Small tuned loop: Difference between revisions
(New page: A ''small transmitting loop'' or ''magnetic antenna'' is defined as a loop antenna of less than one-tenth to one-quarter of a wavelength in circumference. These are normally constructe...) |
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A ''small transmitting loop'' or ''magnetic antenna'' is defined as a [[loop antenna]] of less than one-tenth to one-quarter of a wavelength in circumference. These are normally constructed as a single turn of heavy-gauge wire (AWG #10 or larger), copper pipe or tubing and are tuned to one specific frequency at a time using a series variable [[capacitor]]. | A '''small transmitting loop''', '''small tuned loop''' or '''magnetic antenna''' is defined as a [[loop antenna]] of less than one-tenth to one-quarter of a wavelength in circumference. These are normally constructed as a single turn of heavy-gauge wire (AWG #10 or larger), copper pipe or tubing and are tuned to one specific frequency at a time using a series variable [[capacitor]]. | ||
As these antennas can be constructed in relatively little space, often with diameter of several feet or less, they are used in [[HF]] applications where no suitable location is available for a full-sized antenna such as the [[beam antenna]]. In some cases, these have been constructed in attics or used for [[portable antenna|portable]] operation. | As these antennas can be constructed in relatively little space, often with diameter of several feet or less, they are used in [[HF]] applications where no suitable location is available for a full-sized antenna such as the [[beam antenna]]. In some cases, these have been constructed in attics or used for [[portable antenna|portable]] operation. | ||
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== Tuning == | == Tuning == | ||
Due to the need to maintain low impedance, capacitors used must have effective series resistance of near-zero, despite their handling of large amounts of current and RF voltages which (in any larger than a QRP operation) often reach 4kV or more. As the circuit is largely built from reactive elements, voltages and currents at individual points far exceed those which normally would be associated with the real power being transmitted. The capacitors must also be variable over a wide range to allow the loop to be manually tuned to one of various frequencies and bands in the HF spectrum. | |||
Vacuum variable capacitors are very well suited to this task, but may be costly even from industrial-surplus sources. A possible alternative is an air-variable capacitor in which all connections to individual plates are welded and there are no moving connections to the rotor. | |||
== Feedpoint matching == | == Feedpoint matching == | ||
Revision as of 17:48, 10 April 2009
A small transmitting loop, small tuned loop or magnetic antenna is defined as a loop antenna of less than one-tenth to one-quarter of a wavelength in circumference. These are normally constructed as a single turn of heavy-gauge wire (AWG #10 or larger), copper pipe or tubing and are tuned to one specific frequency at a time using a series variable capacitor.
As these antennas can be constructed in relatively little space, often with diameter of several feet or less, they are used in HF applications where no suitable location is available for a full-sized antenna such as the beam antenna. In some cases, these have been constructed in attics or used for portable operation.
Theory of operation
A wire loop inherently acts as an inductor with a radiation resistance of a fraction of an ohm. In order to keep other resistance and impedance from becoming predominant, the resistance must be kept to a minimum in transmitting antennas by the use of heavy wire or tubing and the inductance of the loop must be compensated using a large series capacitor.
This yields an antenna which operates as a resonant LC circuit at one specific frequency with an impedance very close to zero; at other frequencies, impedance rises sharply and the antenna is unresponsive.
Tuning
Due to the need to maintain low impedance, capacitors used must have effective series resistance of near-zero, despite their handling of large amounts of current and RF voltages which (in any larger than a QRP operation) often reach 4kV or more. As the circuit is largely built from reactive elements, voltages and currents at individual points far exceed those which normally would be associated with the real power being transmitted. The capacitors must also be variable over a wide range to allow the loop to be manually tuned to one of various frequencies and bands in the HF spectrum.
Vacuum variable capacitors are very well suited to this task, but may be costly even from industrial-surplus sources. A possible alternative is an air-variable capacitor in which all connections to individual plates are welded and there are no moving connections to the rotor.
Feedpoint matching
External links
- A Small Transmitting Loop Antenna for 14MHz and 21MHz, Lloyd Butler VK5BR, Amateur Radio, November 1991.
| Antennas | |
| Design | Beam * Dipole * Dish or Parabola * DDRR * Log Periodic (LPDA) * Loop * Mobile and portable * Omnidirectional * Panel * Quad and Quagi * Screwdriver * Small tuned loop * Vertical * Yagi-Uda * Wire and random wire antennas |
| Installation | Antenna Tuners * Capacity hats and loading coils * Cavity filters * Coaxial Cable * Feedlines * Rotators * Towers and Masts * VK2ACY - G5RV coupler |
| Theory | Front-to-back ratio * Impedance matching * SWR * Tower design * Vertical Antenna efficiency * Wire comparison tables |