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Howdy,
Is the overall effectiveness of a loading coil dependent more on wavelength and therefore the length of wire to wind, or on the number of turns (other deficiencies aside)?
Howdy,
Is the overall effectiveness of a loading coil dependent more on wavelength and therefore the length of wire to wind, or on the number of turns (other deficiencies aside)?
If wavelength, then what is a practical fraction of wavelength to use?
For example: I have 4″ schedule 20 PVC pipe … 4″ ID, 4-1/4″ OD. Assuming a minimal amount for insulation on 18 AWG stranded wire @ 5/8 wave and a 9′ copper antenna tube.
The length of 5/8 wave @ 1.650 mHz = 2916/1.650*.625 = 1104.54″
9′ Antenna tube = 9*12 = 108″
Length of wire need to make overall 5/8 wave = 1104.54-108 = 996.54″Circumference of 4-1/4 ID PVC pipe = 4.25*3.1416 = 13.35″
No. of turns = 996.54/13.35 = 74.65 … call it 75 turnsSeems about right to me. But I have no idea if my simple formula works. What say ye?
Total posts : 45366That should be 4-1/4″ OD PVC pipe, not 4-1/4″ ID
Total posts : 45366The loading coil is tuned to resonance with the antenna capacitive reactance. For a 3 m monopole, the antenna capacitance is about 30 pF, which has a capacitive reactance in the vicinity of 3000 ohms near the top of the AM BCB. The loading coil has an inductance in the vicinity of 300 uH.
The tuning to resonance has to be exact. Rough tuning is often done with coil taps, and fine tuning is often done with an adjustable tuning capacitor.
Total posts : 45366Thanks Ermi,
I’m looking more for repeatable formulas to determine the number of turns of x-awg wire on x-diameter form to get in the ball park of a given frequency in the AM band above 1600 kHz.
In spite of taps and playing with lead wire length, I couldn’t get my first loading coil to resonate. As you may know from other posts, I doubled the form diameter, increased wire gauge, and wound at best guess for a fixed antenna length, based on a design I basically copied, for 1710 kHz. It worked surprisingly well with no adjustments and no RF ground. But I couldn’t mount it permanently at the test location, so now I’m trying something else.
I’ve seen some nice air variable caps:
http://www.oselectronics.com/ose_p96.htmPlus, I can adjust the 9′ copper tube portion of antenna length +/- 8″. The loading coil is 10.5″.
Total posts : 45366The best-known approximate formula for the inductance of a single-layer coil was developed by H. A. Wheeler in in 1928. It is very accurate.
L(uH) = [(r^2)(n^2)]/(9r +10l)
n is the number of turns. r is the radius of the coil in inches. l is the length in inches. This is a “low-frequency” formula because it does not take the self-capacitance of the coil into account. At RF frequencies, the self-capacitance causes the measured inductance to be higher than the actual inductance. Self-capacitance reduces Q. Smaller-sized coils have less self-capacitance than larger-sized coils, but smaller coils require the use of smaller-diameter wire for the same inductance, which also reduces Q.
Most important: Do not close-wind magnet wire on the coil form. Close-winding very significantly reduces the Q because of the “proximity effect.” Leaving about one wire diameter between turns will reduce the proximity effect.
Total posts : 45366That +/- 8″ adjustment may be just the ticket once you get the coil close.
Our City’s TIS AM station uses an antenna which is about 10′ long but the loading coil is an integral part of the antenna. Sort of like a center load antenna but the coil is almost 3′ long starting about 4′ from the bottom.
The tip of the antenna is adjustable for length and moving it 1/4″ can make a big difference tuning for minimum SWR.
One of my antennas uses adjustable radials (similar to a Top Hat) to final tune the antenna.
Total posts : 45366The tip of the antenna is adjustable for length and moving it 1/4″ can make a big difference tuning for minimum SWR.
Wow! … 1/4″, eh? My final tuning plan was to run the antenna down as far as it will go which will leave me about 8′ and clamp a 2′ telescopic antenna to the upper tip. When it reaches resonance, hopefully inside that limit far enough to stay legal (but if not, I can tap a coil turn), I can measure the final length, pull off the telescopic antenna, and adjust the main.
But now, I’m considering doing all that just with fairly tight tolerances, and use a nice air cap. That way I can tune at the TX which will be mounted right under the antenna, but inside the cabin on the underside of the cabin roof. Only the thickness of the cabin roof between (need really good seals ;)).
One of my antennas uses adjustable radials (similar to a Top Hat) to final tune the antenna.
Hmm … interesting. I guess that means they aren’t buried. You are altering capacitance, right?Where are they in relationship to the antenna pole and TX? How many are there? How does one adjust radials?
Just curious … My system will be on a boat in a marina, so radials are actually moot. Using the saltwater for counterpoise.
BTW, we are supposed to have some sort of TIS for the ferries on 530 AM (there are signs in the terminal lots), but I don’t see any antennas as you describe, and I’ve never heard anything on 530 kHz here. However, in the main terminal over at Anacortes, it works. I’ll have to look for the antenna next time I’m on the mainland and see if there is something similar here.
Total posts : 45366Hi Ermi,
Thanks for the Wheeler formula. That will help a lot. I think I remember seeing it in old physics books, but forgot about it.
Most important: Do not close-wind magnet wire on the coil form. Close-winding very significantly reduces the Q because of the “proximity effect.” Leaving about one wire diameter between turns will reduce the proximity effect.
Hmm. May be true of magnet wire, which is only lacquered, although many articles suggest close-wound coils even with that. In my case, I decided to go with insulated wire. I suspect the insulation works to provide enough separation. I think the larger gauge also reduces resistance, i.e., the signal isn’t reduced as much on the way to the stick as with smaller gauge wire.
Perhaps that’s why the Manteca Magnum design appears to get good results.
Total posts : 45366I built a center loaded antenna. There is an article on my MRAM page. Look for the antenna story.
My coil didn’t have enough turns and the length of the antenna was already at the limit. The radials added capacitance to resonate with the coil. As I say in the article it was “by guess by golly”. See the article.
The way I figured out where the antenna was for frequency was to step my transmitter frequency until my field strength meter peaked. Based on that I knew the antenna needed either more inductance or capacitance. The radials were added and adjusted to length to peak the signal strength at the desired frequency by adding the right amount of capacitance.
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