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June 9th 06, 05:00 PM
posted to rec.radio.amateur.antenna
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Buyer Beware - Cobra Ultralight
Cecil Moore wrote: Michael Coslo wrote: Sounds like time for Cecil to join in. I know his system uses switchable lengths of twinlead, but perhaps he might have something to offer here in a one length feedline system. Things are slow here at the GED office today so I modeled the Cobra-Jr and Cobra-Sr. The Cobra-Jr suffers from feedpoint impedance problems similar to a G5RV used on 75m. The feedpoint impedance on 3.7 MHz is about 16.5 + j459 ohms giving a 450 ohm SWR of 56:1. For my variable feedline method to work with 450 ohm line, the SWR must fall between 4.5:1 and 18:1. A dipole that is 1/2WL on the lowest frequency of operation usually meets that requirement. The G5RV length doesn't and the Cobra-Jr doesn't. One thing interesting about the Cobra-Jr is that if it is made 62 feet long instead of 73 feet long, it becomes resonant around 3.85 MHz with a feedpoint impedance of about 16 ohms. That's an SWR of only about 3:1 on coax. or ~1.3:1 using a 1:4 balun. That's a half size antenna for 75m that is only about 1.5 dB down from a full size 1/2WL dipole. -- 73, Cecil, http://www.qsl.net/w5dxp |
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