[TowerTalk] Common-mode current on feedline

[Chuck Counselman]

At 3:20 PM -0400 6/26/03, N2TK, Tony wrote:
>Question in regard to Common-mode chokes.
>I use Force 12 B-1 baluns on 10-40M. On 80 and 160M I use 50 of the small
>beads on RG141.  All the Buryflex and hardline is tied together and 
>buried a few inches in sand for about 125'. It all ends at Ice 303 
>lightning arrestors (302 arrestor for 2M) on a grounded aluminum 
>panel in my basement....Should I be using Common-mode chokes 
>anywhere? What benefit would they buy me? Where would I put them?


The Force 12 baluns and the 50 small beads _are_ common-mode chokes. 
I don't know how much choking impedance they provide; but, in 
conjunction with the capacitance to ground of your 125 feet of buried 
coax, plus the ground-connection at your entry panel, they're 
probably all you need.

That's my gut feeling and it's as much of an answer as I can give 
without knowing the impedances of your Force 12 baluns and 50 small 
beads.  With this impedance info and all the relevant geometry and 
dimensions -- of your antennas, tower(s), etc., etc. -- I could do a 
NEC-4 simulation; but _by_far_ the best way to answer your question 
would be to _measure_ the currents on your lines with a 
transformer-type current probe.

Unless you're experiencing trouble with RFI from your TX to stuff in 
your house, or from stuff in your house to your ham RX, I suggest 
relaxing and forgetting about adding more common-mode chokes.

RFI from stuff in your house to your ham RX includes buzz from 
AC-power-line-connected devices such as solid-state-electronic lamp 
dimmers and switches, compact-fluorescent lamp ballasts, switching 
power-supplies and battery-chargers in all sorts of appliances (the 
worst one in my house is in the base of my rechargeable electric 
toothbrush!), televisions and CRT computer monitors, security/alarm 
systems, etc.

If you _do_ have RFI trouble, then the first thing to try is a 
common-mode choke on the AC-power cable (including the protective 
ground wire!) feeding your hamshack.  If there is more than one such 
cable, put a common-mode choke on every one.

Also put a common-mode choke on every other cable (e.g., telephone, 
Ethernet, ADSL, or TV-cable-modem) that connects your hamshack with 
the rest of your house and/or the world beyond.

73 -Chuck, W1HIS