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