Few quetions about a choke....

[Oatmeal]

Ok, Ive been reading on a few other forums about the length of coax that's used for making a choke.....some guys say to use 18-21ft for it, some says use less, who is right on what length of coax needs to be used ?...

I was told, if your going to make a choke, and if you didn't make it with 18-21ft of coax, there wasn't any use making one, any truth to this ?...

Seems like for the 11 meter band the graph said to use 5ft of coax for the choke, to be used on 11 meters....

2nd question, how far from the mast pipe does the choke need to be, or does this make any differences ?...

Last one, how far below the antenna does the choke need to be mounted on the mast pipe ?....

Sorry guys, but Im a little confused here...

 

[W5LZ]

There's no easy method of figuring the length of cable required for a choke. What it amounts to is a coil of sufficient size to provide the amount of inductance to 'choke' off the CMC currents on the outside of that cable. The diameter of the coil, winding spacing, and the number of windings all contribute to that. So does what that coil may be wrapped on, air, iron, ferrite, mast, who knows what else.
Chokes are frequency sensitive, they have different amounts of reactance at different frequencies (something like resistance to current flow).
Typically, that 18 to 21 feet is just more myth/B.S., it depends on what kind of coax is used along with all of the other things above.
I don't have the site address but there's a table of the types of coax and the effective frequency ranges according to coil size. I'll bet someone can post it here.
- 'Doc

 

[northern35s]

No myth or kidology, but a good source of tested data:

Common-mode chokes

 

[Marconi]

No myth or kidology, but a good source of tested data:

Common-mode chokes

35, I've asked on this forum before, if anybody has unitized any ideas noted in this report, and I've never seen one response.

Have you tested any of these ideas? If so, what did you do and how did it work out?

 

[Oatmeal]

This is what Im looking at using for time been....

Antenna will be either my Imax 2k, or SP 500....
Height around 23-27ft....
Coax for now, RG 213 .....
Used on the 11 meter band..

 

[northern35s]

35, I've asked on this forum before, if anybody has unitized any ideas noted in this report, and I've never seen one response.

Have you tested any of these ideas? If so, what did you do and how did it work out?

I use toroid chokes on my portable 1/4WL 20m vertical, I used one on my 10m 4ele beam, and on my 3 element triband beam, I also used an air wound choke on my Hustler 5BTV, however that was before I realised it's limitations and suffered bad common mode on 40/80m.

I don't use these antennas without a choke, when installing my antennas I don't consider them optional, however when I first tested the portable 1/4WL antenna on the back of my car I purposely didn't use a choke and relied on common mode current as a counterpoise, the display on my IC7000 flickered due to the presence of common mode on the coax, I've seen it before and expected it, when I added proper grounding straps to the antenna and car roof line, then choked the coax, no flickering, common mode be gone

 

[Marconi]

I've never found an air wound coaxial choke I've made or two 50 ferret chokes, one I made and one I bought...ever seem noticeably to do any good. They might have been working, but I found it hard to tell. Maybe I just didn't have enough CMC to matter. I never used a choke design from G3TXQ article however.

I think one time I reported what I thought was Sigma 4, install got quieter, and I saw a little change in my bandwidth curve, but after a short time the noise returned. So I changed my mind, and had the thought that the noise event I noticed...was all about conditions changing, and not due to the choke.

I have often noticed, however, that when I added a coke in-line rather than winding the choke using some of my feed line, that I saw the match appear to change at the TX'r end of the line however. After a while and with some help from Bob85, I think we determined that what happened was my frequency changed...due to feed line transformation and a bit of mismatch at the feed point.

I have never had a radio that could detect CM currents at my station, but I do keep a free standing field strength meter, and when I see it working noticeably I can detect a problem and check it out. Most of the time I have found something loose (a construction issue) on the antenna.

I have never tested the choke idea regarding changes in height, but I suspect that height may be a variable that needs to be considered. However, such changes in feed line length could have been the reason I was chasing my tail trying to figure out what was going on, in those case.

I have tried to model this idea, but that gets real complicated and it is easy for me to get off track.

 

[hotrod]

heres a good one K3dav.com/rfchokecoil.com

 

[northern35s]

I have never had a radio that could detect CM currents at my station, but I do keep a free standing field strength meter, and when I see it working noticeably I can detect a problem and check it out. Most of the time I have found something loose (a construction issue) on the antenna.
.

Excessive common mode current will completely shut down an IC7000, when I said flicker if you can imagine what tvi looks like and then imagine what that would look like on the colour display of an IC7000, that's what I mean by flicker

 

[Road Squawker]

Excessive common mode current will completely shut down an IC7000, when I said flicker if you can imagine what tvi looks like and then imagine what that would look like on the colour display of an IC7000, that's what I mean by flicker

z'actly correct,..... the 7k is VERY sensitive to any stray rf,

it can and will drive the PA into oscillation and let the magic smoke out:blink:

 

[Oatmeal]

Another question about these air chokes....

Ok, this choke, when you make 4-5 wraps around a 4" piece of pvc pipe, what kind of affect does it have on the swr ?.....

Reason asking, my swr on my Imax is a 1.1 or less across the 11m band....

Even checked the swr on 28.0450 and the swr was still below a 1.2..by the way, this is without using a antenna tuner...

And how far off the mast pipe does one of these chokes need to be mounted ?..

 

[Marconi]

Another question about these air chokes....

Ok, this choke, when you make 4-5 wraps around a 4" piece of pvc pipe, what kind of affect does it have on the swr ?.....

Reason asking, my swr on my Imax is a 1.1 or less across the 11m band....

Even checked the swr on 28.0450 and the swr was still below a 1.2..by the way, this is without using a antenna tuner...

And how far off the mast pipe does one of these chokes need to be mounted ?..

Oatmeal, you will hear a lot of ideas for your question here no doubt.

IMO, the theory indicates: if you make your coil choke using part of your current feed line, you should see little to no difference at all and such a device is said to be transparent.

However, depending on what your actual FP match might be, if you build a choke and effectively add more feed line to the system in the process, you might see a change.

I believe such changes are due to some mismatch that responds with feed line transformation effects. In my experience, I see the results being the resonant frequency changing, and that does appear as if the feed point match has changed.

Theory also tells us that a choke should be at the feed point, not just close, but right at the feed point. I don't know what difference this distinction makes, but I suspect this being "a close idea sorta' thing," it's just like a lot of other theories an the CB ideas that stream from such theory...the idea really doesn't matter too much...if it works for you at all. This is not saying the theory of the choke does not work, I just question the fix-all remedy CB applications claim.

I have yet to be really convinced that these air wound chokes make all the difference claimed at the CB level of our radio hobby.

 

[W5LZ]

Why would you want a choke at the antenna's feed point? Mostly because that choke reduces or stops the feed line from radiating. If you don't put it at the feed point then whatever feed line is between the feed point and the choke can still radiate, which is what you are trying to prevent, right?
Then again, there are times when having a radiating feed line can be beneficial, so placing that choke away from the feed point can also be useful.
Does a choke affect SWR? Not if the antenna system doesn't already have a problem. It only affects what's on the outside of the feed line, not what's on the inside where you are looking at that SWR. Can changing the length of the feed line make a difference in SWR? I think you know the answer to that already. So adding a few feet of additional feed line to the existing feed line could certainly make a difference in SWR, right? But it's not the -choke- that's causing that change, it would be the same change if you just added that additional length without coiling it up. That's an indication that the antenna on the end of that feed line ain't 'right' anyway.
Seems like it just goes in circles sometimes, don't it??
- 'Doc

 

[Oatmeal]

10-4, yea Ill have to add some coax to make this move..where the ant is now, Im using 50ft of RG 213 coax, and to make the move Ill have to add to it...


I know, we all like using one solid piece, but Im not going to be able to do that here, from the ground up to the antenna Ill use a solid piece here, but at the bottom area of the mast pipe, Im going to have to use a barrel connector, and this too will be where it will be unhooked when a storm comes in too...

Im going to come off the barrel connector with around 94ft and run this to the shack...this will be a total of 144ft of coax Ill be using.....Id rather have a one solid piece but cant do it...Im sure this isn't a good idea, but to make this move this is what I have to do....or leave the antenna were it is for awhile..

If I use a barrel connector at the bottom area of the mast pipe, will this be ok for awhile, or a no, no, to doo ?...

 

[wavrider]

10-4, yea Ill have to add some coax to make this move..where the ant is now, Im using 50ft of RG 213 coax, and to make the move Ill have to add to it...


I know, we all like using one solid piece, but Im not going to be able to do that here, from the ground up to the antenna Ill use a solid piece here, but at the bottom area of the mast pipe, Im going to have to use a barrel connector, and this too will be where it will be unhooked when a storm comes in too...

Im going to come off the barrel connector with around 94ft and run this to the shack...this will be a total of 144ft of coax Ill be using.....Id rather have a one solid piece but cant do it...Im sure this isn't a good idea, but to make this move this is what I have to do....or leave the antenna were it is for awhile..

If I use a barrel connector at the bottom area of the mast pipe, will this be ok for awhile, or a no, no, to doo ?...

Sounds like it will work just fine.

No problem with a barrel connector as long as it is weather proofed so water intrusion can cause damage.