RF chokes

[Beetle]

Ken -

I think it was Belden, some years ago, that did some testing on the "inside surface" vs. "outside surface" of the shield, using a continuous copper foil taking the place of a woven shield in some RG-8 that they made specially for this experiment. This was back when I was still in high school, and our electronics teacher explained it by saying that the inner surface of the coax, whether solid or woven, contains the FIELD, rather than just moving electrons. This field exists between the center conductor and the entire inner surface of the shield, not just at discrete points.

I would have no idea of where to find the results of this test (I'm not even completely sure it was Belden). It would tend to explain why ferrites can effectively block current from traveling on the outside but not affect the field on the inside, though.

 

[ken white]

Beetle, I guess it really depends on how we define the current flow inside the coax.

In order for the RF components to not "leak" from within the inside of coax the shield, the shield must be at least several RF skin depths thick and then the shield will contain any magnetic field generated by the current traveling within the center conductor. Since a current generates a magnetic field, and since a magnetic field generates a current, then yes there needs to be two equal and opposite currents, within the dielectric, that exist between the shield and center conductor to ensure RF does not leak outside of the coax.

Since we know current doesn't flow through an ideal capacitor, and since the center conductor/dielectric/shield forms a type of capacitor, then the changing center conductor current generates a magnetic field which then creates localized charge movement between the center conductor and shield (theoretical capacitor plates). This rapidly changing charge which is a function of frequency and electrical wavelength appears localized (limited to a fraction of the coax length) between the shield and center conductor. The charge does not flow lengthwise for the entire coax length since there is theoretically no difference in potential at the coax shield ends - both ends grounded.

If you look at the plates of a capacitor, one plate will gain an excess of charge while the other plate has a reduction of charge and this charge will be equal and opposite as the charge tries to balance. This charge distribution will occur within the localized conductor length since the signal is a function of frequency and wavelength. The movement of charge per second is current, so current does flow, or more correctly cycle, between points of a section of the shield length within a length of coax, but it does not flow throughout the entire length of the shield.

In order for current to flow along the length of the shield, each end of the coax shield would need to be at a different potential. This occurs when the coax is connected to a dummy load and also when the antenna feedpoint impedance is not exactly 50 ohms resistive.

So this is what I mean when I say that current must flow through the entire shield. When current flows though the entire shield, the shield radiates because the coax is normally greater than 1/4 of an electrical wavelength long and the shield will therefore act as an antenna.

So, does my explanation of how RF currents flow within a section of coax make sense?

:?

 

[Beetle]

It seems to, Ken, but it still doesn't explain why ferrites work so well in choking external radiation from the shield without having the least effect on the field/current flow on the inside of the shield.

I should have mentioned in my earlier post that Belden (or whoever it was) determined that feedline radiation from their special 100% copper shielded coax was significantly less than that of an identical length of a production run of RG-8/U, with identical connectors and switched A/B to the same antennas with known degrees of mismatch. In all cases, as I recall the teacher's telling it, adding a 1:1 balun resulted in near-complete elimination of the feedline radiation.

 

[ken white]

It seems to, Ken, but it still doesn't explain why ferrites work so well in choking external radiation from the shield without having the least effect on the field/current flow on the inside of the shield.

Beetle, actually my explanation does, I just need to try and explain it a little differently.

The currents, or magnetic fields, existing between the center conductor and shield inside the coax are shuffled back and forth within a limited distance inside the coax.

Imagine an RF signal traveling on a piece of wire that is physically greater than 1/4 of an electrical wavelength. At different physical points on this single wire (theoretical capacitor plate) there will be varying amplitudes of voltage since this transmission medium is electrically long, just like an antenna.

So if we look at the center conductor as a capacitor plate, there must be varying levels of charge moving between the center conductor and shield with the charge coming from the more negative potential - shield or center conductor. Since the signal is moving, which is what a traveling waves does, then the charge is moving relative to the shield and center conductor. Because the RF waveform is cyclic in nature, a sinusoid, the currents (or moving charges) move from one point to another within the shield and then back again, a standing wave. So while the shield current inside the coax is flowing between the center conductor and shield locally, it is not flowing along the shield and therefore it can be considered a constant current. Since this current is a constant value at any one point, the ferrite bead won't offer any opposition to the current so the signal is not affected.

However, when a current flows through the entire length of the coax shield due to a difference in potential at each end, the ferrite beads which are really RF chokes, stop the coax from radiating because the current varies with time when measured at a single point.

As I stated earlier, common mode chokes do not fix any coax shield current issues that are nearly always generated at the antenna. Ferrite beads or common mode chokes will stop the coax from radiating, but these fixes will not stop the current from going back to the source via another path which is now generating ground noise.

 

[Lazybones1222]

That's just what I was thinking. 8)
:roll:

 

[4600turbo]

What constitutes a better installation in a mobile situation?

That is a good question and there are many factors to consider. First, have you even determined if the noise is radiated or conducted?

If the noise is radiated, maybe a change in location will generate less noise into the sensor and/or its wiring.

Maybe the antenna is not tuned correctly which means the feedpoint impedance is not near the ideal 50 ohms resistive value and very little of the energy is actually being transmitted which is causing the circulating/common mode currents at the base of the antenna. Does the antenna have a nice flat metal area perpendicular to it to use as a ground plane?

Maybe the Amplifier or CB is not grounded to the engine block using a single point ground connection. Daisy chain grounds generate all sorts of noise and can wreak havoc with logic levels. Are the CB and Amplifier hot leads connected directly to the battery, they should be. Are the CB/Amp power or ground leads running parallel to any of the sensitive electronic device wiring? If so, these high current wires could be coupling using magnetic fields during keying, reroute the wires. Etc, etc...

The list of what constitutes a poor installation can be quite long.

Noise problems are rarely fixed by throwing parts at the problem without first trying to understand where the noise is coming from.

I couldn't positive but about 90% sure it's radiated. If it was conducted wouldn't it be from the hot wire or ground. I don't believe it's conducted.

This installation is on a Semi-truck. so there is no ground plane per-say , unless it's ground planing off my trailer. In other words there is no metal piece under the antenna.

The antennas is tuned about as good as i can tune it using the SWR as an indicator. 1.2 : 1.

As far as the grounding no they're not grounded to the engine block. Only to the chassis. I could probably ground both radio and amp to the same ground. No the wire are not running parallel to any sensitive electronics.

The amp and the radio is connected to the battery where the battery cable comes into the fuse box. They are connected to the cable before the cable go into the fuse box.

This problem is actually caused by the radio more than the amp. I say this because it was doing it before I put in the amp to a certain extent. Now it just seems that the sensor does react quicker than it used to with just the radio.

It could be that the sensor is getting old since it's been on my truck for 1405200 miles.

So I will explore all the possibilities and I thank you for such a good read I've learned a lot with this post

 

[ken white]

If you key up the entire system when the antenna is removed and it is connected to a dummy load, do you still have a problem?

If not, the antenna is causing the problem so try relocating the antenna and see if you can reduce the magnetic fields the sensor is coupling with.

If so, then the problem could be conducted or coax radiation.

Test the CB and amplifier with a short section of coax at the amplifiers output.

If the problem is still there, the problem is considered conducted, even though the power leads may be radiating electromagnetic fields and coupling into other circuits. Try running the power and ground leads, twisted togther using tie wraps to hold the twist, directly to the engine compartment. Connect the (+) lead directly to the battery's (+) terminal and connect the (-) lead to the engine block - this connection point will already have the alternator and battery (-) connected to it. The reason for the (-) lead connection at the engine block vice the battery's (-) terminal is because all of the engine sensors and alternator use this a the reference connection. Also, check and make sure the engines computer which is typically connected to the tub, has a good braid wire connection to the engine block as well. (1,405,200 miles!!!! Wow, the sensor could be bad...)

If the problem went away, the coax is radiating and additional grounding at the antenna with braid wire may help, or the coax will need to be re-routed so it doesn't couple into the sensor that is causing the problem.

 

[4600turbo]

If you key up the entire system when the antenna is removed and it is connected to a dummy load, do you still have a problem?

If not, the antenna is causing the problem so try relocating the antenna and see if you can reduce the magnetic fields the sensor is coupling with.

If so, then the problem could be conducted or coax radiation.

Test the CB and amplifier with a short section of coax at the amplifiers output.

If the problem is still there, the problem is considered conducted, even though the power leads may be radiating electromagnetic fields and coupling into other circuits. Try running the power and ground leads, twisted togther using tie wraps to hold the twist, directly to the engine compartment. Connect the (+) lead directly to the battery's (+) terminal and connect the (-) lead to the engine block - this connection point will already have the alternator and battery (-) connected to it. The reason for the (-) lead connection at the engine block vice the battery's (-) terminal is because all of the engine sensors and alternator use this a the reference connection. Also, check and make sure the engines computer which is typically connected to the tub, has a good braid wire connection to the engine block as well. (1,405,200 miles!!!! Wow, the sensor could be bad...)

If the problem went away, the coax is radiating and additional grounding at the antenna with braid wire may help, or the coax will need to be re-routed so it doesn't couple into the sensor that is causing the problem.

Thanks, 8)