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*Subject*: [amsat-bb] Question on 1/4 wavelength transformer*From*: "Floyd Sense" <sentek@xxxxxxxxxxxxxx>*Date*: Tue, 25 Sep 2001 14:09:22 -0400

I'm looking at ways to feed a pair of crossed 70cm yagis so I can switch the polarization for satellite work. I've researched the Satellite Handbook and ARRL Antenna Book and these questions arise from those readings. First, the antenna in question consists of a pair of 10 element yagis on one boom, with a 1 inch offset between the vertical and horizontal elements (NOT a 90 degree spacing!). The yagi driven elements are gamma matched and I'll assume for now that they're supposed to be adjusted for a 50 ohm match. While this antenna is believed to be Cushcraft, there's no documentation available, although I have the manual for the Cushcraft A144-20T Twist, which is identical in construction, but for 2 meters instead of 70cm. Now the questions: 1. Forgetting the requirement for switchable circular polarization for the moment, the Cushcraft literature shows, for "Axial Radiation", connecting the 50 ohm feedline to a "Tee" and then running an unknown length of RG-59U to the two gamma matches. I assume that this length is an odd multiple of 1/4 wavelength at the operating frequency (won't odd multiples of 1/4 wavelength perform the same transformation as 1/4 wave?). I don't understand why matching section is 72 ohm line. With a single antenna and a feedpoint impedance of around 110 ohms, the 72 ohms would make sense to transform the impedance to that of the 50 ohm feedline. Perhaps the gamma matches on these driven elements are actually adjusted for a 110 ohm match and then it would make sense to me. What do you think? Does the fact that the two antennas are being connected to a single feedline factor into the impedance chosen for the matching sections? 2. On page 10-10 of the Satellite handbook, Figure 10.11: The figure shows feed systems for crossed yagis and again the 72 ohm transformation section appears, so, same question as above. I want to use system (A) in that diagram, since the (B) and (C) apply to crossed yagis with a 90 degree offset. In (A), I'm puzzled by why the 1/4 wave delay line is 95 ohms. By the way, in these three systems the yagis are assumed to be 50 ohms resistive. If you can shed any light on either or both of these situations, I'd be very grateful. K8AC Floyd Sense - Angier, NC ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

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