Topband: ADSL line "mute" when TX on 160

BillWB8FPQ at BillWB8FPQ at
Mon Oct 22 12:48:23 EDT 2007

Rune, Paul --
All of the advice below assumes that the shack is as neat and clean as you  
can make it.  Balanced feedline for balanced (symmetrical) antennas and  good 
coax (hardline or good quality, well-shielded flex:  RG-214) for  unbalanced 
(asymmetrical) antennas, good (low-Z at 160) ground.  This means  short, fat 
conductors tied to a driven ground field (size and aftertreatment  dependent on 
local ground conductivity).
The DSL signal on the phone line is differential mode.  You don't  mention 
how the modem is connected to the computer.  Hopefully it is  E10/100, also 
differential, and not some single ended connection.
The RF is probably getting into the modem common mode.  That is, the  same 
polarity on all of the I/O lines.  Importantly, this includes the  computer 
interface and the power (full voltage or wall wart, either AC or  DC).
It is important to treat ALL of the lines going into/out of the modem with  
common mode chokes (all wires in each cable going through the same hole in the  
core, at least one choke per I/O cable (More than one may be necessary.)   
This configuration will have minimal or no effect on the differential mode  
signals, so there is no need to have a sharp cutoff between the modem range  
(<1.2MHz) and 160.
The chokes should be as close as possible to the modem. It is most  important 
to use a very high initial permeability material, such as J or 77 (VHF  stuff 
like 43 is practically useless down here.)  Additionally, the modem  itself 
may not have a well designed PCB and its probably in a plastic case, so  it may 
pick up RF either inductively (magnetic coupling) or capacitively  (electric 
coupling).  At these frequencies, inductive dominates, so  magnetic shielding 
is more effective, therefore I would try a steel enclosure,  grounded, of 
Good luck,
Bill Gilmore  WB8FPQ
Electromagnetic Compatibility Engineer
DaimlerChrysler Corporation (Retired)

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