Loop Stick Ferrite Physical Connections

[i_hear_you]

I'm building a ferrite loop stick comprised of two larger "rods" of 12 bundled rods, stacked end to end.  Each bundle of 12 is currently held tightly together with electrical tape on both ends.  I lined them up, then wrapped some more electrical tape over the joint.  There is the slightest of mechanical play between the bundles, but not within the bundles.

As I was checking the loop for proper inductance together with my air variable, I was also rotating the radio + ferrite assembly to test changes in signal.  I noticed some occasional wild oscillations of signal while handling the assembly, and I assume it is due to the imprecise and intermittent physical connections where the bundles meet in the middle, right where the coil currently sits. 

Any input on whether I should:

1) Move the loop to one end where it would then reside on a solid segment of ferrite ,
2) Put some Penetrox A between the bundles,
3) Figure a way to make that center joint mechanically stable.

Regarding these points:

1) I tested sliding the coil along the ferrite.  I don't know the EE theory here, but it seems the gain was highest with the coil in the middle, and I think I read that the inductance changes based on the position.
2) It seems intuitive to use a conductive paste between ferromagnetic materials, but I can't shake the feeling that's not how it works.
3) It will be tough, but not impossible, to get some stabilizing dowels inside the plastic cuff I am using.

[Josh]

Please see here;
https://swling.com/blog/2017/01/a-new-approach-to-fsl-antenna-construction/
https://dxer.ca/articles/92-gary-debock
http://www.thomasn.sverige.net/7_Inch_Affordable_FSL.pdf
https://www.youtube.com/watch?v=WYuikQj7yPA

itd be fun to watch a guy try to explain one of those fsls to airport security

[i_hear_you]

Thanks for the links, Josh.

I was familiar with the FSL from some reading while researching the RGP3 ferrite loopstick.  I opted to go with the RGP3 because it was less expensive and much more compact. 

I've decided to "splint" the the junction of the two bundles with small dowels or tongue depressors and be done with it.  After writing my post, I made the well-documented AM input jack modification to my pl880 (a five-wind loop is added to the end of the internal ferrite antenna and soldered to the unused sleeve connection of the jack, and the ground.) I added three winds to the end of the RGP3, and I tested with this "direct" connection. The mechanical intermittence was not noticeable as I steered the antenna.

I was blown away by the results. It was hotter with this connection than with proximity inductance. I took the opportunity to slide the coil along the ferrite and find the spot with maximum signal pickup, which probably unsurprisingly is about dead center.  I haven't had a chance to A/B yet, but it seems to pull in a better signal than the 36" cross-sized loop I made with the cardboard package bracing for a mirror my wife had purchased.  The tuning is extremely sharp and I may need to find an oversized knob or even spend on a vernier.

[Josh]

Nice it's working out. The thing about the fsls that interested me most is spacing the coil from the cores. Have you tried doing that to see if there's a difference in performance? Most loopsticks have the coils pasted right to the core, when the radio they're housed in is tiny, I can understand why.

[i_hear_you]

spacing the coil from the cores

In fact I do mean to try this before I commit to the final enclosure.  I more thoroughly experimented with coil position along the ferrite core length last night, and the sweet spot seemed as hard to hit as did tuning the cap without a vernier.  Based on a mental running average of the DBU and DBV reported by the pl880 it seemed the highest gain was achieved just off of center.  However, a new variable entered in:  because the coil sleeve doesn't currently fit snugly over the ferrite core, the central axis of the coil can pitch and yaw out of parallel with the central axis of the core. 

If the wire distance from the ferrite isn't noticeably detrimental to performance, I am considering trying to engineer a sliding mechanism for the coil to create a second form of tuning in addition to the usual variable cap.  Think about the pump on a shotgun:  I envision the core tightly packed inside a PVC tube, and another PVC cuff wound with the coil that fits snugly on the outside of the tube and slides along two metal rails, and these will make the appropriate connections to the coil and cap to complete the LC circuit.

It may not be worth all the extra work, but it sounds fun to try and fabricate it.

[Josh]

I think I recall someone saying that the sweet spot's different for each end of the band, and especially if the stick is used for sw as well.

I have an ancient loopstick from a tube rig that has the coil spaced over the entire core, the core being 7 or 8 inches in length or longer. They just wrapped the coil around the core and then coated the entire works in wax to secure it. Been meaning to pair it up with a varicap and some pvc tubing to make a small, desk sized passive ambc sig booster.

[i_hear_you]

I was doing some more loop research and found a good read:

https://pdfs.semanticscholar.org/4736/4a3b4a94109482271f3ff3d249d780acac23.pdf

It's the first time I've encountered a description of how ferrite actually increases signal pick up:

"If an air-core loop is placed in a field, in essence it cuts the lines of flux without disturbing them. On the other hand, when a ferrite (magnetic) core is placed in the field, the nearby field lines are redirected into the loop. This is because the reluctance of the ferrite material is less than that of the surrounding air, so the nearby flux lines tend to flow through the loop rather than passing it by."

It essentially bends more of the M component lines of the RF wave into the loop, wild stuff!

[i_hear_you]

The paper goes on to discuss leakage flux, and says that with a short coil on a long rod, Q and inductance are maximized with the coil centered on the rod.  If the coil is extended to the length of the rod, Q will go even higher but inductance will drop.  This can be fixed by adding turns on the rod.