It's been a long time since I've done electronics but if I remember right then then the best antenna would be equal to the wave length of the signal recieved, so a 27mhz signal has a wavelength of approx 11m

Now you can't have a 11m antenna on your RC tank so the best you can do is go for a fraction of that 1/2, 1/4, 1/8, 1/16 or 1/32 which'll be 34.375cm, so you could have a 17cm antenna with a 17.375cm coil inside the tank.

If you want to find out the wavelength of your transmitter then go [link=http://www.sengpielaudio.com/calculator-wavelength.htm]HERE[/link] type in the frequency of your transmitter into section 2. "Radio and light waves in a vacuum" Rember that transmitters are in Mhz so 27Mhz is 27,000,000hz.

This'll give you an approx figure (unless some of you live in a vacuum) as air density will shortens wavelength.