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*Subject*: Re: [amsat-bb] HELP please AO40 RHC Crossed Yagi connections*From*: "John P. Toscano" <tosca005@xxxxxxxxxx>*Date*: Tue, 25 Dec 2001 10:56:18 -0600

Darren Rainer wrote: > Hi all,I am currently setting up for operation on AO40 but am a bit > unsure how to connect up a F9FT Tonna 435MHz 2 X 19ele crossed Yagi > for RHC polarisation. Now both yagis on the boom are 50 ohms. The > horizontal Yagi is positioned 20cm in front of the vertical Yagi on > the boom. To get circular polarization, the horizontal and vertical elements need to be 1/4 wavelength out of phase. At 435 MHz, 1/4 wavelength is about 17.25 cm. So your elements are already offset to produce an elliptical pattern, but seem to not be spaced quite optimally for true circularity at 435 MHz. A spacing of 20cm should give a circular pattern at 375 MHz. Are you sure the spacing is 20cm, or was that just an approximate guess? To match the impedances, there are two basic choices. Transform the impedance of each 50 ohm antenna to 100 ohms, and then put the two 100 ohm loads in parallel to make 50 ohms, -or- connect the two 50 ohm antennas in parallell to make 25 ohms and transform the 25 ohm impedance up to 50 ohms. The first method is what you are describing. Odd quarter-wavelengths of 75 ohm coax will transform 50 ohms to approximately 100 ohms. To compute the exact values, Zcoax = SQRT(Zin x Zout), so the ideal coax impedance would be SQRT(5000) = 70.71 ohms. Most people would probably use RG-59 and PL-259 "UHF connectors" to a SO-239 TEE, but if you can find 75 ohm cable to fit N connectors, an N TEE might be better. The TEE should be of the 50 ohm variety, since that is the impedance of the feedline connected to the common port of the TEE (going to the radio). Oh, and the quarter-wavelengths of coax must take into account the velocity factor of the coax, so they won't be exactly a quarter wavelength long. The second method is arguably better. You use only standard 50 ohm cables and connectors, and use a 2-port power divider to do the impedance transformation between 25 ohms (antenna side) and 50 ohms (radio side). The coax jumpers from each antenna to the 2-port power divider can be any length, so long as they are EQUAL in length. And if you need to make one of them longer to compensate for the fact that your elements are not quite at the right spacing, you just add in the extra length. No need to worry about 1/4 wavelength matching, or velocity factor, etc. The power divider looks like an aluminum pipe (round or square in cross-section) with a single N connector on one end, and two N connectors on the other end. Inside the pipe is a piece of tubing (usually brass or copper) of optimal diameter, which is connected to the center pins of the three N connectors. It functions as a quarter-wavelength air-dialectric coaxial transmission line whose impedance is a function of the diameters of the inside (brass) and outside (aluminum) tubes. To transform Zin=25 ohms to Zout=50 ohms you want the power divider to have an impedance of SQRT(25 x 50) = SQRT(1250) = 35.4 ohms. If the outer sleeve is square aluminum tube (easy to mount N connectors to), and the innter sleeve is round brass or copper tube (easy to find, easy to solder to), and D is the inner width of the outer tube, and d is the outer diameter of the inner tube, the formula for impedance is Z = 138 log(1.08 x (D/d)). If the outer tube has D = 7/8" (0.875 inches) and the inner tube has d = 1/2" (0.5 inches), the impedance would be 138 log(1.08 * 0.875 / 0.5) = 138 log(1.89) = 1.38 x 0.276 = 38.15, which is pretty close to the desired 35.4 ohms. If you aren't handy building such a device, they are available commercially: http://www.downeastmicrowave.com/Antennas.htm Part number 70-2PD ....................................... $58 US http://www.directivesystems.com/pricelst.htm Part number 70-2PD ....................................... $58 US http://www.m2inc.com/products/acess/pwrdivs.html Part 70cm 2 Port Power Divider ........................... $65 US http://aesham.com Part 70CM2PORTPD ......................................... $66 US Hope that helps. John (KB0ZEV) ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**References**:**HELP please AO40 RHC Crossed Yagi connections***From:*Darren Rainer

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