RF choke "choke balun" questions

[loosecannon]

hi all,

im considering adding an RF choke to my mobile installation.

many times these are referred to as "choke baluns", but i dont want anyone to be confused about what this is for.

it is for common mode currents; not impedance transformation.

even though my current setup is for CB, i am purposely asking these questions in the ham portion of the forum because i believe the hams here have more experience using RF chokes and such.

my questions are:

1. what length of coax will i have to use to form the coil for the choke to work on 11 meters (cb freqs)? (i will be using LMR-240UF coax)

2. is it better to have more turns on a small diameter coil form, or to have less turns on a larger diameter coil form? (i realize it is a small consideration, but i am still curious which one is preferable)

3. will it still work if the choke is 4-5 feet away from (below) the feedpoint?

my install consists of a base loaded antenna (wilson 5000) mounted to the center of the roof, a texas star amplifier (DX400) and a radio (PC78xl)

my goal is to reduce the common mode currents i believe are flowing back down my coax and re-radiating inside the car.

i am open to all input and past experiences regarding RF chokes in mobile environments.

thanks for your time,
LC

 

[hookedon6]

correct they are NOT a balun. they don't correct any balanced to un balanced conditions or transform any impedance mismatch. all they are is a coil with current flowing thru it to isolate the coax braid.

here is a good thread to read: http://www.worldwidedx.com/cb-antennas/30217-proper-coax-choke-mobile.html?highlight=choke+balun

 

[Robb]

Rather than messing with unsightly coils of coax next your antenna, try a snap on ferrite choke. They cost about $3-5 and can be had online. Shop around a bit - you will find them. I bought a couple at a surplus electronics place for use on my base station and my mobile. I don't think I spent more than $6 for both. Of course, if you are going to run a bit of power, you may need a few. They just snap together over the cable - no mess - no fuss. Put them as close to the antenna as you can - at the end of the coax. Done.
Just a thought...

 

[W5LZ]

The size/diameter of that coil isn't critical, depends more on the size/stiffness of the coax used than anything. Length of coax to use? Something like 10 to 15 feet ought'a do it for 27 Mhz. Where to put it? As close to the feed point of the antenna as you can get it. Whatever length of feed line between the antenna and the choke is still going to have common mode current on it, so you want it to be as short as possible/practical.
I can tell you from experience that one or two of those ferrite chokes are not going to be enough. Think in 'hand fulls', sort of, the more the better. If you can find the ones that will just slip over the coax, making up a jumper of them about 3 - 4 feet long is NOT unreasonable. Another option is to make a 'tuned' sleeve from braid. Close to about 6 - 7 feet long for27 Mhz, slip it over the coax near the feed point and ground the other end to the vehicle body. NOT very practical, but it can be made to work.
I think I'll quit before it gets totally ridiculous. The choke thingy isn't a bad idea. May or may not help, but it won't hurt.
- 'Doc

 

[loosecannon]

thanks for the input so far.

being that this antenna is mounted to the center of the roof in my car; whatever ferrite i use will have to fit between the metal and the headliner.

what about winding the coax around a "toroid" (ferrite donut)?
i might be able to squeeze one of these in at the feedpoint.

what mix of ferrite would be best?

what size toroid?

how many turns?

thanks for the advice!!!
LC

 

[BOOTY MONSTER]

good stuff

 

[Robb]

The ferrite snap on choke I'm talking about is about 2 inches long and an inch or so on the outside diameter. I have a Wilson 1000 on my vehicle and have just one ferrite on it. Can't say that it works or it doesn't. But I don't have any effects of RF phenomena in the vehicle. I'm running a radio that is putting out 4-18 watts.

The inside diameter of these snap on ferrites can be either big enough for the .40" Belden 9913Flex coax or the .25" RG8X mini on the Wilson 1000. They come in different sizes where I get them. If I add a few more to each application, it won't cost that much.

 

[DXman]

This place as some kits that will work for you.
Toroid cores for amateur radio

 

[K8PG]

Go to the Amadon Website and choose the correct core-put 2-4 together over the cable
Making a rf coaxial choke -Id say 8-10 turns of mini 8X.I feed my yagis with coaxial
chokes-they can do wonders- it actually is a 1-1 current choke to shield the inner conductor-to the outer shield in a magnetic field.I get better SWR curves and improved
radiation along the plane of the antennas elements(or wire/whip antenna)Their should be
no impedence-missmatch with this type of feeding arrangement.....50 ohms-75ohms-Zl

Paul-K8PG

 

[loosecannon]

thanks for all the advice guys.

i placed an order with Amidon about three days ago for some Mix 31 ferrite beads.

i got 15 of the 1 inch long ones that slip over the coax.

i will post back here with the results after i put them on.

might be a little while though, i currently get one day off a week and this week its all about yard work.

later,
LC

 

[gameover]

thanks for all the advice guys.

i placed an order with Amidon about three days ago for some Mix 31 ferrite beads.

i got 15 of the 1 inch long ones that slip over the coax.

i will post back here with the results after i put them on.

might be a little while though, i currently get one day off a week and this week its all about yard work.

later,
LC

so what happened?

 

[wavrider]

so what happened?

6 year old thread, but the results probably are no cmc on the coax shield

 

[Slowmover]

Our beards have turned grey and hang to the tabletop, awaiting that reply transmission from the Crab Nebula . . .

.

 

[Alexis Mercado]

I wonder what the re-radiation did in his car that he requested advise regarding choking a mobile antenna. I can understand choking a base antenna, but a mobile one ?