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Tapering open wire feedlines?
Roy Lewallen wrote:
Pete Bertini wrote: As an academic exercise, for example using a tapered line to match an OCD with a known FP impedance of 200 ohms down to a 1:1 balun at the 50 ohm coax line transition point. What is the recommended minimum length of cable needed to perform a four to one characteristic impedance change in the open wire for relating to the wavelength? Pete It's not practical to make a 50 ohm open wire line. The wires would have to be extremely close together. But as to "recommended" minimum length, it depends on who does the recommending and what your criteria are. The length of line necessary to transition from one impedance to another depends on at least three things: the amount of transformation required, the type of taper (e.g., linear, exponential, Klopfenstein), and how close the impedance match has to be (usually specified as input reflection coefficient). There's a brief treatment in Johnson, _Antenna Engineering Handbook_. In Brown, Sharpe, Hughes, and Post, _Lines, Waves, and Antennas_, it's stated that the tapered section must be at least a quarter wavelength long, and that if less than that, "it will fail in its matching function". In Skilling, _Electric Transmission Lines_, ". . . a 2 to 1 change of characteristic impedance many be accomplished by using a tapered section that is 1 wavelength long. A good impedance match results at this frequency and at all higher frequencies. Results are fairly good if the tapered section is as little as 1/2 wavelength long, particularly if the impedance ratio required is less than 2 to 1. It is not worth while to use a tapered section much shorter than 1/2 wavelength, for a taper that is 1/4 wavelength or shorter will give nearly as much reflection as will the abrupt change of impedance at a mismatch. Those frequencies for which the length of the taper is a multiple of a half-wavelength are transmitted with the least reflection. This may be a guide in choosing the length of the taper." The author goes on to state that linear and exponential tapers give practically the same results, and summarizes with a recommendation that a tapered section be between 1/2 and 1 wavelength long at the lowest frequency of use. Roy Lewallen, W7EL I've seen essentially the same ... bigger than 1/2 (or 5/8) wavelength works well. BUT... that's for a "good match".. if you're willing to tolerate a "not so good match" I think you can get much shorter. There's a lot of antenna designs that use this sort of scheme to match, say the 100 ohm impedance of a helix to 50 ohms, over a distance on the order of 1/4 or 1/5 wavelength. Whether it's truly a tapered transmission line or a semi-distributed lumped L and C is probably a good question. Certainly, one could use a string of lumped L/C networks to make a synthetic transmission line that has mechanical length much shorter than 1/2 wavelength. So, when does a series of lumped LCs become a transmission line? You also see tapered transitions in microstripline all the time, and they're often shorter than 1/2 wavelength. But maybe that's more in the nature of a "distributed matching network" rather than a pure transmission line. There's a goodly amount of empiricism in both antennas and stripline design. Especially if you're talking about an balanced pair with separation smaller than the diameter/width of the conductor, it's probably not a "simple" transmission line. |
#22
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Tapering open wire feedlines?
I saw hints that the application of the tapered line matching section was
to a HF off centre fed dipole, and an assumption that as a multiband antenna, it had a feedpoint impedance of close to 200+j0. Most users of OCF dipole systems are expecting to be able to use the antenna on many bands without an ATU, and accept significant tradeoffs in peformance for that objective. It is unlikely that the feedpoint impedance of the dipole itself is sufficiently tight to worry too much about the broaband peformance of a tapered line matching section, much less the practicality of constructing low loss tapered two wire lines with Zo approaching 50 ohms. It is only when a system perspective is taken, that some of the component level discussion seems to be an interesting issue, but poor application. Owen |
#23
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Tapering open wire feedlines?
On Jun 22, 1:57*pm, Owen Duffy wrote:
It is only when a system perspective is taken, that some of the component level discussion seems to be an interesting issue, but poor application. In a multi-band system with high SWRs, are there any configurations where a tapered line would be a good application? -- 73, Cecil, w5dxp.com |
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