The 707, 727 and 747 use the same basic concept probe (high voltage) antenna. It's at the top of the fin pointed forward in the 707; pointed aft in the 727. The 707 and 727 use the Collins 180R-12 antenna coupler, similar inside to above, and the 747 uses the 180R-17, which is just the same only different.
Some couplers have a beryllium copper tape about 0.5 cm wide, but I can't remember which ones. I'm pretty sure the 180R-17 uses the variable inductor on a drum, as above.
If you've ever watched the wingtip of a 747 at work, you know what torture those electromechanical couplers have to endure. They are usually failed or ready for overhaul by 2,000 hours.
The retrofit leading edge (high current) antenna was developed by a Brit engineer at Douglas for the DC-10, concurrent with similar development on the L-1011, and availability of high current couplers. The notch (high current) antenna up the vertical was developed by Eastern Airlines for their 727 fleet, and sold widely. Boeing eventually adopted that one for their 727 and 737 production, and on into the 747-300. All current generation airliners use the same high current antenna and coupler concept.
GB