It surprises me they could transmit an analog signal back while keeping its quality secret from the Soviets. I wonder if they did succeed at that, or just at keeping the rest of us in the dark.
It wasn't like the Soviets could just turn their TVs to the right channel. I doubt the Soviets knew what frequency they were using, and analog video is still "encoded" in the radio waves. These signals still required that specialized $300,000 equipment (which I'm sure was top secret at the time) to make sense of.
The height of the antenna would give away the frequency. But the soviets had good spy penetration into the US industrial base so it's likely they already knew anyways.