i'd like to see a external inline whip/coil design for the sstran amt 5000 to use with the intenral coil bypassed.
it should have a low loss tapped air wound coil using a low dialectic coil form with litz wire. would like to see two designs. one with and one without a large air variable cap.
then i would like to see someone with the means to do so test this thing using the external coil design over a large buried radial system and see how well this transmitter operates with such a configuration.
this transmitter has been out for more than a year now, lets get cracking on some design mods and improvements and experiments.
This is the year to build and test an outdoor antenna with radials, using an AMT5000-EX, but I plan to use the internal toroid coil.
You must believe that an external loading coil might produce a better result?
I have not seen someone else suggest that idea, but I would like to hear more about why you believe it to be worth doing.
A difference in performance between an internal and an external loading coil would result if the losses of the two coils are not comparable. There is some data regarding the losses of an external air core coil but I have yet to see anything about the losses of the internal coils used in either the AMT-5000 or the AM-1000. The internal air coil used in the Procaster could be used in such a comparison.
Robert's suggestion for an on the air test is appropriate but this could also be done on a bench where conditions and measurements are more controlled.
Neil
much larger (than internal coil) air wound coil would surely mean lower loss especially when placed over a large ground plane i am a cave dweller and don't have access to property for these experiments.
Common knowledge tells us that a large air coil will have low loss compared to a toroid core coil but this may not be the case. Consider three factors: the length of the wire, the capacitance between windings, and the hysteresis loss in the core material all can increase loss but the addition of a core loss for a toroid coil could be offset by the fewer turns needed for a given inductance which reduces the loss in the winding and the interwinding capacitance to the point where the toroid coil could have less loss than the air core coil.
Speculation, being what it is, solves nothing but it does point the way for experiment and questions. I have seen reports of Q factors for powdered iron core coils as well as for typical air core coils which suggests that the powder iron core loss is less than that for air coils. A good experiment as Robert suggested would be interesting.
Neil
You don't need property to test the idea, as Radio8Z points out, since the only real objective is to compare coil efficiencies the bench testing could be done in the cave.
i have on occasion seen 10" toroids. wonder how well one of them wound with litz wire would work?
My guess is that nobody has ever tried a 10" toroid/litz wire combination with Part 15.
You may be the first.
it's just an idea i thought up. i lack funds right now to buy anything. i lack the room to do anything. someone else will need to do the experimenting as i can't right now.
Robert, I still want to pursue an experiment with a toroid inductor and have a T184-2 core which is wound and ready to test but the problem is that my plate is full with other things at present. That being said, the first chance I get I will bench it for Q and I can compare it with the Q of the air core coil I presently use.
Results are pending and I'll post when I have them.
Ben Tongue has done tests and simulations using ferrite and iron powder rods in linear coils and reports getting high Q values. One way he mentioned for minimizing the stray capacitance is to use a small diameter rod in a large diameter coil. It is a long read found here. He has published other coil data and a web search should find them.
Neil
Litz wire is very effective below 500 kHz and it is rarely used above 2 mHz as it is much less effective. Most Part-15'ers usually work at or above 1.6 mHz., near the end of it's useful frequency range to avoid losses..
What would be the big advantage of using Litz wire compared to solid copper? It might help some if one was designing at the lower end of the band..
Why would Litz wire be less efficient above 500 kHz? Litz wire frequency ranges are dependant on the number and fine-ness of the strands (more and finer for higher freqs), perhaps where skin effect comes into play. In my admittedly limited knowledge base, if you pick a type appropriate for a given frequncy range, in theory it should be at least a bit more efficient than copper.
The skin effect is caused by the magnetic field produced by electrons moving in a conductor and acts to push the current to the surface of the conductor which increases the resistance to RF because the conductive cross sectional area is reduced. The idea behind Litz wire is that by using many small insulated conductors each has its own skin effect depth which is greater than the radius of the wire so the whole wire is conducting and when these are combined in parallel the resistance effect is less than for an equal cross section of a single conductor.
As the frequency increases the skin effect begins to act on the small conductors since the skin depth becomes smaller than the radius of the conductor. There is also the influence of neighboring wires' magnetic fields which comes into play to further distort the current distribution.
When low loss coils are needed the conductors are silver plated so the cross section seen by the RF current is comparatively low resistance. A similar example is solid center conductor coax or hard line for VHF where the conductor is steel for strength and lower cost and is surface plated with copper. This gives the conductivity of copper with the strength of steel since due to the skin effect the current is flowing mostly near the surface of the conductor which is copper.
Neil
Yes, LITZ wire is better then magnet wire dor part 15 AM antennas. As for ferrite~! It sucks for part 15 antennas outside. The antenna would need almost constant tuning as temperature changes, even slight ones, have a big effect on antenna tuning. Yes, they can give high Q but not as high as a properly designed wire coil. (Correct dia to length ratio, spacing and wire sizes plus dialectric.) Then you take into account the detuning from temperature changes and you loose any advantage of their Q. Bottom line, stick with a high Q wire wound coil. All the wire coils I see have a length twice as long as their diameter. A much better coil can be had with a diameter twice as wide as its length.