A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Örjan Sandström
Date: 2017 Mar 31, 13:49 -0700
Frank, some ramblings on antennas.
The ground is just "the othet half of the antenna" if we "dumb it down" that is simplifying it to the brink of lying but...
basically any conductor will pick up signal, how effective at any given frequency has to do with a number of factors, mainly length, capacitive and inductive coupling to environment, the conductors self capacitance/inductance...
Main trouble with short antennas is losses in mismatched impedances between circuits and weird nearly inpredictable pickup pattern.
for strong locals it is ok to clip a lead to say a car chassi or another largish conductor, as the energy levels are relatively high, but for distant weak stations, not so good
problem with single wire "no ground" antennas for crystalset is that the "return path" (current needs to circulate) for the radio waves will go throug mainly capavitive coupling to everything around it (inefficient), if done right it can make for sharper tuning though (look up mystery set and selectivity enhancement circuit).
some antenna types do not need ground - dipole, loop - A dipole would require ridiculously long antenna at 500kHz, roughly 300m, at 5MHz 30m, and it needs to be vertical for most AM...
Loop is possible, but is directional and will fade in out with swaying, ferrite loopstick is slightly better as it is compact but still directional.
I have held sloping wire antennas aloft 50m with a large kite, and used a 50 m 2.5mm^2 insulsted eire dragging through water as counterpoise ground, got ~800km crystalradio Dx that way (transmitter in Oslo me just west of Tromsø) .
As to powered primitive emergency radio.
Uhm, what about a hybrid feedback regen, they are sensitive down to noise floor and in some bands below If made/adjusted with care.
power requirement slightly lower than superhet as no local oscillator is needed.
Likely needs option of both internal ferrite loop (long narrow with sharp null for direction?) and external antenna.
antenna and regen section isolated from each other, not so much for any radiated signal, more to avoid different antennas and likely swaying mast/antenna to pull it off frequency by varying the load impedans feeding it back to the weakly oscillating coil...
I think a grounded gate jfet antenna preamplifier is about as simple as it gets for that.
Any radio for say 4-5 bands (few khz each side of 2.5, 5, 10 and ?) can be fairly simple to set up with rotary switch for channel selection and two back to back diodes set up as fine tune varactors. additional bands for news?
A superhet will also be ok, I have just built two and not designed or even tweaked any design beyond aligning stages so do not expectmuch input from me beyond what I have read in books.