High Power Magnetic Loop Antenna Theory and Construction By Ted Robinson, K1QAR
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- Опубліковано 15 жов 2024
- Twenty years' experience has taught me how to make high power magnetic loop antennas that use self supporting copper tubing with a bolt-on vacuum variable capacitor , that is lightweight, yet takes the power. A manual stepper motor control makes tuning the antenna easy. A Russian style doorknob capacitor matching extends frequency range of my magnetic loop antenna.
This presentation was made at the QSO Today Virtual Ham Expo. For more information go to:
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Interested in 80/40 bands mostly. Building mine with 6" semi-Rigid aluminum Duct. They have 3", 4", 6" and 8" diameters. and yes Russian Federation 15KV Vac Variable. Going initially with 24 ft. circumference. The Vac Variable can easily fit inside the duct so makes a very nice package. Where are the inefficiencies in the Feed? What about a few turns around the duct and use Open Wire to a balanced tuner? How high of power can I do with 15KV?
Any reference for capacitive feeding? It's the first time I encounter it.
I'm building a loop using a hula hoop, don't laugh, covering with silver plated 1 inch copper braid which has been gently expanded to fit over a 1 inch loop at 30" diameter for 21 MHz. Comet 5-125 pF vacuum cap rated for 4.8 KV RMS for 100 watts CW power, will tune 21 for 85% efficiency, 14 for ~50%. Silver plate won't help much unless 20 microns thick and most silver plate to wire is only 1 micron, but it looks nice. The manufacturer, Continental Cordage, refuses to answer my emails asking for the plate thickness spec of their 178S braid, so might have more than I think.
Will you report back on how using the braid works on your hula hoop?
It's 66Pacific and their calculator although good, keeps telling me longer circumferences which will work are not ideal. What? i.e. 15 feet circumference at 21 MHz which is a bit more than 3/10 of a wavelength is not ideal? Appears they believe the loop must be less than 1/4 wavelength, not fully true, up to 7/10 of a wavelength will still preserve the nulls with the antenna acting like a MLA. Also, below, you will find a loss resistance, what is that based off of and does it include skin effect? The value will be different depending if copper or aluminum conductor.
Input Values:
Length of conductor: 15 feet
Diameter of conductor: 1 inches
Frequency: 21 MHz
Transmitter power: 100 watts
Antenna efficiency: 97% (-0.2 dB below 100%)
Antenna bandwidth: 166 kHz
Tuning Capacitance: 15 pF
Capacitor voltage: 2,502 volts RMS
Resonant circulating current: 5.05 A
Radiation resistance: 1.89 ohms
Loss Resistance: 0.068 ohms
Inductance: 3.76 microhenrys
Inductive Reactance: 496 ohms
Quality Factor (Q): 126
Distributed capacity: 12 pF
Antenna "circumference": 15 feet
Loop antenna Side length: 1.88 feet
Antenna diameter: 4.5 feet
Comments:
((Their red highlighted warning which is wrong)): The specified conductor length of 15 feet is not ideal.
Conductor length should be between 5.68 and 11.4 feet at the specified frequency of 21 MHz.
For highest efficiency, the conductor length for a small transmitting loop antenna should be greater than 1/8 wavelength (greater than about 5.68 feet at the specified frequency of 21 MHz).
((This statement also wrong)): To avoid self-resonance, the conductor length for a small transmitting loop antenna should be less than 1/4 wavelength (less than about 11.4 feet at the specified frequency of 21 MHz).
I was almost unable to follow the talk, unless speeding it up to 1.5x. It must be my fault as I am not a native English speaker: the irregular and frequent pauses affected my comprehension.
We are looking at some new technology for cleaning up audio pauses and filler words. This might help for comprehension by non-native English speakers. Did you look at the transcription in English?