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coax for choke

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KB6M, Jul 14, 2009.

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  1. KB6M

    KB6M Ham Member QRZ Page

    I am going to be building a multi band-dipole here in the next couple of weeks.
    I have a 50' piece of rg8 to feed it with, and it will take just about 45' to reach the antenna. So, I don't have any left to make a balun/choke. From reading here, a choke should be a coil of about 20' of coax, Used to isolate the antenna from the feedline. How important is it to use a choke, and can I use other coax (rg58) to make it? I have plenty of rg58, so if there is any benefit to to it, I will use it.

    My thoughs are that 50ohm coax is 50ohm coax. I will just get more loss for that 20 feet. I just don't want to cancel the benefit of using a low loss coax for something that won't help the performance of the antenna.

    Thanks for any replies,
    Greg
     
  2. AC0GR

    AC0GR Ham Member QRZ Page

    Be sure that the dielectric is not foam. The center conductor will migrate over time in the tight turns.
     
  3. KC5FM

    KC5FM Ham Member QRZ Page

    Balun

    Is there a problem that you need to solve by installing the device?

    A Balun is used to solve problems, not because there's coax lying around that has no use. :)

    No problem? No balun.

    I've never used one. That is not to say they are not useful.

    73
     
  4. KB6M

    KB6M Ham Member QRZ Page

    5FM,
    I cut and pasted this from your link:

    As the feedline becomes part of the antenna, currents can flow from the line into the mains and on TV cables, metal masts and yagi booms, causing a variety of EMI problems that can be very difficult to trace. Frequently these problems are simply due to unbalance - and the solution is the humble air choke

    In a nutshell, I want to avoid the above problems. I guess, I should have mentioned that I am devoting a day of vacation to installing the antenna. I work strange hours and family commitments use up most of my free time. But, I can operate during the "family" times. I just can't be out on the roof or climbing trees. Once installed/tuned, I won't be able to pull it down very easily. The idea of installing a choke was more proactive than reactive. Are the problems, that the choke is designed to cure, common enough that I should go ahead and install it. Or, are they so rare that it would be a waste of time.

    Sorry for leaving out part of the story,

    Greg
     
  5. K7FE

    K7FE QRZ Lifetime Member #1 Platinum Subscriber Life Member QRZ Page

    A coax balun is a specific length (1/2 wavelength with the coax velocity factor taken into account) for a given frequency or band. It is not effective in most multiband situations.

    I would suggest a ferrite choke 1:1 balun. It will have a much broader bandwidth. You could make one that will cover all of the HF bands. Walt, W2DU describes this type of balun in many of his publications. http://w2du.com/
    [​IMG]

    Walt is a frequent poster here. Perhaps he will jump into the discussion.

    73,
    Terry, K7FE
     
  6. K7NNO

    K7NNO Moderator QRZ Page

    Agreed. Unless your antenna is at least 1/2 wavelength above ground you wont notice any difference.
     
  7. K4SAV

    K4SAV Ham Member QRZ Page

    A choke is usually a requirement on a directional antenna like a Yagi, in order to not degrade the front-to-back of the antenna. A choke can reduce the common mode currents on the feedline, reduce RFI, and reduce the amount of noise received due to common mode currents. Some of those may not be important to you on a dipole. A dipole has no front to back, feedline radiation on a dipole is not a problem unless it causes RFI, which could be minimal if you run low power. Received noise due to common mode currents is usually not a problem unless the antenna has very low gain, or your house AC ground is very noisy, or the coax cable happens to lie right next to a noisy line. With most antennas the band atmospheric noise usually masks the noise that runs up the feedline. So you can try it without the choke, and knowing what a choke can do, you can then add the choke if you experience one of the problems a choke can cure.

    As far as using RG58 I would want to know what this multi-band dipole is, and the maximum SWR on the different bands. You are probably going to need about 15 ft of RG58. At low SWR the loss will be negligible. That may not be true with high SWR.

    Jerry, K4SAV
     
  8. AI3V

    AI3V Ham Member QRZ Page

    You are going to be very hard pressed to find a set of conditions where the radiation from the feedline is a problem, when the actual radiation from the antenna, inches away is not.

    99% of all "feedline radiation" complaints can be cured without a balun or choke by getting the antenna up higher(away from the interference).

    By all means, try the antenna with, and without a balun and or choke, just don't be surprised when you cannot tell any difference.

    Rege
     
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    Greg,

    You'll get a variety of views on the efficacy of common-mode chokes. One reason is that the problems they solve may or may not be severe, depending on many factors, including the local environment.

    Without a choke you have not built a dipole, you have built a "tripole" - the third leg being the outer surface of the coax braid. That third element may cause problems, or it may not, depending amongst other things on the length of the coax and how well it is grounded at the shack end. Here's an extreme case of what the current distribution would look like when the coax is a half wavelength long and connected to an excellent ground. Forget any skew of the dipole pattern - do you really want that much RF at the shack end:

    http://www.karinya.net/g3txq/temp/dipole_coax.png

    Reciprocity tells us that the "tripole" is potentially bad news on Rx also; we've placed one end of one of the tripole legs inside the shack close to a lot of equipment radiating noise!

    You should note that the choke at the feedpoint will prevent conducted current flowing back down the braid outer, but it wont stop current being induced on the braid as a result of being within the radiation field of the dipole - that will likely be more of a problem if the coax does not come away symetrically. You would then need chokes at other positions, which would likely be band-dependent.

    You may get lucky and find that none of these problems affect you, but the potential for problems is greater on a multiband dipole because the feedline length is unlikely to be benign across several bands.

    The problem with air-cored baluns is that they only provide good choking impedance over about a 4:1 frequency. So if you intend working over a wider bandwidth you need to be thinking of a ferrite-cored choke instead; that will also result in a much shorter length of coax and negligible losses.

    PS: an Old Timer local to me never uses a choke - he says he's never had a problem in 50 years of using dipoles. Same guy never wears his seat belt when he's driving ;)

    Steve G3TXQ
     
  10. KL7AJ

    KL7AJ Ham Member QRZ Page

    Good point. The only way to know for sure is to actually MEASURE common mode currents....very easy to do with a simple inductive current probe. I've gotten to using this on every new installation. Quite revealing!

    Eric
     
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