12-07-2021, 09:04 PM
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)
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)