I know I’m the annoying new guy to part15.us, but I have a probably stupid question…If I’m winding a piece of PVC with wire to make a loading “coil” for my Part 15 AM station, and I have 200 feet of wire on there, but it is only 3.5 feet “high” is it “legal” or do I even add it in to the “height” or is it just a loading coil? I’m really confused on EXACTLY what constitutes an antenna and a “coil.” As usual, any help would be GREATLY appreciated…
Dave
KSOL Radio
1000 AM
92.1 FM
880? AM (soon)
Carl Blare says
Undefined
Whether to count the loading coil as part of the length of the antenna with respect to 15.219 is not specified in the Rules, and opinions that have been given here on part15.us have varied.
The loading coil does not radiate, so I would personally not count it as part of the “legal” length, and there is another thing I do, and that is that I DO NOT stand the coil upright so that it’s full length exists at the base of the antenna……
I mount loading coils long-ways at the base of the vertical stick, which has been recommended in the ARRL Handbook (American Radio Relay League) as a way of slightly improving antenna performance. With the coil put long-ways the vertical measurement would be a small 3″ or so.
radio8z says
Another Opinion
As Carl mentioned interpretations on this vary so here is another one. On the basis that the loading coil does radiate the length of the coil (the physical length of the winding, not the length of wire on the coil) needs to be counted in the 3 meter limit. Practically, a typical coil which is 3 inches in diameter and 3 inches long would only take 3 inches from the 3 meter “budget”. For my installation I included the coil length.
You mentioned a 3.5 feet number and I ask is this the length of the coil? Though this will work it would make the length/diameter ratio rather large. The recommendation for “air” coils is to keep the L/D ratio around 1.5. With less than 200 feet of #18 (for example) you can wind about 76 turns on a 3 inch diameter form and the coil will be close to 3 inches in length with an inductance around 260 uH which will tune to resonance in the 1600-1700 kHz part of the band in a 3 meter antenna system.
Here’s a nifty calculator which I have found useful for coil design:
Link to Inductance Calculator
Another Link to a post in which I detail how I built an antenna system which might give you some ideas to design your own.
Neil
Carl Blare says
Link Needs Fixing
Neil,
Check the link to the inductance calculator, I think it needs fixing.
radio8z says
Thanks Carl, Links Fixed
Thanks for the alert on this. I should have followed my own advice about posting links which is to preview the posts before saving. Turns out I encoded them wrong.
Neil
RFB says
All In The 3M
Everything inside the 3 meter measurement is considered the antenna. Anything outside of that is a violation.
RFB
kc8gpd says
i personally believe that
i personally believe that anything that fits within the 3m height, symmetrical capacity hat radials, symmetrical ground radials and loading coil is legal. as rfb pointed out anything outside that is not.
whether the fcc inspecting you or not will take that interpretation is a whole other ball game.
Carl Blare says
The VH Antenna
I made up the name VH for an AM part 15 antenna idea I might try someday, based on something said by MICRO1700 somewhere in these threads.
MICRO1700 recalled a statement he overheard said by an FCC inspector, something like, “If it (the antenna) fits in a 10 X 10 cube, it’s legal”.
Hey!
So here it is. The VH antenna rises vertically to a height of 10-feet, makes a sharp right angle at the top and extends horizontally for a distance of 10-feet. A whole new concept. 10X10.