12-07-2021, 09:41 PM
Interesting thought. I haven't played with loops at HF but I will put a few numbers into the play... Numbers often make it easier to see where it is going.
You suggest a two-turn loop 1000mm dia spaced about (eg) 100mm end-end { by the time you take the heliax physical diameter into account } (ie length) resonates at 15MHz.
The effective L should be about 7.15uH, and should have an effective composite inter-turns C across the L of about 16pF.
To make it work at 14MHz, increasing the diameter to 1100mm (still 2 turns) changes the L to about 8.0uH and the resonant frequency drops to about 50KHz above the lower 20M band edge.
Varying just the turn spacing (further apart) such that it becomes 15pF (in lieu of 16pF) {while also slightly reducing the L value} moves it back up to 14.5 MHz. (ie Variable capacitance plus var inductance )
So the mechanics required are to hold one end solidly and use a worm-drive to expand or contract the two-turn 'coil' length to the other not-quite-so-solid other end. Driving the worm with a servo motor would provide fine frequency adjustment - just like with the screwdriver-style antennas. To test the idea, simply use a length of 6mm threaded rod driven by a battery drill in lieu of a more complex worm assembly plus some threaded insulating/plastic hardware.
No extra variable or fixed tuning C's required so no breakdown voltage issues, just the heliax. It is then a matter of how to 'drive' it efficiently.
73 Doug
Next day edit : a two turn loop at 1200mm dia gives 8.8uH L value, f = 13.8MHz with 15pF. XL= 770 ohms
Expanding that from 100mm to 125mm length : f = 14.075. XL=755 ohms.
Expanding that from 100mm to 150mm length : f = 14.330. XL=740 ohms.
You suggest a two-turn loop 1000mm dia spaced about (eg) 100mm end-end { by the time you take the heliax physical diameter into account } (ie length) resonates at 15MHz.
The effective L should be about 7.15uH, and should have an effective composite inter-turns C across the L of about 16pF.
To make it work at 14MHz, increasing the diameter to 1100mm (still 2 turns) changes the L to about 8.0uH and the resonant frequency drops to about 50KHz above the lower 20M band edge.
Varying just the turn spacing (further apart) such that it becomes 15pF (in lieu of 16pF) {while also slightly reducing the L value} moves it back up to 14.5 MHz. (ie Variable capacitance plus var inductance )
So the mechanics required are to hold one end solidly and use a worm-drive to expand or contract the two-turn 'coil' length to the other not-quite-so-solid other end. Driving the worm with a servo motor would provide fine frequency adjustment - just like with the screwdriver-style antennas. To test the idea, simply use a length of 6mm threaded rod driven by a battery drill in lieu of a more complex worm assembly plus some threaded insulating/plastic hardware.
No extra variable or fixed tuning C's required so no breakdown voltage issues, just the heliax. It is then a matter of how to 'drive' it efficiently.
73 Doug
Next day edit : a two turn loop at 1200mm dia gives 8.8uH L value, f = 13.8MHz with 15pF. XL= 770 ohms
Expanding that from 100mm to 125mm length : f = 14.075. XL=755 ohms.
Expanding that from 100mm to 150mm length : f = 14.330. XL=740 ohms.
Doug VK4ADC @ QG62LG51
http://www.vk4adc.com
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If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.
http://www.vk4adc.com
This Forum is only going to be as interesting as the posts it contains.
If you have a comment or question, post it as it may trigger or answer the query in someone else's mind.