Topband: Thanks for the listening and share of a common mode current experience

[Jim Brown]

On Fri, 8 Jun 2007 15:21:06 -0700 (PDT), Eduardo Araujo wrote:

>2  Even if we are at the right point 200 or 300 ohms
>may not be good enough.

The first place to put a choke is at the point where you want 
common mode current to be zero -- where the coax connects to the 
antenna. In general, you want the impedance of that choke to have 
a very large resistive component. I would try to aim for 4,000 
ohms. At your operating frequency, 1.8 MHz, and at the frequency 
of the broadcast station, that requires a lot of turns through a 
#31 or #78 material. 

There are two ways that noise and broadcast signals can get into 
your radio. One is common mode on the feedline, and the choke 
suppresses that. The other is the antenna itself, which puts the 
noise inside the coax (a differential mode signal). A high pass 
filter tuned for about 1.7 MHz will take care of that part of it.  
If you are using a power amplifier, the filter should go between 
your transceiver and the amplifier. 

73,

Jim Brown K9YC