I have been experimenting with receive antennas for shortwave and amateur radio and want to confirm my understanding of common mode interference and what chokes actually do and do not do. And this question is in a receive only context, not transmit.
Suppose one is dealing with a choke where the coaxial feed line is wrapped around a toroid. And let’s suppose that the toroid mix, the toroid size, and the number of turns of coax on that toroid is appropriate for the frequencies of interest and is of a sufficient degree of impedance.
So the I part of all this I think I understand is if there is common mode on the shield of the coax, this can both act as an antenna screwing up the radiation pattern of the actual antenna and also bring in noise and interference into to the radio so long as that noise and interference is not directly reaching the antenna. So putting a choke at the radio helps with this.
The part I am not sure about is when a choke is used at the antenna feed point and is the part I want to confirm.
Would it be fair to say that if local interference is directly received by the antenna itself, no choke can remove that? And that the reason for this is that if the noise is directly received by the antenna then it is differential in nature and not common mode?
Would it also be fair to say (given the above) that the reason one would use a choke at the antenna feed point is to prevent common mode noise on the outer shield of the coax from going upstream onto the antenna itself and then once there, becoming differential mode and then going back downstream inside the coax to the radio?
Any comments about whether this is right or wrong would be appreciated.
Thanks!
Suppose one is dealing with a choke where the coaxial feed line is wrapped around a toroid. And let’s suppose that the toroid mix, the toroid size, and the number of turns of coax on that toroid is appropriate for the frequencies of interest and is of a sufficient degree of impedance.
So the I part of all this I think I understand is if there is common mode on the shield of the coax, this can both act as an antenna screwing up the radiation pattern of the actual antenna and also bring in noise and interference into to the radio so long as that noise and interference is not directly reaching the antenna. So putting a choke at the radio helps with this.
The part I am not sure about is when a choke is used at the antenna feed point and is the part I want to confirm.
Would it be fair to say that if local interference is directly received by the antenna itself, no choke can remove that? And that the reason for this is that if the noise is directly received by the antenna then it is differential in nature and not common mode?
Would it also be fair to say (given the above) that the reason one would use a choke at the antenna feed point is to prevent common mode noise on the outer shield of the coax from going upstream onto the antenna itself and then once there, becoming differential mode and then going back downstream inside the coax to the radio?
Any comments about whether this is right or wrong would be appreciated.
Thanks!