Loading coil
| Loading coil | |
|---|---|
| What it is | An inductor that makes a physically short antenna resonate |
| Why it is needed | A short antenna's feed point is capacitive; the coil cancels that reactance |
| Positions | Base, center or top loading |
| Key figure | Coil Q: the higher the Q, the lower the loss |
| Typical uses | |
| Mobile | HF whips, Hamstick-style antennas, screwdriver antennas |
| Fixed stations | Shortened verticals and dipoles for 80 and 160 m |
| Companion | Capacitance hat (top hat) |
| Drawbacks | Narrow bandwidth and coil loss |
A loading coil is an inductor placed in an antenna to make it resonate on a frequency where it is too short to resonate by itself. It "electrically lengthens" the antenna. Loading coils make it possible to operate the lower HF bands with antennas a fraction of full size: nearly every HF antenna on a car, many portable antennas, and shortened verticals and dipoles for the 80- and 160-meter bands use one. The price is narrower bandwidth and some lost power.
How it works
An antenna shorter than its resonant length, such as a quarter wave for a vertical or a half wave for a dipole, has a feed-point impedance that is partly capacitive reactance, and the shorter the antenna, the larger that reactance. A coil's inductive reactance is the opposite kind, so a coil of the right size cancels it, leaving the antenna resonant with a mostly resistive feed point that can be matched to coax.
The coil does not make the antenna radiate like a full-size one. A short antenna has a low radiation resistance, the part of its feed-point resistance that represents power actually radiated. An 8-foot whip on 40 meters, for example, has a radiation resistance of only a few ohms or less. Anything else in the circuit with resistance, including the coil, the ground system and the connections, competes with it for the transmitter's power.
Base, center and top loading
Where the coil sits makes a difference.
- Base loading puts the coil at the bottom, at the feed point. It is the easiest to build, adjust and weatherproof, but the current falls off steadily from the base to the tip, so the radiation resistance stays low.
- Center loading puts the coil partway up. The section below the coil carries nearly full current, which raises the radiation resistance and improves efficiency. A coil higher up needs more inductance, and so has more loss of its own, so the best position is a compromise, often somewhere near the middle.
- Top loading keeps the current high along the whole radiator. It is usually done with a capacitance hat, a set of spokes or a wire ring at or near the top, rather than with a coil, since a coil right at the tip would have to be very large. A hat above a center coil lets the coil be smaller and is one of the most effective arrangements for a short antenna.
Q and efficiency
A coil's quality is described by its Q, the ratio of its reactance to its own loss resistance. A coil with 1,000 ohms of reactance and a Q of 200 has about 5 ohms of loss resistance. Efficiency is the radiation resistance divided by the total resistance. If an antenna's radiation resistance is 2 ohms, and the coil and ground together add 10 ohms of loss, only 2 parts in 12, about 17 percent, of the power is radiated.
High-Q coils are wound from heavy wire or tubing with the turns spaced apart, on low-loss forms, and kept away from nearby metal. Water and dirt on a coil lower its Q.
A high-Q coil on a short antenna also makes it sharply tuned. On a short mobile whip for 40 or 75 meters, the range between 2:1 SWR points may be only a few tens of kilohertz. An antenna of this kind that shows a broad, flat SWR curve is often a lossy one, since loss resistance broadens the response.
Hamsticks and other loaded antennas
Hamstick-style antennas are monoband HF mobile whips about 7 feet long, named after one manufacturer's trade name and now made by several companies. Their loading is spread along most of the length as a helical winding on a fiberglass rod, with a short adjustable steel whip at the top for tuning. They are inexpensive and light enough for a magnetic mount, and two of them on a center mount make a compact dipole for portable use. Their efficiency on the lower bands is low, as with any short antenna.
Screwdriver antennas use a large coil in their lower section, with a motor that moves a contact along it. The operator retunes from inside the vehicle and covers several bands with one antenna. Fixed-station antennas use loading coils in shortened 160- and 80-meter verticals and dipoles, often combined with capacitance hats, and the same principle appears in the traps of a trap dipole, which act as loading coils on the lower band.