some interesting stuff for AM like home made sheilded loop, measure AM modulation with a scope, etc....
http://radiomagonline.com/tutorials_tips/engineers_notebook/
I assume the antenna is for reception, not transmission at midband freqs. Also it should be coupled rather than direct. However, if you move to the mHz regions, it can be practical. We've covered a bit of ground already determining that a magnetic loop antenna will not work for TX'ng at 100mw. I thought about faking it (for those with space problems, or whose apartment residence won't allow placement of a loading coil/10 ft. antenna system) by making a pseudo-loop antenna with a detached break with a loading coil connected on one end.
We already know that an inline loading coil is quite a noticeably more effiicient. Magnetic induction seems stronger if it is on-center with the vertical antenna component. Therefore I'm not sure my idea will work well for Part 15 AM TX'ng.
However, having said that, my friend Dick Rich of Falcon Radio Research (50 years in business, government contracts, etc.) still insists a magnetic antenna is easier to load, and only requires a little RF network to achieve resonance, regardless of final power. Looking over the formulas, I determined that a larger copper pipe, say 3/4" or even 1", formed into a 3' dia. circle or even a square, will yield better results because of skin effect and less resistance.
I wish I had more time to play with these ideas (I need sponsors to afford more automated gear) and also with a large format fractal antenna ... which would look like a very nice garden accessory ... albeit a lot of work to construct (57 circular pieces of copper tubing of varying diameters, soldered together in a certain pattern). No idea how the FCC would look at it, though 😉
If you don't have the table in the link cited handy there is a well known way to measure modulation percentage. Count the scope divisions between the maximum peaks (A) and the minimum valleys (B). The percent modulation is 100((A-B)/(A+B)).
Also, a couple of points re Ken's post above:
We already know that an inline loading coil is quite a noticeably more effiicient. Magnetic induction seems stronger if it is on-center with the vertical antenna component.
I don't follow this. Do you mean a situation where the loading coil is axially aligned with the vertical antenna element? If the coil is a very small fraction of a wavelength in dimension it seems orientation wouldn't matter in terms of the radiated field when combined with that of the vertical radiator. If you are referencing inductive coupling then the tx. and receiving coils should be axially aligned.
...my friend .... still insists a magnetic antenna is easier to load, and only requires a little RF network to achieve resonance, regardless of final power.
Has your friend collected data to confirm this? If true wouldn't we see this used for AM BCB applications? What does final input power have to do with resonance?
I determined that a larger copper pipe, say 3/4" or even 1", formed into a 3' dia. circle or even a square, will yield better results because of skin effect and less resistance.
This sounds like the DDRR antenna (search this site for discussions about this) which is designed to be physically near 1/4 wavelength in circumference and is not practical at BCB frequencies.
Neil
Also, a couple of points re Ken's post above:
"We already know that an inline loading coil is quite a noticeably more effiicient. Magnetic induction seems stronger if it is on-center with the vertical antenna component."
I don't follow this. Do you mean a situation where the loading coil is axially aligned with the vertical antenna element? If the coil is a very small fraction of a wavelength in dimension it seems orientation wouldn't matter in terms of the radiated field when combined with that of the vertical radiator.
Yes, axially aligned. It seems to be more efficient overall. But, if you are correct, then my hope of building a "loop that isn't a loop" antenna that will resonate and have decent range has risen. My idea was to make a circline antenna from soft copper tubing.
It wouldn't be electrically joined. Where the ends come together, one will have a coil turned on a PVC pipe connected to the pipe on it's end both physically and electrically, while the other end of the pipe will be connected to plastic fittings except that part of it has copper adapters and an attached threaded section with double nuts for adjusting the overall length of the copper pipe.
The end sections would have fabricated plastic brackets. The TX mounts to that as well, and the whole thing can be bolted to an overhang. In effect, except for the coil, it is an ordinary electric antenna with the exception that it's circline instead of straight and vertical.
Then ... How would one create a reasonable ground for it?