WHERE TO USE RF-CHOKES
We often hear the expression, "Less is More"; In this case, "MORE is MORE".
By now most people understand the need for using a good Choke-Balun (Current Balun) at the feedpoint of their antenna. If you still don't understand why, please watch this short video:
It is also necessary to use an RF-Choke at the house (outside of the house, inside of the house, or both). Personally, I use one just outside of the house, and again at the first place the coax touches the station - in my case, the antenna matchbox.
Here is what I do, and what I recommend:
ANT+Choke ¦ -Coax-Coax-Coax*- ¦ Choke ¦ House ¦ Coax ¦ Choke ¦ Station
*If you have a very long run of coax between the antenna and the house, it is advisable to break it up with RF-Chokes every 50 to 75 feet - similar to how we break up [steel] guy lines for metal towers with insulators.
You are probably wondering why I use so many chokes.
The simple answer is, don't ask, just do it.
Or.... read below.
If you are not happy with the simple recommendation above, then I suggest you read this quote from an email sent by Jim Brown (K9YC), to the Ten-Tec group on Contesting.com:
One fact that is being missed in this discussion is that the transmission line as a common mode element of the antenna system, and, like any antenna, current varies along the length of every element based on the boundary conditions.
The most common "boundary conditions" are the open circuits at the ends of conductors that force the current to near zero (limited by capacitance at the ends to free space and the antenna's surroundings).
When we add a common mode choke with a high choking Z, we force current to near zero, creating another boundary condition AT THE POINT WHERE THE CHOKE IS PLACED.
A choke at the antenna feedpoint forces near zero current there, but the feedline may still [be] connected (as common mode element) via the tuner to ground. A choke at the tuner can [create] another near-zero condition.
With a choke at the tuner and no choke at the feedpoint, the feedline is still part of the antenna, and is vulnerable to picking up noise on receive. This can be clearly seen (and studied) by a simple NEC model of the antenna that includes the feedline and the choke(s) using the Load function in NEC.
My tutorial on chokes shows how to determine the parallel RLC equivalent circuit of a choke based on measurements of its impedance vs frequency. NEC computes currents vs length for every element of the antenna, and can display them graphically and as table.
For more information from Jim/K9YC on this topic, please read his excellent Powerpoint presentation on Coaxial Transmitting Chokes.