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*Subject*: Phasing CP (was:Re: [amsat-bb] A question about 75 Ohm connectors)*From*: Jon Ogden <na9d-2@xxxxxxxxxxxxx>*Date*: Thu, 13 Feb 2003 23:05:06 -0600*In-Reply-To*: <200302122326.h1CNQjt16367@panix1.panix.com>*User-Agent*: Microsoft-Outlook-Express-Macintosh-Edition/5.0.3

on 2/12/03 5:26 PM, Doug Faunt N6TQS +1-510-655-8604 at faunt@panix.com wrote: > I think I was unclear: One element would immediately at the feedline > tee, the other element would be 1/4 wavelength of coax away from that > point. Doug, I looked up some info in the Satellite Expermenter's handbook on feeding CP yagi arrays. There are basically two choices. In one choice the antenna elements are physically located next to each other at roughly a common feed point. In the second, the feed points are physically separated by a 1/4 wavelength. I'll deal with the first case since that's what you have. OK, each antenna's impedance is 50 Ohms. You'll need to join these two impedances at a tee and then feed that into a common 50 Ohm transmission line. Therefore, the impedance of each branch of coax feeding each antenna needs to look like 100 Ohms so that when combined it makes 50. You probably understand that already (I'm explaining in detail for those who don't.). We can create an impedance transformation from 100 Ohms to 50 Ohms by using a 1/4 wave transformer of 75 Ohm coax. So working out from the tee, we have 100 Ohms. We then run through a 1/4 wavelength of 75 Ohm coax on each branch and we are now transformed on each branch down to 50 Ohms. But we have one more problem. We now need to feed the arrays 90 degrees out of phase. We do this by adding an additional 1/4 wavelength of 50 Ohm coax to one of the branches before feeding the antenna. This will give us the necessary impedance and the 90 degree phase shift. Now you may need to add extra coax in order to be able to physically and practically reach the feed points. You do that by adding equal lengths of 50 Ohm coax to each branch. But remember from transmission line theory that if you have a 1/2 wavelength piece of coax, it does not matter what it's impedance is as long as the impedances on each end are the same. This way, the side of the branch that is fed only with the 1/4 wavelength of 75 Ohm coax can be lengthened by adding an additional 1/2 wavelength of 75 Ohm coax. The side of the branch that has both the 1/4 wavelength of 75 Ohm and the 1/4 wavelength of 50 Ohm can be lengthened by adding an additional 1/2 wavelength of 50 Ohm coax. By using the same characteristic impedance for the extension that is at the end of each branch it makes assembly easier. Remember though that you need to make sure that it is a 1/2 wavelength ELECTRICALLY not via physical measurement. Be sure to take into account the velocity of propagation of the coax when creating the 1/x wavelength sections in each case. Does this make sense? I hope I worded it well enough! 73, Jon NA9D ------------------------------------- Jon Ogden NA9D (ex: KE9NA) Citizen of the People's Democratic Republik of Illinois Life Member: ARRL, NRA Member: AMSAT, DXCC http://www.qsl.net/na9d <- Updated on 1/22/03!!! "A life lived in fear is a life half lived." ---- Sent via amsat-bb@amsat.org. Opinions expressed are those of the author. Not an AMSAT member? Join now to support the amateur satellite program! To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**Follow-Ups**:**Re: Phasing CP (was:Re: A question about 75 Ohm connectors)***From:*Doug Faunt N6TQS +1-510-655-8604

**References**:**Re: A question about 75 Ohm connectors***From:*Doug Faunt N6TQS +1-510-655-8604

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