RF choking  | | 
09-18-2009, 08:52 AM
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Quote:
Originally Posted by moleculo  I'm sure that's where it came from, but how does that help the average person figure out the diameter and number of turns of coax to use to form an effective choke at 27Mhz? | Ah - for that you'll have to ask Frank. BUT, --- you can measure the inductance of the shield of a piece of coax wrapped in a fixed coil - if you have the proper test equipment - or you can carefully measure dimensionally and apply the proper formula to be found in the ARRL Handbook or any one of other references and come up with a calculated inductance. Then just apply the inductance value and frequency to the formula to determine inductive reactance. The DC resistance of the shield should be virtually zero, and the distributed capacitive reactance should be very low compared with the inductive reactance, so the choke should be primarily inductive (as you'd expect) and as effective as a more conventional RF choke would be.
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09-18-2009, 10:40 AM
| | Delta | | Join Date: Sep 2009 Location: London UK
Posts: 45
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whoops, double post..
Last edited by delta; 09-18-2009 at 10:42 AM.
Reason: double post
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09-18-2009, 10:40 AM
| | Delta | | Join Date: Sep 2009 Location: London UK
Posts: 45
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Quote:
Originally Posted by W5LZ delta,
"The number of turns is not critical because the inductance depends more on the length of the wire (coax) than on the number of turns, which will vary depending on the diameter of the plastic pipe that is used."...
I think you will find that this just isn't the case, the part about the length being more important than the size and number of turns in that coil. Other wise, why bother winding that feed line into a coil at all. The size/shape of that coil, length, diameter, and turns spacing also plays a part in the amount of inductance/inductive reactance produced by that coil. That inductive reactance is what gets rid of the CMC I think, since it doesn't 'radiate', which is the whole point of this.
"The problem with scramble-winding is that the first and last turns of the coax may touch each other. This creates two complications. The distributed-capacitance of the balun is increased and the RF-lossy vinyl jacket of the coax is subjected to a high RF-voltage."...
I'm sure that's probably true, but just exactly when would that decrease in distributed-capacitance and that induced RF-voltage become a problem? Before that choke does what it's supposed to do, or at some point after? (A choke isn't very efficient to start with, and while I've never encountered all circumstances of using a choke, I haven't found this sort of problem with the times I have used a choke.) That outer jacket is supposed to be RF-lossy, or an insulator. Then again, I may be misunderstanding what's meant by that.
I think it's already been said, but in case I'm mistaken, that coaxial choke is not a balun in any sense of the word. It doesn't have any of the properties that make a balun a 'balun'. It may make you happy to call your cat a dog, but until I hear it bark... 
- 'Doc | Crickey me Doc i'm a bit confused now, so what would you recomend me doing regarding a Rf choke? I've just sent off for the interceptor 10K. Do i use a RF choke or not, if yes what would you suggest me doing? You seem to know more than i do and i'm all ears for any help you have. This is a great forum and you guys are what makes it great. Thanks for your time.
Last edited by delta; 09-18-2009 at 12:38 PM.
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09-18-2009, 10:58 AM
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I used to have an I-10K up, and an Imax before that. You will probably find that you don't need the choke with the I-10K, but it certainly won't hurt anything. Both the ARRL Handbook and the ARRL Antenna book have a table that tells you the number of turns and length of coax to use for various frequencies and types of coax. If someone else doesn't beat me to it, I'll post it later when I get home and have time to look it up. |
09-18-2009, 11:07 AM
| | Senior Member | | Join Date: Apr 2005 Location: West Coast - Washington
Posts: 671
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Delta,
I would try the 10K first before worrying about the choke.
I have a outdoor TV antenna sitting just 12 feet below my 10k and I have never caused any TVI, even using 200 watts and that after 2 yrs now.
The 10k sure smoked my I max 2000!!! I have never regretted installing the 10k!!
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09-18-2009, 11:47 AM
| | Supporting Member | | Join Date: Apr 2005 Location: Oklahoma
Posts: 3,075
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delta,
Like the others have said, try it and see if you need a choke at all. Putting one in line very seldom hurts anything so you could always do that as a 'just in case' thing.
You will be mounting that antenna on a mast of some kind, so if it isn't too small, why not just wrap that choke around the mast at the feed point? That metal inside the coil should increase the coils inductance and therefore inductive reactance some, so you might not even need as many turns in that coil. What the #$##, use the same number of turns, can't hurt anything.
- 'Doc |
09-18-2009, 12:55 PM
| | Delta | | Join Date: Sep 2009 Location: London UK
Posts: 45
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Thanks guys for all your replies. I will put the I-10K up and try it without the choke and see what i get. I don't get ant tvi at the moment so i'm not expecting any with the i-10k either. I just thought having a choke might help things a little, eg; nothing on the feedline.
Ok my tower is a three section tower, the botton two sections are a steel triangular lattice design at about 40ft , the third section is a 20ft aluminum 2 inch mast. When it's cranked up to the top where i mount the antenna, it's then about 57ft in total plus the hight of the antenna. I like what your telling me about the interceptor 10K, you must rate it as a 5/8 wave gp antenna.
thanks for your time guys...
Last edited by delta; 09-19-2009 at 03:43 AM.
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09-19-2009, 08:07 PM
|  | Member | | Join Date: Sep 2009 Location: Hope, BC
Posts: 67
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Is this the way it should be?
RG213 11' 11"
I'm gonna put this in my Imax 2000 and use a barrel connector into my existing feedline.    |
09-19-2009, 10:26 PM
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Posts: 845
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the coil looks good, but without a solid form to coil around, it may warp a bit in the heat, and that electrical tape has a tendency to lose its "stickiness" in the cold.
i would add a few cable ties into the mix to keep the shape the way it is now.
the thing i want to mention to you is your PL-259 connectors.
this is by no means a judgement on you or your abilities, but those connectors are soldered on very poorly.
it looks like they were put on with a 30-40 watt soldering iron, which just doesnt have the heat to solder the connector the shield.
there are many places online that will show you how to do it properly, and what tools you will need.
the way you have it now, you are running the risk of getting water in your coax which can be disastrous.
dont let your connectors be the weak link in your antenna chain.
make sure you read some of the threads in this section regarding RF chokes and isolating antennas from their support structures.
later,
LC
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09-19-2009, 10:59 PM
| | Member | | Join Date: Sep 2009 Location: Hope, BC
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I soldered to the braid and etched the outside of the 259 between the solder holes and drew the solder from the holes to the etched area because I have had the solder in the solder holes work loose and wiggle inside the holes.
This way I hope there is a good electrical connection.
Should I tape the connections or maybe blowtorch a big heatshrink over them?
What happens with water in coax? | | |
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