Since the antenna adds a fair amount of weight, I needed a light weight base. The Lego Vision Command expansion set comes with a couple of interesting clear rectangular pieces which I used in this design. Bigger...

This is the basic frame. The gap in the front is where the light sensor goes. Bigger...

The tires have been added. Bigger...

This is the fully assembled base. Bigger...

This is the antenna feeler unit, which attaches to the top of the RCX. It basically has two long feelers which are held against the switches using an elastice (a Lego elastic). When the feeler runs into something it moves away from the switch. Bigger...

View from the side. Bigger...

View from the front. Bigger...

View from the other side. Bigger...

Here we see the antenna feelers mounted on the base. Bigger...

This is the starting position. With the antenna feelers in this position the bot still only occupies a 10 cm x 10 cm square. Bigger...

From the other side. The back of the bot in on the right. At start time, the bot backs up, which kicks the feeler out. This causes the top portion to fall down, making the antenna's horizontal. At this point they're no longer held in by the inverted L piece (seen about mid-feeler) and the elastics cause the feelers to snap into position. Bigger...

Rear view while in the starting position. The feeler snugs in between the tire and the gear, and friction holds it in place. Bigger...

Here we see the feelers in their fully deployed position. Bigger...

Front View with the feelers deployed. Bigger...