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Foxhunter
Intermediate Member
Username: Foxhunter
Post Number: 114
Registered: 4-2008
| | Posted on Wednesday, September 03, 2008 - 12:49 am: | 
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In another thread recently about "Modifying a Maco V 5/8" I was asking about coiling coax for the placement of an "RF choke" just below the antenna feedpoint. Known as (so I've read) an "RF Choke"---aka---"Solenoid Current Balun"---aka---"Air-Wound Coil Choke".
Although no one should have common-modes even to begin, unfortunately they do exist and occur in some installations. I know they are the symptom of an underlying problem/issue and if by some chance they do occur in my installation I want this addressed before it even shows up. I am not planning on using 5-loops or so through a ferrite-choke. Or should I? Or do both?
I'm wanting to prevent **even the possiblity** of common-mode currents travelling back down the shield of the coax. Although I am often trying to save money (as this is becoming an expensive hobby) I will be upgrading to a premium-quality coax here soon for my Maco installation.
I know I am not teaching anyone anything here but at higher powers, the slightest of problems (that one might not even notice at lower or mid-level power) will be greatly exaggerated/amplified/magnified. I do not want to have any problems with this, as I have had in the past with this at one point. I'm trying to do what I can myself at lower-costs, as I can't just go out and buy what I need commercially right now. Improvising/Homebrewing is supposed to be part of the hobby to begin with too.
I've read some conflicting/confusing information a little on one or two points with making an RF Choke of this type. I've read a highly technical sheet recently that stated I can wrap either 6-FT---OR---"a proper length" of coax **TIGHTLY AROUND THE MAST** just below the antenna feedpoint. It goes on to say "purists" will insist on wrapping the coax EVENLY SPACED with a one coax-diameter space/void in-between each turn.
Another technical article on RF Choke construction says NOT to use the metal mast, but to TIGHTLY wrap the coax around **A NON-METALLIC PVC PIPE-SECTION** instead, and mount it at the antenna feedpoint.
*Which/what is right?
*On the metal mast or PVC pipe?
*Tight wraps or with spacing?
*What diameter mast or pipe for the choke to be effective is minimum or maximum?
*My mast is 1.25-inch OD heavy-walled radiator pipe. If I can use the mast for the choke, how many turns or how many feet of coax should be used?
Reading all of the above, would anyone be willing to share some advice or information regarding this type of installation? |
Tech833
Moderator
Username: Tech833
Post Number: 1693
Registered: 8-2002
| | Posted on Wednesday, September 03, 2008 - 11:55 pm: |
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If your 5/8 wave antenna feedpoint match is good (SWR is low), then the coax coil choke is a waste of time and cable beacuse there will not be any common mode currents in play.
It only serves as a band-aid approach to fixing an antenna when the SWR is not good.
More ham and CBer 'voodoo' engineering. |
Foxhunter
Intermediate Member
Username: Foxhunter
Post Number: 120
Registered: 4-2008
| | Posted on Thursday, September 04, 2008 - 8:06 pm: |
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Tech 833 I hold you in high regard. Well I want to stay "out of the jungle" so-to-speak with the voodoo and superstition and/or pseudo-science. Although I am reasonably new to radio I am taking it more seriously. I am trying to understand it all but it hasn't "come natural" to me---like it could or may to those who have electrical or engineering aptitudes.
Among some other things I am very knowledgeable in Astronomy, Cosmology, Archaelogy and History. Depending on some of our viewers political/military leanings this may not mean much but I scored very high this past year in the Electrical section of my ASVAB/Military Entrance Examinations (high score all sections).
I'm trying to "sort through" what I read from credible sources but it is hard as I see even Extra-Class Hams arguing over some very technical issues and schools-of-thought.
Yes I do agree that a choke is somewhat of a band-aid if one has the need for it. Common-modes are symptomatic of an underlying issue. I guess that is likened to "treating the symptom and not the disease". I was encouraged to it's possible use by such devices as the "W2DU in-line isolator" and similar products made commercially. There must be many who do have the need for them to prevent coax-line radiation and currents as well as the need to surpress interference issues. I was considering the use of one as a "preventative measure" only. Not sure if I'll use one... |
Tech833
Moderator
Username: Tech833
Post Number: 1699
Registered: 8-2002
| | Posted on Friday, September 05, 2008 - 12:04 am: |
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An 'extra class ham' is someone who memorized answers to a test and passed it. Nothing more. because I pass a test and get a driver's license, does that make me an expert driver? of course not. Same thing applies.
Although many hams like to wear their class of license as a monument to their intelligence on all radio matters, the truth is, they are wrong. If I am only a general class licensed ham, does that mean I know less about how radio works than an extra? No, it only means I have not yet taken the extra exam.
As for the choke, in the case of hams using antennas to cover large frequency ranges, sometimes outside of the antenna tuning curve, the choke will help keep RF out of the shack. In the case of CB, the bandwidth is very narrow, and your antenna should be tuned for the band you want to use. If you try and use your CB antenna for 20m, you might need a choke to keep RF out of your shack. If you use your tuned CB antenna for the CB band, it will not be an issue.
On a properly tuned 5/8 wave antenna used for only one narrow band, if the match is perfect, there will not be any current flowing over the outside of the cable back into the shack as long as the cable is well grounded to the feedpoint and mounting structure. |
Foxhunter
Intermediate Member
Username: Foxhunter
Post Number: 124
Registered: 4-2008
| | Posted on Friday, September 05, 2008 - 8:33 pm: |
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Ok I do agree with your example of the Licensees and it is no guarantee of expertise at times. They may know more than your average "cber", but that's not to say that some Extras may not be under false impressions or mistaken schools-of-thought. I read some of your archived postings regarding the frustrations you'd experienced trying to speak to some of them at radio clubs
As far as for generally monobanded antennas such as the 11-M types I was under the impression that Class C amplification or just plain high-power could cause problems a choke could help reduce/resolve.
Well if I won't need a choke or balun I won't get I'll be tuning the antenna centered at 27.205 Mhz.
Thanks for keeping me straight and I hope you'll check in at times because I know for sure I'm going to have a few more concerns as I'm progressing. I appreciate your helpfulness and patience. Thank you! |
Tech833
Moderator
Username: Tech833
Post Number: 1702
Registered: 8-2002
| | Posted on Saturday, September 06, 2008 - 1:02 am: |
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A choke can also create problems. In your case, it wouldn't change anything.
I know some CBers that are way smarter than most extra class hams.
Class C amplification has nothing to do with the antenna or tuning. Class C just means that the amplifier is biased above cutoff. |
Patzerozero
Senior Member
Username: Patzerozero
Post Number: 4382
Registered: 7-2004
| | Posted on Sunday, September 07, 2008 - 7:57 pm: |
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If you try and use your CB antenna for 20m, you might need a choke to keep RF out of your shack-i DO use my maco v5/8 with my ic735, & approx 3:1 swr on 20m. i get NO tvi to any of the rg6 quad coax fed tv's in my house, hd or not, EXCEPT i DO HEAR rf through the tv next to the radios in the shack (has not been rewired w/rg6 quad yet). my 746pro does only marginally better through the built in tuner, 1:1 tuned swr on 20m, though the rest of the house is still good. while redoing the 6 & 2m loops a few weeks back, i DID add a coil of about 15+ turns to the maco, til i get a chance to move & remount it higher-too much coax bunched up on the roof. lo & behold, the rfi to MY tv while on 20m was more then cut in half on the 746 pro, and if i back it down to about 80% power on its meter, i get NO rfi.....11 meter shows no xmit/rcv/swr changes  |
Foxhunter
Intermediate Member
Username: Foxhunter
Post Number: 128
Registered: 4-2008
| | Posted on Monday, September 08, 2008 - 2:59 pm: |
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I was considering the use of either a "coax choke" made by wrapping turns of coax around the mast or buying a pre-assembled unit such as a Balun because I wanted to surpress in any way the coax acting as a radiating or receiving antenna.
It would appear that common-modes currents causing "radiating coax" with higher/fluctuating SWR readings or TVI----is a common problem judging by the amount of people plagued with this type of problem. Perhaps it is due to a fault in the installation to begin with---I'm unsure. That may be so.
The reason I mentioned Class C amplification is I thought that type of amplifier contributed more to this type of problem than differently biased amplifiers.
I was really looking at the "coax choke/solenoid balun" as another form of "filter" in a sense, to eliminate feedline radiation, or the coax in any way acting as an antenna on either transmit or receive.
That is what I was thinking when I first began to look into the possible use of one for my final installation. |
Fishbones182
New member
Username: Fishbones182
Post Number: 4
Registered: 4-2009
| | Posted on Friday, April 24, 2009 - 11:29 pm: |
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As stated if your using an antenna made for the band your on you really dont need it. The size and numbers of coils in your choke depend on the operation frequency. A good current balun to cover hf and cb bands would be about 8 feet of feed line tightly coiled in a 6 inch circle then taped tightly together. As far as rfi most of those problems occur from cheap radios and amps.
Most of these amps intended for use on cb use a very wide portion of the spectrum. Am operation is about 6 khz wide and with alot of these export radios and cheap amps out there they are wider than that. One thing to keep in mind the mic gain doesnt have to be wide open to get the full potential of the audio out of your rig. Thats another cause of RFI is for alot of people they are over driving their equipment and causing splatter. One more tid bit of information for you a ferrite choke or beads if you are going to use them is to make sure you use the correct ferrite for the job such as mix 43. There are several ferrite bead mix's so do some research before you buy to be sure your getting the ones you want. 73 and good dx |
