Some years ago I found a rf release on ebay. Made in Japan, Prinz I believe was the brand - definitely from the seventies. The solenoid/receiver is about the size of pack of cigarettes with a telescoping antenna and the transmitter, slightly larger than a bic lighter, has a simple on/off switch and a four foot telescoping antenna. It actually works quite well as I can trigger it from at least 100'. I sometimes use this rigged to a sinar 5x7. Neat stuff, but not as reliable as an air release like those posted above.