Home |
Search |
Today's Posts |
#20
|
|||
|
|||
Tapering open wire feedlines?
"Roy Lewallen" wrote in message ... 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 Roy Thank you Roy for taking the time to post. That's exactly what I was looking for, and answers my questions completely. My curiousity is satisfied. Pete |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Open wire feedline | Antenna | |||
OPEN WIRE LINE | Antenna | |||
Open wire feedlines | General | |||
WTB: 2" open wire spreaders | Equipment | |||
WTB: 2" open wire spreaders | Equipment |