It takes a ton of beads to get significant choking action on the lowest bands and that gets expensive and heavy.
This depends on the beads used and the amount of 'choking' (Z) needed from the choke? I have used large heavy beads from Amidon (number escapes me right now) and using 6 or more gives the required impedance needed for 80-10M. But you are right .... they are heavy (each bead weighs a few ounces but they're not that expensive).
Gene W5DQ
As a general statement about ferrites, any energy that the ferrite is going to "eliminate" is going to end up as heat in the ferrite material. Think of 1.5 KW through a coax with even 20% of that being eliminated by the ferrite, that is 300 W of heat that would need to be dissipated in a tiny area. Sooner or later you are talking about heat that can actually melt the feedline insulation, somewhere around that area you are going to cross the curie temperature of the ferrite material and change the permeability of the core material (oftentimes this is a permanent change). Once that happens then all bets are off, you do not know what you will end up with. You can have something that initially looks like a success and then after a half-hour of rag chewing the SWR is much worse than anything you had seen before.
That rule about heat applies to any ferrite, even on conventional wound designs for impedance matching.
While some folks luck out and have bags of different cores and materials to put a dozen or more toroids on a feedline most folks may only have 2-3-4 toroids hiding away in their junk box. The ferrites strung along a length of feedline can get expensive and sooner or later you cross the point where it is cheaper to actually wind a balun and stick it in a plastic box with SO connectors on either end.
True if you have SWR that's thru the roof but with SWR close to normal and using the cores as a choke to prevent return currents off the outside of coax, there isn't much heat buildup. While I don't run 1.5Kw, I do run 500W and at that level, 8 of the Amidon FB-77-1024 provided enough current choke to remove all RF from the shack coming back on the outside of the coax driving my 6BTV on the lowbands. Without it I had some, not alot, but some RFI in the shack. I took a piece of 1.5" PVC pipe and ends, drilled holes in the end of the caps and ran the coax thru one end, thru a stack of 8 cores taped together and out the other end and added the PL259 to the feedline. Made a nice compact choke and with it laying on the base of the vertical, it is out of the way and I don't worry about the weight.
Referring to the graph at
https://www.amidoncorp.com/specs/2-15.PDF, using the #77 material curve and a filter conversion factor of 3.7x, I calculated that for 80M, 8 of the FB-77-1024 cores provide a Z of appx 500 ohms. On 40M, Z is around 660 ohms, which is where I had the worse RFI and coincidently, the most interest of operating so it all worked out for the best. The curve for Z in this choke rises to a peak around 890 ohms at 10M. The cost for this choke was around $20 for the 8 cores (when I bought them) and the rest from home repair junk box. I am sure this setup would work at full power providing the SWR at the choke poinit wasn't above a 2:1 or so. It works for my needs and that is what is the bottom line here.
Gene W5DQ
