03-20-2012, 05:00 PM
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 | Extraterrestrial Admin | | Join Date: Apr 2005 Location: High Sierras Near Yosemite Nat Park
Posts: 4,472
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Quote: |
there's no such thing as a "quarter wave coil"
| In the case of that antenna, and the A99/Imax 2000 antennas, yes it is used as part of the matching network.
It is a common misconception that you can unwind a ( for example) a coil in a center loaded antenna and the length will be the "missing " length of a 1/4 wave or 5/8 wave antenna.
This is From Tom`s (W8JI) Web page that can be found @ Mobile antennas, short verticals, loading coil loss,and loading coil current Quote:
Common myths about inductor behavior:
We often find inexperienced builders of 5/8th wl antennas think the "loading coil" needs to contain 1/8th wavelength of wire in order to make the 5/8th wave antenna a "3/4wl resonant antenna". They think, through wire length alone, the wire creates a low feed impedance by making the antenna three-quarters of a wave long electrically. In other cases claims are made one half-wave of wire wound on a compact form causes a 180-degree current delay, making a compact coil useful for phasing in a collinear array.
The basic flaw is the above ideas do not account for what actually occurs in a coil. The flawed viewpoint is current goes in one end, winds its way around through the physical length of wire in the coil, and after a time delay caused by the copper path length current appears at the other end. There is a physical mechanism that prevents what we might intuitively think happens from actually happening. A coil or loading inductor has magnetic mutual coupling between turns. The physical mechanism is the magnetic field in the coil!
What Really Happens
When current flows in the transmitter-end of the coil, a magnetic field is created. This time-varying magnetic field causes charges in the other turns to instantly move. This effect ripples through the length of the coil at light-speed, just over 186,000 miles per second. As long as the magnetic flux coupling is high, the delay through the coil is the speed of light over the physical length of the coil. In an inductor with good flux coupling from end-to-end, the electrical time delay in current is very close to the physical length of the coil expressed in degrees at the operating frequency. Note that this time delay is NOT the phase relationship between voltage and current, but the delay time of current appearing at each inductor terminal. More on this appears later in this text.
| Tom Has loads of good reading on his Antenna pages.
73
Jeff
__________________ Radio-active 
NASA/SOHO there is no dark side in the moon, really. As a matter of fact it's all dark
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but mess with my family and I become ^can not stand it^ 
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