Can someone more knowledgable tell if this coil ( http://www.mcmelectronics.com/product/287-3666) could work with the AMT3000 and the usual 10 foot monopole. It's rated at 260 microhenries, which according to this chart ( http://www7.brinkster.com/yvesroy/10ft-antenna.asp) is nearly the ideal inducance for 1690kHz. I'm a noob so I'm probably missing something here...unless everyone is just building magnet wire coils for their health.
It might work but there are two things I would note. One is the core material. Not all materials are low loss and suitable for operation at broadcast band frequencies.
The second would be that you may have to remove, tap, or add a few turns to do the final adjustment for resonance. This is not a big deal however, just be aware of it.
At that price it may be worth getting one to try.
Toroids are used in some of the AM transmitters as loading coils but the question, again, is whether this coil has the right core material.
Neil
That's about what I figured. Guess I'll just have to try it.
This toroid will likely perform extremely poorly as an antenna loading coil because it is constructed using a very high permeability ferrite core which will be extremely lossy at MW frequencies.
A "powdered iron" core is the only type that will give relatively low core losses at RF frequencies. There are a range of powdered iron core materials that are formulated for use in specific frequency ranges. Off the shelf pre-wound powdered iron coils are practically nonexistent. However, you will see a broad selection of off the shelf high permeability toroids for use in EMI filtering and power supply applications. Core loss is not much of a factor at DC and low frequency AC mains frequencies.
So why do we see high permeability ferrite cores used as rod antennas and IF transformers in receivers? The answer is that core loss is a function of both the permeability and the RF current flowing through the coil. In receivers, the RF current flowing through the coil is down in the microamp range and what core loss occurs can be compensated with extra amplification. A much larger RF current flows through a transmitter loading coil. The extra current is sufficient to cause very high core loss unless the core material is carefully selected. This extra core loss in a loading coil can't be compensated by extra amplification without violating FCC 15.119 rules.
I'd suggest taking a look at these :
http://www.bwantennas.com/coils/mini.htm
FWIW, I'm going to build out my own system...and speaking of that, I can use some advice....(next post...wait for it ;))
I'm a cheap bastard, so I'm going to make one myself.
Make sure you get the clips as It does make things better.
Here is a link http://www.part15.us/forum/part15-forums/general-discussion/am-antenna-construction-and-installation to a write up of how I constructed my antenna system including the loading coil. You'll get the general idea and don't have to do it exactly as did I.
I did find in my experimenting that white PVC drain pipe did not work well as a coil form but the clear acrylic tube did. The PVC tended to lower the Q quite a bit. I also wound one with insulated #14 house wire and it had a low Q so I ended up using #16 enameled magnet wire. I was able to buy a pound of this for $12 so it didn't break the bank.
Rather than using taps, I chose to add a 5 turn coil in series with the loading coil and on the same form. Tuning is achieved by sliding the 5 turn coil for resonance. I prefer this over using taps since it avoids connection problems with clips and corrosion. Here's a photo of the coil and tuning coil: http://postimg.org/image/eu2gfj6n1/
Neil
Neil: Could you provide a drawing to complement the photo? TNX
John,
Give me a couple of days and I will put something together with more details.
Neil
Thanks Neil.