[TowerTalk] Common Mode Choke Locations

[Lux, Jim]

On 1/26/23 6:28 PM, David Gilbert wrote:
>
> I'm not trying to promote choke overkill, but look at it this way. If 
> it is a truly high impedance CMC at the antenna, the shield of the 
> remaining coax run to the shack is essentially a receiving antenna 
> with an open end at the antenna.  If the antenna is a vertical with 
> radials, the coax becomes just another radial.  I don't understand why 
> there wouldn't be additional currents on the coax shield in those 
> cases ... over and above the differential currents the coax is 
> carrying.  If so, depending upon the circumstances, it seems you might 
> want a CMC at the shack end of the coax as well.
>
> I don't use one there myself, but I've often wondered whether or not I 
> should.
>
> 73,
> Dave  AB7E 


I use chokes at various places along the feedline - it keeps the shield 
from being an unexpected parasitic element AND it helps with RFI 
reduction.  Sometimes it might be superfluous - if you're running the 
coax along a rain gutter, not much point in choking the coax when the 
gutter is there as a parasitic conductor.

Consider a case where there's an RFI source midway along the coax, 
putting a choke at where the coax comes inside keeps that RFI from being 
conducted into the room.

A couple of decades ago, I bought something like 100 or 200 2.4" mix-31 
cores (it's a big heavy box in the garage) - So I just put them here, 
there and everywhere on general principle. It's easier to just over 
choke than to go hunting later.

The other place chokes are nice is on power lines both 120V and 12V, and 
ethernet cables. Ethernet is balanced pairs, but there's often common 
mode noise from the box the ethernet is connected to. Reach into the box 
of cores, put a half dozen turns through it, done.