ARCHIVE: Australian Ham Radio Discussion Forum ( AHRDF )

Hi all,
I'm amazed this topic has not been discussed before.

I was looking at the 1m dia two turns loop antenna of 1/2" heliax when I noticed, it resonated at around 15MHz.
The coil had self capacitance with the turns spaced about 50mm. 

I ask, why not bring the spacing closer to bring the resonance down into the 20m band?

This brings me to why I am writing.

The hardest problem with loop antennae is tuning.

High voltage variable capacitors are difficult and having to adjust a single ended capacitor while "hot"
is even more difficult. So here in this discussion I'd like to see innovation in this subject, tuning.

Tuning methods:

Variable capacitance
================ Really not that easy. But with the two turn loop, the "hot" ends are back down near a zero voltage point
and if the conductor spacing adjustment is all that's needed, not too hard.

Variable inductance 
=============== Hardly ever discussed. However, in the Halcyon days, the Variometer was the go,
a smaller coil was placed inside the main coil and it was rotated to either enhance the magnetic field or oppose it.

Perhaps, a section of the loop could be more flexible to aid in tuning? There's been discussion here and the use of braided
coax discouraged. Well, why not use what Spot Welders and RF welders have done for years, copper strip? 

Fixed capacitance
==============   Scrap heliax is readily available and it's breakdown voltage is huge. Also it's dielectric loss is very low,
primarily designed for 100MHz and up, I'm fairly confident 20KV  would not stress 1/2" heliax one bit. (comments here please)

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.