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Why are chokes usually placed at the feedpoint, can't common-mode currents occur below the choke as well?

I understand it's placed at the feedpoint to keep everything balanced by eliminating common-mode currents on the outside of the shield, but i thought they can occur below the choke too (unless i've misunderstood?)

For the purposes of this discussion let's assume the antenna is a dipole and is fed by coax.

Thanks :)

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    $\begingroup$ Hello and welcome to ham.stackexchange.com! $\endgroup$ – rclocher3 Jun 8 at 17:26
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Common mode current occurs due to coupling between the antenna and the coax shield, so if you place the choke at the feedpoint, you break the circuit and most of the common mode current will be blocked. (The circuit is from the antenna, though the coax, and back to the antenna via RF.) You shouldn't get any significant amount of common mode current further down the coax.

However, nothing is perfect, and some common mode current may leak past the choke, especially if it is at a nodal point. If this is the case, placement of a second choke somewhat further down the line (say, 1/8 wavelength velocity factor corrected) may hit a maximum and block more current.

Alternately, if you don't place a balun at the feedpoint, then the coax between the balun and the antenna can become part of the antenna and will radiate. If that length of coax is resonant (say, 1/4 wavelength), then this will have minimal impact on SWR shouldn't cause more common mode current past the balun. It will, however, change your radiation pattern, which could increase or decrease your gain, depending on the direction of interest. This is one way to soften the directionality of a beam where you want to get some omnidirectional radiation, or even add a band to an antenna. (For example, the carolina windom uses a 1/4 wavelength of coax with a choke for 10m.)

Placement of a choke at the transmitter might also help, but it only helps common mode from getting into the transmitter -- it may still be on the coax.

There are two primary reasons why we care about common mode current at all. First, common mode current on the coax in the shack is dangerous -- it means you are radiating in the shack, but also you could get RF burns from touching surfaces carrying that common mode current. Secondly, common mode current sourced from the feedpoint is power that is not going into the antenna, and may be radiating in directions we don't want and causing high swr. Other sources of common mode current should be negligible and inconsequential.

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  • $\begingroup$ A choke won’t break the coupling if the coupling is largely electromagnetic. Even a (nearby) completely disconnected feedline can pick up antenna currents on the shield and thus alter the antenna pattern. $\endgroup$ – hotpaw2 Jun 9 at 11:02
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You are right that common-mode currents can occur elsewhere. A common-mode choke with an ideal infinite impedance essentially "disconnects" the common-mode of the feedline, but common-mode currents can still find their way onto the feedline through mutual coupling through the electromagnetic field.

Fortunately, in most antenna and feedline arrangements this coupling isn't very strong. For example, a dipole is usually symmetrical around the feedline's axis. In this arrangement, there is no electric field along the axis of the feedline, and so there is no coupling. For a monopole, the feedline exits below the ground plane and is thus "hidden" from the antenna fields.

Of course real-world installations are always a little bit away from these ideals, and thus there might be a little coupling between the antenna fields and the feedline common-mode. But in most cases this coupling is negligible.

In cases where it's not negligible, it's certainly possible to install additional chokes on the feedline to break it into segments that won't couple as effectively. This just isn't common practice since it's not a common problem.

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  • $\begingroup$ Good point, something I hadn't considered. However, if you you have common mode current that is induced on the coax separately from the power at the feedpoint, then either it is negligible, or your RF field is so strong that any metal object in the area could carry current, and that's not somewhere I want to be, like the radiation hazard zones immediately surrounding high power commercial broadcast antennas and cell towers. $\endgroup$ – user10489 Jun 9 at 13:14
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    $\begingroup$ @user10489 The coupling can be significant while still being nonhazardous. Consider a Yagi with a transmit power of 200 mW. Certainly nonhazardous at any distance, due to the low power. But the parasitic elements are very significant due to the currents excited on them if the Yagi is any good. Now if you manage to arrange an antenna such that the feedline is parallel to a dipole as is a parasitic element in a Yagi, you're going to have significant common-mode currents, without any particular hazard. $\endgroup$ – Phil Frost - W8II Jun 9 at 14:57
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Dr Chuck Counselman, W1HIS, stresses the advantages to lowering your station's receiving noise floor in his paper on Common Mode Chokes. It's important to note that two high-impedance points created by chokes on a coax shield create a conductor that will be resonant at some frequency. Significant currents can be induced on that conductor by wideband EMI sources like noisy consumer electronics, power lines, solar-panel inverters and industrial machinery as well as by narrowband sources like industrial RF generators, commercial broadcast and ham radio stations. Those currents will (re)radiate and be picked up by your station antenna and receiver, eroding all the time and treasure you invested in building up your station's capabilities.

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  • $\begingroup$ You seem to be implying putting two chokes on a feedline creates a resonant section, and this will somehow make noise worse. But firstly, isn't a feedline with just 1 or even 0 chokes already a resonant section, but with the high-impedance points defined by the ends of the cable? And secondly, why does the noise need to be picked up by the resonant section and (re)radiated to be problematic? Wouldn't the antenna pick up this noise even without the resonant section? Surely a passive section of feedline can't amplify noise, right? $\endgroup$ – Phil Frost - W8II Jun 9 at 14:53
  • $\begingroup$ An isolated passive section of feedline (or any wire in the near field) of the right length for some frequency can act as a director or reflector element of a multi-element antenna. A multi-element antenna will usually have non-isotropic gain in some direction, possibly aimed right at your RF interference, or a null aimed at your signal of interest. That's why balanced antennas with orthogonal feeds are much easier to analyze. $\endgroup$ – hotpaw2 Jun 9 at 16:49
  • $\begingroup$ The implication here is that if you have the unfortunate mutual accident of creating a resonant section for an existing RFI source in exactly the right place your antenna receives in for a frequency both your receiver and antenna filter poorly, you can increase your noise floor. While that may seem like a lot of coincidences that have to all happen at once, when it does, it will suck and you'll have a hard time figuring it out. $\endgroup$ – user10489 Jun 10 at 11:07

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