See design below for more details. (large image)
What is it? - Applications - Design & variations - Other variable inductors
* I am being called a terrorist,
please help! *
| What is it? Imagine if
you could twist a telescopic antenna into a coil. That is the basic
idea. Eliminate unwanted coil turns by rotating them into each other.
It is a high Q variable inductor.
Other variable inductor designs have problems with Q, or efficiency, over part of their range. Using only part of a larger coil causes loss in the unused section of coil. Screwdriver antenna type coils have proximity losses because of the tube that "swallows" the coil. The Telescopic Coil swallows it's unused sections into it's self so it keeps looking like a higher efficiency coil! |
|
Here is a 3 section Telescopic Coil shown adjusted at appx. 1.2, 1.5, 2, and 3 turns:
See design below for more details. (large
image)
Would it be better to call this a "Telescopic Variable Inductor",
"Telescopic Inductor", or something similar?
I wanted a short antenna for my vehicle that would tune to all the HF bands, 1.8 Mhz up to 30 Mhz. The efficiency of a high Q coil design is considered necessary to prevent power loss. Looking at the internet and antenna handbooks it appears that the best performance is with a properly designed single frequency coil. The telescopic coil gives you an equivalent of a single frequency coil that can be made shorter with less turns, or longer with more turns.
Another application would be for a multi-frequency compact yagi beam
antenna. High Q is necessary as the radiation resistance gets worse
with compact or close spaced designs.
The prototype Telescopic Coil in the above photo was made from #10 wire, 1/4 and 3/8 inch copper a/c tubing. The 1/4 inch tube slides into and out of the 3/8 inch tubing. The wire slides inside the 1/4 inch tubing. Contacts are placed at the end of the larger tubing to connect to the inner conductor.
This prototype is far from ideal. I could only bend the metal by hand well enough for about one turn of each. A practical coil could be made with more sections of thinner tubing, set up by machine for precision. Tin or a better coating should also be applied. A holder and other mechanical aspects of adjusting the coil also need to be considered.
Wire only Telescopic Coil variation: A variation on the Telescopic
Coil idea would be to "simulate" the tubing idea with wire or other conductor
arrangements. For example, a wire coil with a contact at the end
could be pushed inside a larger coil with a contact at the start.
The contact on each coil would touch the other coil at the overlap point.
This would cause the overlap regions of each coil to look like inductors
in parallel, the non overlapped parts of the coils in series with the paralleled
overlap regions.
Inductors in series gives you a total of their inductance, in parallel they have less inductance. See the following diagram:
Telescopic Coil #2: At (a.) is the schematic symbol for
a Telescopic Coil made of wire only, the movable inner coil shown on the
bottom. At (b.) is a diagram with a cutout showing the contacts from
one coil to the other.
Effects when adjusting wire version: 1. When fully extended it appears to be one long series inductor. 2. Part way it appears to be inductors in series, then in parallel, then series. 3. Fully collapsed it appears to be two parallel inductors.
Both the tubing and wire versions "telescope". The end effect is to reach the same goal of a better Q for a variable inductor. The tubing version is simpler to understand and predict because the unused tubing is shielded by the larger sections that it slides into. The wire version may have undesirable effects when collapsed due to each parallel coil having a self resonate frequency. This is why I did not mention using 3 or more levels of the Wire Telescopic Coil - you quickly reach a point where self resonance of individual sections could actually make the Q worse.
I have not built a wire version. My claim is based on theory. The coils would need holders and mechanics that allow the contact to be made as diagrammed. The inner coil would be easy to make with wire wrapped around a tube. The outer coil could be a tube with a slot cut in it for making contact with wire wrapped on the outside. Or, you could use a tube as a "mold" to hold the wire then apply something like epoxy to the outside, then cut away the tube holding the wire.
(E-mail me if you can provide a better way to describe Telescopic Coil
no.2.)
“Variometer coil”: I liked this design because it required no contacts! It consists of a smaller coil that is varied inside a larger coil. The inductance of the coil's add when they are aligned in the same direction or subtract when in opposite directions. This also results in a varying Q. The Q is best when the coils are aligned in the same direction, but gets worse when the coils are aligned in opposite directions. It may be more practical to use a small variometer coil for fine tuning on a larger inductor coil. See variometer examples on Google Images.
Traditional “variable tapping” coil: This is the old standard. It is recommended that unused turns be shorted in order to prevent transformer effects and arcing. The unused turns absorb power so that they lower the Q compared to a coil that is the necessary length. So in this type of coil the Q is usually best when all the turns are being used.
Screwdriver antenna style coil: The advantage is that the unused part of the coil gets hidden in the tube that swallows it. However the larger amount of metal in the tube near the turns of the coil tends to lower the Q. I discovered this type of coil discussed in US Patent 3281748, dated Sep 11, 1964. See it on Google Patents.
The screwdriver antenna coil could be combined with the Wire Only Telescopic Coil to make a better antenna, perhaps.
Ferrite and other materials that influence the magnetic field:
These are usually only good over a narrow frequency range and at lower
power levels.
My final vehicle antenna may be a variation or combination of the Telescopic Coil(s), screwdriver coil, or other variable inductors. Using a telescopic whip above the coil is already in my plans. This limits me to about 100 watts, but that is all my mobile rig puts out anyway.
I am significantly paralyzed from the waist down, so I am also designing an antenna that can be put up and down from within the vehicle.
Michael Lake, ham callsign KD8CIK, Toledo, Ohio, USA.
See the Main page of HamDomain.com
- or - About me on KD8CIK.com
© Copyright 2006 - Michael John Lake - Make copies for personal use only.
* Other patents with screwdriver type coil: 1911980 on 3-31-1932,