bidirectional, not necessarily because it's inefficient. A bidirectional antenna is usually not an optimum choice. For the same number of elements or same amount of real estate, you can usually make a unidirectional antenna which has a single main lobe of about the same width but 3 dB greater gain. Or, you can have a main lobe of about the same gain as before but greater width, which is an advantage when the antenna can't be rotated. However, this doesn't say anything about simplicity, which is the main attractiveness of a rhombic, along with its bandwidth. Roy Lewallen, W7EL |
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Reg Edwards wrote:
A terminated rhombic is only 50 percent (or even less) efficient. But it's 50% in the bad direction, not the good direction. :-) -- 73, Cecil http://www.qsl.net/w5dxp |
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"Cecil Moore" wrote Reg Edwards wrote: A terminated rhombic is only 50 percent (or even less) efficient. But it's 50% in the bad direction, not the good direction. :-) =========================================== In the most simplistic of terms, a rhombic consists of four 1/2-wavelength wires plus a lossy resistor which gets hot. If the four 1/2-wavelength wires are rearranged to form a dipole, plus a reflector, plus two directors, we have only one good direction in which 100% of the power is radiated. Nothing gets hot. Common sense prevails. No need to refer to Eznec. Even a drunken old-wife would know which arrangement to choose, if only because it saves the cost and fitting of a high power, non-reactive resistor. ;o) But no doubt US Army Field Manuals still call upon rhombics. ---- Reg. |
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In article ,
Reg Edwards g4fgq,regp@ZZZbtinternet,com wrote: In the most simplistic of terms, a rhombic consists of four 1/2-wavelength wires plus a lossy resistor which gets hot. And, in this case, I think the "most simplistic terms" are a misapplication of the way the term is usually used. My recollection is that in practice, rhombics of the sort being referred to have arms which are several wavelengths long. The total double-arm-length of a wire rhombic is often 10 wavelengths or more. In this sort of rhombic, the great majority of the transmitted power is radiated before it reaches the termination resistor. There is little power left to dissipate in the resistor. If the rhombic were unterminated, and the forward-travelling wave were reflected at the end of the rhombic, most of this reflected power would be radiated before it reached the transmitter and were re-reflected. Dissipating the remaining (small) amount of forward wave at the end of the rhombic helps maintain a very high front-to-back ratio. This can be advantageous both when transmitting (no back-spill) and when receiving. I believe that rhombics were popular among U.S. government radio sites for use at coastal sites, for precisely this reason - they were very good at rejecting QRM from landside transmitters, and didn't blast landside receivers with high power. Long, terminated rhombics have another advantage - they maintain a consistent directionality and feedpoint impedance over a wide range of frequencies... rather wider than you can do with a resonant standing-wave antenna such as a reflector/DE/directors beam. Common sense prevails. No need to refer to Eznec. Even a drunken old-wife would know which arrangement to choose, if only because it saves the cost and fitting of a high power, non-reactive resistor. If you're insisting that a "rhombic" may have arms of no longer than 1/2 wavelength, I'd agree. Since that's not the only way to design 'em, though (and is not how some of the better-known ones were designed), I think that your conclusion is overbroad. It's all a matter of serving your needs, whatever they may be. If you're limited on space, and/or want a steerable beam, then a Yagi or similar is probably the best choice. I certainly wouldn't try to put up an effective HF rhombic on my roof! If you've got oodles of space, want to listen (or transmit) only in a single direction, need a lot of front-to-back isolation, and want a broad bandwidth and consistent radiation patterns and a considerable amount of gain, then a long-armed terminated rhombic may be a better choice than the alternatives (e.e. a honking-big LPDA). -- 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! |
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What??? Four 1/2 wavelength wires!! You gotta be kidding!!!
The Rhombic at AGA5HI in the early 70s was close to 6 wavelengths per side. There are/were two different rhombic designs. [Roy may want to comment on this.] A resonant rhombic without a termination resistor is bidirectional. A non resonant rhombic, a traveling wave design that uses a terminating resistor is unidirectional. Gain increases as the length of each side increases. Directivity [gain] is also dependent on the included angle which contributes to radiated beam width. AK Reg Edwards wrote: "Cecil Moore" wrote Reg Edwards wrote: A terminated rhombic is only 50 percent (or even less) efficient. But it's 50% in the bad direction, not the good direction. :-) =========================================== In the most simplistic of terms, a rhombic consists of four 1/2-wavelength wires plus a lossy resistor which gets hot. If the four 1/2-wavelength wires are rearranged to form a dipole, plus a reflector, plus two directors, we have only one good direction in which 100% of the power is radiated. Nothing gets hot. Common sense prevails. No need to refer to Eznec. Even a drunken old-wife would know which arrangement to choose, if only because it saves the cost and fitting of a high power, non-reactive resistor. ;o) But no doubt US Army Field Manuals still call upon rhombics. ---- Reg. |
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Amos Keag wrote:
A resonant rhombic without a termination resistor is bidirectional. Yes and, being open-ended, is a *standing-wave* antenna. Contrary to what has been said here on r.r.a.a in the past, Balanis says: "The current and voltage distributions on open-ended antennas are similar to the standing wave patterns on open-ended transmission lines. ... Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forward and backward) and represented by traveling wave currents If and Ib in Figure 10.1(a)." -- 73, Cecil http://www.qsl.net/w5dxp |
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Cecil Moore wrote:
Yes and, being open-ended, is a *standing-wave* antenna. Contrary to what has been said here on r.r.a.a in the past, Balanis says: "The current and voltage distributions on open-ended antennas are similar to the standing wave patterns on open-ended transmission lines. ... Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forward and backward) and represented by traveling wave currents If and Ib in Figure 10.1(a)." Why would that be of any advantage? Do you believe it is easier to solve for the exact radiated fields by partitioning the standing wave into two components? The real challenge in antenna theory is determining the exact current distribution. Further subdividing or superposing the current is a trivial exercise in comparison, and such manipulation may or may not be useful. In other words, what Balanis says may be true, but so what? 73, Gene W4SZ |
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On Tue, 10 Jan 2006 15:25:35 GMT, Cecil Moore wrote:
Amos Keag wrote: A resonant rhombic without a termination resistor is bidirectional. It may be bidirectional, but it still has directivity in the direction away from the feed. Yes and, being open-ended, is a *standing-wave* antenna. Contrary to what has been said here on r.r.a.a in the past, Balanis says: "The current and voltage distributions on open-ended antennas are similar to the standing wave patterns on open-ended transmission lines. ... Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forward and backward) and represented by traveling wave currents If and Ib in Figure 10.1(a)." If the unterminated rhombic is long enough to be considered a -real- rhombic, it is both a standing wave and a traveling wave antenna. If you want to think of it in transmission line terms, it's a very lossy line. The 100% reflection at the open end is 100% of what's left after the effects of resistive and radiated loss are factored in. The rhombic as the positive attribute of simplicity but that is about the only positive. It takes a huge amount of space (in wavelengths) and it's really a crappy antenna from the standpoint of minor (and some not so minor) lobes. The Laport version is much better, but much more complicated. |
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Gene Fuller wrote:
Cecil Moore wrote: Yes and, being open-ended, is a *standing-wave* antenna. Contrary to what has been said here on r.r.a.a in the past, Balanis says: "The current and voltage distributions on open-ended antennas are similar to the standing wave patterns on open-ended transmission lines. ... Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forward and backward) and represented by traveling wave currents If and Ib in Figure 10.1(a)." In other words, what Balanis says may be true, but so what? The "so what" is the additional knowledge to be gained by not choosing to ignore the underlying physics. When the forward EM wave hits the end of the dipole, what happens? Essentially the same thing that happens when a forward EM wave hits the end of an open-circuit transmission line. The H-field (current) goes to zero and the E-field (voltage) doubles, i.e. the forward wave existing at that point is completely reflected. That explains why the feedpoint impedance of a 1/2WL dipole is 50-75 ohms instead of the physical characteristic impedance of ~1200 ohms. The feedpoint impedance of a 1/2WL dipole is a virtual impedance caused by destructive interference between the forward and reflected voltages, Vfp = |Vfor|-|Vref|, and constructive interference between the forward and reflected currents, Ifp = |Ifor|+|Iref|, and Zfp = Vfp/Ifp It is interesting to note the consistency of the arguments here on r.r.a.a. Someone says, "'A' is true". Someone else says, "No, 'A' is gobbledygook". After 'A' is proven to be true, the argument shifts to, "OK, so what? Those grapes are probably sour anyway." :-) -- 73, Cecil http://www.qsl.net/w5dxp |
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Cecil,
I don't understand the "sour grapes" reference. To the contrary, I believe I have argued with you many times that there are multiple ways to solve a problem. Use standing waves or traveling waves as you choose for computational convenience. The only thing that goes into the field-defining equations is the current, not the waves. Your original message implied that may be some special benefit to a *standing-wave* antenna over a *non-standing-wave* antenna. Other than all of the hand-waving, which seems to somehow be connected to your intuitive thinking, there is no physical difference. In other words, what Balanis says may be true, but so what? 73, Gene W4SZ Cecil Moore wrote: Gene Fuller wrote: Cecil Moore wrote: Yes and, being open-ended, is a *standing-wave* antenna. Contrary to what has been said here on r.r.a.a in the past, Balanis says: "The current and voltage distributions on open-ended antennas are similar to the standing wave patterns on open-ended transmission lines. ... Standing wave antennas, such as the dipole, can be analyzed as traveling wave antennas with waves propagating in opposite directions (forward and backward) and represented by traveling wave currents If and Ib in Figure 10.1(a)." In other words, what Balanis says may be true, but so what? The "so what" is the additional knowledge to be gained by not choosing to ignore the underlying physics. When the forward EM wave hits the end of the dipole, what happens? Essentially the same thing that happens when a forward EM wave hits the end of an open-circuit transmission line. The H-field (current) goes to zero and the E-field (voltage) doubles, i.e. the forward wave existing at that point is completely reflected. That explains why the feedpoint impedance of a 1/2WL dipole is 50-75 ohms instead of the physical characteristic impedance of ~1200 ohms. The feedpoint impedance of a 1/2WL dipole is a virtual impedance caused by destructive interference between the forward and reflected voltages, Vfp = |Vfor|-|Vref|, and constructive interference between the forward and reflected currents, Ifp = |Ifor|+|Iref|, and Zfp = Vfp/Ifp It is interesting to note the consistency of the arguments here on r.r.a.a. Someone says, "'A' is true". Someone else says, "No, 'A' is gobbledygook". After 'A' is proven to be true, the argument shifts to, "OK, so what? Those grapes are probably sour anyway." :-) |
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Gene Fuller wrote:
I don't understand the "sour grapes" reference. To the contrary, I believe I have argued with you many times that there are multiple ways to solve a problem. Actually, you have gone from saying I was wrong to saying that I am right but it doesn't matter. I guess that's an improvement. :-) Your original message implied that may be some special benefit to a *standing-wave* antenna over a *non-standing-wave* antenna. Sorry, I never said anything about a standing-wave antenna having some special benefit over a traveling-wave antenna. It was a problem with the inferrence function, not with the implication function. :-) -- 73, Cecil http://www.qsl.net/w5dxp |
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Unless a self-respecting radio amateur wishes to set up a
point-to-point communications link, he would not consider a rhombic which wastes half the transmitter power in a resistor. How popular are rhombics, such as you describe, amongst amateurs? There are, of course, some rich, perhaps contest participating amateurs, with time any money to burn. I have no objection to their existence. They are very welcome to the fraternity. They demonstrate just what can be achieved. ---- Reg. |
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