Looking back at some previous threads, I couldn't decide where to put this particular topic, so I made a new one.
Looking back at some previous threads, I couldn't decide where to put this particular topic, so I made a new one.
My reading this evening leads me to believe that close-wound coils may not be the most efficient answer for conductance. The reasoning is that close-wound coils must have some sort of separation between winds, either an insulation jacket or lacquer coating, to prevent one wind from connecting to the next and so on, but if close-wound, I would think it would create unwanted capacitance.
If true, then you could reduce the capacitive effect by separating the winds further, IOW, space-wound rather than close-wound.
Further study, however, suggests that spacing the winds could produce some unwanted eddies, which could cause some phase-cancellation issues ... unless the winds were quite widely spaced, in which case the coil could get very cumbersome in length, and perhaps out of compliance if seen as part of the overall antenna length.
I'm thinking there might be a compromise solution. A large diameter coil form, like the 12" one I was thinking about using for a cage monopole.
Because of the larger diameter form, it would support larger diameter wire to reduce resistance, plus spacing one wire diameter/width apart would not have the same overall length problem as it would with, say, a 3" dia. coil form, yet reduce capacitive effect in the coil. Further, since there are fewer turns, there may be less chance of eddies causing phase cancellation. I'm thinking O.A. efficiency should be better, and at flea-power Part 15 projects, every little bit of efficiency matters.
With knee-replacement surgery coming up end of next month, I'll be more-or-less out of commission for awhile. Can't drive, be stuck at a Convalescent Center doing therapy and such. Good time to draw up plans and maybe even a little bedside construction project.
But before I invest in it, I'd like to know what the experts around here think about such a project. Will it be a practical experiment or am I just wishing?
Ken,
You are on the right track with this. In addition to winding to winding capacitance there is also an effect created by the magnetic field produced by the current. This effect repels the current from adjacent windings squeezing it away from the surface of the conductor which results in increased resistance to RF. Spacing the windings apart by 1 wire diameter reduces this effect.
You might be interested in this:
http://www.pa2mrx.nl/UK/coil%20homebrewing_uk.htm
It was found by a web search on "basket weave coil". Do the search and you will find many other similar articles which may help you in your research.
Neil
First of all, Ken, we will all
be pulling for you when you go
through this knee operation and
recovery. I think you will feel
much better when it is all done.
The link Neil posted is great. I
have tried to make basket coils.
I haven't had much success, but I
think you have to have the right
materials, and I didn't.
The crystal set crowd is trying to
do exactly what we are doing, except
that they are trying to tune a very
small signal that they are receiving -
and WE are trying to do it for
transmitting.
I struggle with my crystal sets the same
way that I struggle with Part 15 transmitting.
I have been able to hear DX from at least
500 miles away on my crystal sets at night
with a 150 foot antenna wire. But I can
only DX below about 750 kHz. The local stations
swamp the crystal radio. Best DX is Havana, Cuba
on 570 kHz here in Hartford, CT.
Anyway Ken, when you are recovering, read as much
as you can if you are able to do that. Who knows
what you might find.
And let us know how you are doing if you can.
Best Wishes,
Bruce, Dog Radio Studio 2
If the windings are spaced apart, at some point a coil becomes a "helical wind." True?
It seems to my foggy memory that I may have read somewhere that coils need not necessarily be "round," which is of course the most common form, but could be rectangular, triangular or some odd geometry. I have made a triangular coil but so far have not tapped it to the proper resonance. But so far as I know these various shapes have no advantage over plain round coils.
As a question of pure curiosity I wonder if perhaps some advantage would be gained by avoiding using the same diameter throughout the whole coil, but started with a small diameter and making each winding successively larger, like an ice cream cone.
The loading coils used in Part 15 are generally described as being in-line with the vertical element, however there may be an advantage to mounting the coil at a right angle to the stick, based on this quote from "Radio Handbook", page 19.16, by Wm. Orr: "It was found that mounting the loading coil at right angles to the whip greatly improved the Q."
Bruce, Neil, Carl. et al ...
First, thanks for the kind words and encouragement.
Thanks for the article, Neil. One question I have which it doesn't address: What are the advantages/disadvantages of basket and/or honeycomb coils over a circular wind?
What are the advantages/disadvantages of basket and/or honeycomb coils over a circular wind?
Pro: Reduces turn to turn capacitance. Reduces magnetic "squeezing" of the current in the wires. Raises the coil Q. keeps the coil length short compared to spacing wires on a single layer coil.
Con: Complexity
There may be more for each category but this is what comes to mind.
Neil
I also believe mounting a coil at right angles to
the antenna stick will increase the Q. I have no
data to support this, but it just seems to make
sense to me.
When I shut down the MICRO1690/1700 transmitter
system a few years ago, I LOST the loading coil! It
may be just as well, because it was out in the elements
for a long time.
As I am trying to get it back on again, I am going to
have to start from scratch. I also would like to wind
a coil such that the windings are a little bit apart, based
on your above comments. I haven't figured out exactly
what I want to do yet, but I will have some questions
at some point.
There certainly is a lot to think about regarding coils.
Especially from the crystal set crowd and also the
longwave Part 15 experimenters.
Bruce, Dog Radio Studio 2
Actually some of those coils such as the "spiderweb" coil were often found on the back cover of old tube tabletop radios for the AM antenna. Though the "spiderweb" coil is circular, and the ones found in those radios was oval in shape, the windings in each are the same.
The technique is not new, and these types of coils do work incredibly well. It would be interesting to see how one performs in loading up a 3m stick.
RFB