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The only difference between dual polarization and circular is the addition of an electrical 1/4 wavelength of cable on one of the driven elements. It's actually very easy to achieve. The difference between left hand and right hand CP is which side of the antenna has the longer coax. Since most of the stations you will be working are not using CP, it won't make any difference if you use RHCP or LHCP. You will see the same signal stabilization in either case.

Feeding dual antennas 90 degrees only causes significant directional effects when both elements are in the same polarization. The effect is different when the two elements are physically orientated so they are turned 90 degrees from each other and electrically driven 90 degrees out of phase. Then you have a signal in all polarizations when transmitting and response to all polarizations in receive. Not 90 and 180 degrees. That's what you get when you combine the two with equal lengths and no phase delay.

I've also given more thought to the issue of whether you lose 3 db in receive with CP and have reverted back to my original thoughts on this. I base this on the need to combine the two elements to form either CP or dual polarity. When you combine the two coaxes through a pair of 1/4 wave 75 ohm cables, you have formed a two port, bi-directional splitter / combiner. This transforms the combined antennas impedance from 25 to 50 ohms but gives you a 3 db loss on either of the two ports in RX and TX.

If you just combine the ends of with a "T" connector, you still have a 3 db loss but it's from different reasons now. Two 50 ohm loads in parallel equal 25 ohms and will produce a VSWR of 2:1. Your VSWR meter may not show this much of an increase due to coax loss and the fact that the lengths of coax may not be reflecting the true VSWR at the antenna. That would require using coax lengths that are electrically equal to 1/2 wavelength multiples. Even then the reading will still be hiding some of the reflected power through coax loss.