Re: [IRCA] Cable connectors

[Rick Kunath <k9ao@xxxxxxxxxxx>]

On Monday, September 06, 2010 02:36:01 pm Chuck Hutton wrote:
> I liked your explanation except for the part below. Considering the choke
> impedance to be part of a voltage divider along with the ground
> resistance, you surely need a choke with at least several thousand Ohms of
> impedance. With a poor ground connection (sandy soil), you can need even
> more. I won't bore everyone with the explanation.
> 
> Bottom line: I don't think you can ever get even close to that with the
> over-the-cable chokes. I can recommend only the chokes made with mini-coax
> and high permeability (at least 2000) material.

Ground isn't a part of the issue with common-mode currents on the coaxial 
cable's outer shield surface. This isn't a matching transformer, or other 
circuit that may have a connection to the local ground. The choke impedance 
places itself in series with the currents flowing on the outer surface of the 
coaxial cable's shield surface. There is no ground connection.

Using #31 core material, if you stacked maybe 5 or 6 toroid cores, and 
assuming that you wound the feedline through the cores so as to minimize 
capacitance coupling between turns, i.e. very large loops not wound tight on 
the cores surface and separating the turns radially from each other, you can 
get in the neighborhood of 3k to 4k of series impedance to common mode 
currents on the coax shield at 1 MHz with 7 or 8 turns or so. That'll drop as 
you approach the bottom of the mediumwave band and be worse at longwave. I 
haven't played with other core materials that might work better there. But you 
do have good impedance values up to 10 MHz or so. So it's helpful on the 
tropical bands too. If you need better high frequency performance above 10 
MHz, adding more cores and dropping the number of turns is the way to go. But 
the low frequency performance suffers then. Probably 7 cores and maybe 3 turns 
might be a good starting point for that. The impedance of a choke core, i.e. 
the core and one turn, is multiplied by the square of the turns ratio. If you 
need more impedance, wind more turns. But you have to balance that against 
loss at the higher end of the frequency range because of the various capacity 
coupling effects on the choke design, most of which are dependent on the 
winding style  and cable used too.

If you have an impedance bridge testing the various configurations and surplus 
cores and testing stacked cores and various winding ideas and wiring is worth 
doing.

And don't forget that you can series up these chokes along the ends of the 
cable, and the impedances will add. (series inductive and capacitive 
reactances, because they are of opposite sign cancel each other), but if you 
design with this in mind you can still have good wideband performance. For 
example to get the performance you want you might need one choke to deal with 
longwave and another for mediumwave through the tropical bands. Just wind 2 
separate chokes designed for the frequencies you need and separate them at 
each end of the cable a little. You'd have 2 chokes at each end of the 
feedline. 

Same goes for tubular cores. These can be series placed to increase the 
impedance. Rarely is one core going to be enough at mediumwave. But again core 
material is important here. If you are using #31 material, you might see 
around 1k of series impedance at 1 MHz from 40 1/2-inch long tight fitting 
beads. Chuck makes an excellent point, it takes a lot more ferrite material in 
tubular form to get usable impedance than it does when winding turns through a 
toroidal ferrite core.

The good thing is that if you are testing the chokes with the feedline 
connected to a shielded dummy load, you'll know with testing how things are 
going. Your station receiver will be plenty of tool to tell you how it's 
going. Having an impedance bridge is fun, but in the end, it's how it works in 
place that matters. And a little cutting and trying goes a long way here. 
Sweep the receiver across the range of frequencies of interest when signals 
should be there and see what you do or don't hear leaking in.

One other point is that you can place turns of Ethernet cable through toroid 
cores too, creating chokes to reduce the leakage of crud from an Ethernet 
device like a cable modem or DSL router, an Ethernet switch, or a wireless 
access point. I actually think this works better than using shielded CAT 
cable, as the enclosures and connectors (RJ-45) are not designed to be RF 
tight anyway. And remember the power cords of these devices too. Anything 
leaving the device can act as a radiating antenna. But again, you need to have 
enough impedance in the choke you are placing over the cable at the frequency 
you are trying to suppress, to make the thing work.

The best thing is just to start somewhere and see what happens. Experiment 
with several configurations and work your way to the desired level of 
suppression that you need.

Rick Kunath









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