Looks very helpful, thanks PhilB. I only have 16 AWG enameled magnet wire on hand, which I got because of less AC wire resistance...so you are saying there will be more efficiency with a 18 or 20 awg? The numbers showing are calculated for 1400 kHz, but I'm going with 1650 kHz.
NOTE: According to the frequency searches I've done, that frequency should still be good down the road when I take a shot at applying for a commercial CP (no current window open), so I want to stick with it.
Anyways, 1650 kHz will have a considerably different set of values. I'm going to try to run numbers and see if I can get the L/d ratio in bounds. BTW, I'm using white PVC sewer pipe, OD=4.215".
Previously I made some ratio calculations and came up with 67 turns with fine multi-strand 18 AWG insulated wire (insulation provides some spacing, multi-strand for skin effect) in a modified (to 1650 kHz) Manteca Magnum coil http://filebay1.home.comcast.net/~filebay1/antenna.html, which worked quite well (after tuning the radiator to resonance) on the boat when I had ocean counterpoise, but that's not the case anymore.
First, a couple of comments:
I know you said you are using white PVC sewer pipe, but let me add a caution for the benefit of others reading this that they should choose white rather than grey or black as shown in the Manteca coil pictures. It's been said on the web anecdotally that dark PVC contains carbon pigment that can cause some unknown amount of additional dielectric loss.
Multi-stranded wire will not reduce skin effect relative to solid wire of the same gauge. It's only multi-stranded to increase physical flexibility. Litz wire, however, does reduce skin effect because the individual strands are insulated from each other. This results in an total skin diameter equivalent to a much larger solid wire.
The capacitance for a solid cylinder monopole of 4.215" diameter is calculated to be 55.5 pF.
At 1650 kHz, you will need an inductance of only 170 uH to resonate with your antenna capacitance of 55.5 pF. I originally picked 1400 kHz because I interpreted that you wanted a coil to cover the upper 1/4 of the band.
Here are updated results from the coil calculator for 170 uH coil on a 4.215 in OD (2.108 in radius) form using enamel magnet wire with nominal coating width, and wire pitch = 2 x wire diameter (one wire diameter space between turns). The total wire length is calculated by multiplying the circumference at the wire centers by the number of turns:
#16 wire (.0525 in OD), 54 turns, coil len. 5.67 in, L/d ratio 1.33, wire len. 60.33 ft, wire AC resistance 1.476
#18 wire (.0418 in OD), 47 turns, coil len. 3.93 in, L/d ratio 0.92, wire len. 52.38 ft, wire AC resistance 1.610
#20 wire (.0334 in OD), 43 turns, coil len. 2.87 in, L/d ratio 0.68, wire len. 47.83 ft, wire AC resistance 1.839
For this lower inductance 170 uH coil for 1650 kHz, the L/d ratio falls within the .4 to 1.5 L/d sweet spot for all three wire diameters, so #16 wire is now the best choice because of its lowest AC resistance. According to the Medhurst equation for coil capacitance: C = H * D pF (where H is a constant from the L/d lookup in his table and D is the coil diameter in cm), the coil capacitance will be within a couple tenths of 5 pF for all three coils, so the lower AC resistance of #16 wire makes it the wining choice.
Note that all this complex coil calculation software will result in an approximate inductance (some say within about 5%). Also, the antenna capacitance calculation was done for a solid cylinder of 4.215 in diameter and 118 in long. Your cage monopole may be different depending on its construction. The 55.5 pF capacitance calculated for a solid cylinder will be approached with a maximum number of vertical wires around the circumference. So, when you wind the coil, you should shoot for somewhat more than 170 uH to cover the likely lower capacitance of a practical cage monopole. Don't let this deter you. I think maybe 8 vertical copper wires equally spaced around the circumference will bring you close to the capacitance for a solid cylinder. The full 55.5 pF would definitely be achieved with fine-mesh copper screen or sheet metal formed completely around the 4.215 PVC pipe.
Also to consider is that you may not have a way to adjust the capacitance of the cage monopole by adjusting its length. It might be good to use Neil's (Radio8z) coil adjustment scheme by making the bottom few turns just loose enough to enable adjusting their spacing to fine tune the coil inductance to get peak resonance. This would be in addition to providing a few taps on the coil (one per turn) to allow for the unknowns of the real coil and real antenna.
According to some other calculations I made, the UNLOADED Q of your coil will be in the region of 700 to 800. If you use a good ground radial system with 16 30 ft radials or better, you can probably expect a ground loss resistance of around 5 or 10 ohms depending on your local earth conductivity. This will bring your LOADED Q down to 260 and 160, respectively, and your audio bandwidth will be 3155 Hz and 5281 Hz, respectively.
Finally, don't use my results without checking them! I may have made a mistake somewhere in my entries. You should go through the same calculations to cross-check my results. If you find discrepancies, let me know and we can make corrections.
This has been a great exercise for me to increase my understanding of the intricacies of creating optimum Part 15 coils. I appreciate the motivation and opportunity. I have always wanted to do this but never got around to it.
If you get this all worked out, I'm sure everyone here would appreciate seeing a report of your results and any problems/solutions you encounter along the way.
If anybody wants to, they can Google
"Dave's Spider Coil Page." It is very
interesting, and I think Spider weave coils
are the same as basket weave coils. (??)
Anyway, it's fun to look at, and there is
a calculator there.
Bruce
For PhilB...
Thanks for your tremendous effort and help. There are a few anomalies associated with the design I'm trying to work out. Also, I'm about to purchase AMT5000 kit (although, even tho' I know how, I HATE soldering...it's the only way to go for the present work), so I will email offline if you don't mind, to look over the antenna design, tuning parameters, and ground radial setup.
Is that OK?
I meant email offlist (not offline).
Similar to spider web, the diamond weave...another one of the "exotic coil" windings.
IF my basic understanding is correct, there are two things I think I understand about coils...
1.) the length of the wire used to wind the coil sets up a resistance which has a value in ohms just the same as a resistor;
2.) the nearness of windings, which places the wire side-by-side with itself, exchanges some magnetic energy resulting in capacitance;
Now I'll hazard a guess... I guess that "inductance" is a value derived from the resistance and capacitance working together(?)
But here's a question I've never heard addressed at all...
What affect does the constant circular spinning of the electrical signal as it goes around and around and around have on the signal? I would think that among other things the capacitance factor across the sides of the coil constantly changes phase with every cycle.
Ken,
Sure, I'll be glad to help if I can.
Resistance describes the opposition to DC signals. Reactance of a coil or capacitor describes the opposition to AC signals. Impedance describes the total opposition to AC signals when combining DC resistance and AC reactance.
The length and guage of the coil wire affects the DC resistance of the coil.
The physical attributes such as length, diameter and spacing of a coil affect the coil inductance which is the reactive component. As frequency increases, reactance increases although the inductance stays the same for a given set of physical attributes.
Combine DC resistance with capacitive and inductive reactance and you get Impedance, the total opposition to an AC signal.
When a tuned ciruit such as an antenna is resonant, the reactances cancel. This is how the loading coil reactance cancels out the antennas capacitive reactance.
Impedance describes the total opposition to AC signals when combining DC resistance and AC reactance.
Wouldn't it be expected that the resistance of a conductor to the flow of r-f current is higher than for direct current, because r-f current is confined by "skin effect" to a thin layer on/just below the outer diameter of the conductor, where direct current uses the whole cross-section of the conductor?
I would expect that may be why large diameter tubing is used for high current RF applications.
For round copper wire, electrical grade purity, at 20 degrees Celsius:
DC resistance = 0.022/Sqr(D) ohms per metre.
RF resistance = Sqrt(F)/12/D ohms per metre, provided SD < D/5
Skin depth, SD = 0.066/Sqrt(F) millimetres.
Resistance temperature coeff = +0.4 percent per degree C.
Where F is frequency in MHz and D is diameter in mm.
The science of electronic coils is more complicated than my mind.
But no one specifically addressed the question of whether spinning in continuous circles has a particular influence on radio signals as they twirl through a coil.
I would expect that if you spin in circles you would get dizzy however I don't think that affects radio signals in coils.
A 3-meter, base-fed, 1/2" OD monopole needs a coil with about 287 µH of inductance to resonate it at 1650 kHz.
Below is a paste-up from the coil calculator at http://hamwaves.com/antennas/inductance.html and another site, showing the specs of an appropriate coil.
Its effective series resistance at 1650 kHz is about 5.5 ohms, while its DC resistance is only 0.63 ohms.
