A Dipole _and_ a loop
A loop antenna for 14MHz and a dipole for 7MHz need the same length of wire. To fit a 7MHz dipole in my garden it ends up as a v-shape. Can we combine these facts to make an antenna that's usable on at least 7MHz and 14MHz (and maybe 21MHz, and 28MHz)?
Here's S11 for the dipole.
Popping outside and joining the ends of the dipole together, here's S11 for the loop (dipole in blue too). There's hope!
If we pop a trap in the middle, it'll look like a loop at 14MHz and a dipole at 7MHz, right? Could we use a series resonant circuit to make the loop only on 14MHz?
Minor diversion here, cos I've got lots of RF on the coax outer. Time for a balun I suppose, but I'm lazy: what about this clamp on ferrite? A few turns of the coax around the clamp on ferrite seems to help but numbers are nice.
Found an old germanium diode, a sensitive panel meter, and another clamp on ferrite in the pile of junk and knocked up a little meter. Not calibrated of course, but a pot to set the sensitivity - full-scale - five - without the ferrite, down to three with. S11 looks the same too.
Here's the 'after' shot, terrible quality but it's late and dark. Use your imagination for the before, the needle points at five. Five turns of the coax around a similar ferrite. Yes the coax is tiny, and what's more it was free: from an old GPS antenna.
