Sidebar Sidebar
  • You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

RF chokes

4

4600turbo

Active Member
Aug 27, 2006
151
0
26
RF Chokes how do they work?

Where do I install them?

Will it stop the problem I'm having with RF. My radio is messing with a booster sensor on my truck.

Just wondering if a RF Choke or two will help

I got 4600 turbo 12' of coax to a Fatboy amp then 18' to a 10k antenna.

Will putting a choke on the coax near the radio and the one on the coax near the amp help?

Will putting an RF choke cause any problems with TX and RX?
 

4600turbo said:
RF Chokes how do they work?

Where do I install them?

Will it stop the problem I'm having with RF. My radio is messing with a booster sensor on my truck.

Just wondering if a RF Choke or two will help

I got 4600 turbo 12' of coax to a Fatboy amp then 18' to a 10k antenna.

Will putting a choke on the coax near the radio and the one on the coax near the amp help?

Will putting an RF choke cause any problems with TX and RX?
Click to expand...

RF Chokes are nothing more than an inductor. An RF Choke blocks an AC signal while allowing DC to pass.

RF Chokes are typically used on the DC supply of the device affected by noise. Most times an RF Choke is placed in series with the power lead with a capacitor placed across the power lead and ground to create a 2nd order low pass filter which creates clean DC.

However before you start filtering the DC supply, you should first determine whether the RF noise is conducted or radiated and go from there.

Ideally the best way to get rid of any interference is to try and remove the interference at the source. That way you won't end up trying to fix everything it affects.

Most noise is caused by insufficient grounding or by daisy chaining the grounds and creating ground loops. I would take a close look at the installation and make sure the grounds are all connected single point to the engine block. You should also use a scope to see what the DC supply looks like at the sensor.

Chokes added to the feedline are probably not a good idea...
 
So RF chokes are used mainly for current running through the hot wire. Like a filter for alternator noise.

Why do they make them for coax cables then? :?:

Is it to keep RF from leaking out of the cable? :?:

I've seen different sizes of chokes. Some for rg8x and rg8u coax.

Am I using the wrong term. :oops:

Maybe I should use only 100% shielded coax. Maybe this will stop the problem. since I only use 95% shielded coax from the radio to the amp.

My radio has a built-in amp.
 
4600turbo said:
So RF chokes are used mainly for current running through the hot wire. Like a filter for alternator noise.

Why do they make them for coax cables then? :?:

Is it to keep RF from leaking out of the cable? :?:

I've seen different sizes of chokes. Some for rg8x and rg8u coax.

Am I using the wrong term. :oops:

Maybe I should use only 100% shielded coax. Maybe this will stop the problem. since I only use 95% shielded coax from the radio to the amp.

My radio has a built-in amp.
Click to expand...

Are you asking about a common mode coaxial choke? If so, it may mask the symptoms but it doesn't fix the problem.

Common mode currents are currents that are undesired when using an unbalanced antenna system such as a monopole whip. Ideally all of the current is radiated into free space when sent down the center conductor. If the antenna is not installed correctly, then some of the current is returned back to the transmitter via the coax shield or other ground return paths.

So while a common mode coaxial choke may stop some of the current from going back to the transmitter through the coax, the current will still find another path back which means the antenna issues are still not fixed.

Connect the radio through the coax to a dummy load and see if there is still have a problem when keying. Since a dummy load hopefully doesn't radiate, the currents will be predominately common mode.

If the problem exists when keyed, it may be that the coax is radiating or it may be conducted noise through the power leads or a grounding problem.

If the problem disappeared, the issue is with the antenna location, installation, or tuning.

Now connect the dummy load through a very short piece of coax at the radio and key. If the problem goes away now, chances are the coax was radiating which means the coax may not be up to the power being used, there may be a grounding problem, and/or the antenna installation needs to be fixed.

If the problem is still there, then the noise is probably conducted and a DC power filter may work, but chances are there is a ground issue at the transmitter.

Just throwing parts at a problem probably won't fix it. Take your time and analyze the problem at hand and try to isolate it.

Good Luck
 
A coax "choke" only affects RF current flowing on the OUTER surface of the shield. Assuming you're using good quality coaxial cable, one of these "chokes" won't have any measureable effect on the flow of RF current on the inside, where it's supposed to be.

A coax feedline, though it might appear to have only two elements (center conductor and shield), actually has THREE. The center conductor, the inside surface of the shield and the outside surface of the shield...and these last two are independent of one another, even though they're only separated by a few thousandths of an inch. RF is crazy.

With some antennas, RF can travel on the outside surface of the shield, where it's generally not wanted. A "choke" can block this renegade RF and not allow it to get down into the shack area. On the other hand, some antennas rely on RF on the outside for their operation. These are not antennas that I want at my station.
 
Beetle said:
A coax "choke" only affects RF current flowing on the OUTER surface of the shield. Assuming you're using good quality coaxial cable, one of these "chokes" won't have any measurable effect on the flow of RF current on the inside, where it's supposed to be.

A coax feedline, though it might appear to have only two elements (center conductor and shield), actually has THREE. The center conductor, the inside surface of the shield and the outside surface of the shield...and these last two are independent of one another, even though they're only separated by a few thousandths of an inch. RF is crazy.

With some antennas, RF can travel on the outside surface of the shield, where it's generally not wanted. A "choke" can block this renegade RF and not allow it to get down into the shack area. On the other hand, some antennas rely on RF on the outside for their operation. These are not antennas that I want at my station.
Click to expand...

ok this is what I thought I could do with my system.

I'm mobile. the antenna is a 10k predator.
I've got mini 8 coax from radio[with a 2x2290's] to the 4pill amp. From the amp to the antenna i got rg8/u coax.

Do you think that this particular antenna is like the on you mentioned, using the outside surface of the shield?

Where would i install this choke near the antenna or near the radio and the amp? I would like to get rid of any renegade RF.

I'll also check my grounds on my amp.

Thanks
 
Near the antenna feedpoint. You might want to put one (or more) snap-on ferrites, available at most Radio Shacks, right at the feedpoint or as close to it as you can get. If you have to make a coax choke, don't coil the cable to less than a four inch radius (8 inch inside diameter) to avoid problems down the road.

Since "RG-8/U" cable hasn't been listed in MIL-C-17 for decades, my spidey-sense tells me that whatever it is you have isn't really as good as you might think. Lots of fly-by-night manufacturers slap together some junk with a 0.405" outside diameter and call it "RG-8", but it won't begin to meet any testing to MIL-C-17 criteria.

RG-213 would be a good alternative. That's currently listed. Get it from a name-brand source, such as Belden or Brand-Rex.
 
Beetle said:
Near the antenna feedpoint. You might want to put one (or more) snap-on ferrites, available at most Radio Shacks, right at the feedpoint or as close to it as you can get. If you have to make a coax choke, don't coil the cable to less than a four inch radius (8 inch inside diameter) to avoid problems down the road.

Since "RG-8/U" cable hasn't been listed in MIL-C-17 for decades, my spidey-sense tells me that whatever it is you have isn't really as good as you might think. Lots of fly-by-night manufacturers slap together some junk with a 0.405" outside diameter and call it "RG-8", but it won't begin to meet any testing to MIL-C-17 criteria.

RG-213 would be a good alternative. That's currently listed. Get it from a name-brand source, such as Belden or Brand-Rex.
Click to expand...

Thanks I'm going to get the RG-213 coax. and do that from the radio to amp and out to the antenna. Then if I still have the problem I'll go with the snap on ferrite's. :D
 
Lazybones1222 said:
Beetle said:
some antennas rely on RF on the outside for their operation. These are not antennas that I want at my station.
Click to expand...

Some antennas? Which antenna?
Click to expand...

Any antenna that requires a balanced feedline, such as a dipole, though a balun could be used.

Also, I don't buy into the whole current flow on the inside of coax versus the outside of coax. If current is flowing in the shield, it is flowing in the entire shield (inside and outside) since the braid is woven and every wire spends some small distance in the inner part of the shield and then in the outer part of the shield throughout the coax length. So somehow the current would have to be discontinuous and jump from one wire to the next to flow in just the inner or outer part of the shield. Does this seem plausible?

Coax is designed to transfer RF energy from a source to a load. The inner conductor carries the energy and the shield is connected at ground potential to prevent the RF from leaking which means to contain the RF signal from point-to-point while keeping the losses minimal.

If the antenna is not tuned correctly, the feedpoint impedance of the antenna is not near the desired 50 ohm resistive which means the voltage at the base of the antenna is not at a minimum so current will be generated (coupled) into the shield.

I think the question should be why is there common mode currents in one installation and not another?

The fix is not a common mode choke but rather a better installation...
 
The fix is not a common mode choke but rather a better installation...[/quote]


What constitutes a better installation in a mobile situation?

Better grounding of the amp, radio, or antenna, or all the above? I've got everything grounded to the chassis

Is it grounding that causes RF to feed back to the amp and maybe cause RF leakage. I thought it was the SWR.

What about the RF that is radiated from the antenna itself.
Will this be enough to cause the problems I am having? Since there is nothing in its way to stop if for radiating towards the sensor; fiberglass roof.

The cab is metal but it has a fiberglass roof.

For that matter there is nothing to keep it from radiating on me!

That why I wear a foil aluminum hat when I talk on the radio :shock: 8) :LOL:
 
4600turbo said:
What constitutes a better installation in a mobile situation?
Click to expand...

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.
 

dxChat
Help Users
  • No one is chatting at the moment.


      Top Bottom
      Back