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Beads, Ferrities ?..

Discussion in 'CB Antennas' started by Oatmeal, Jan 18, 2015.

  1. Heavy Metal

    Heavy Metal Active Member

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    Cool, but how many turns and how wide of turns for 9913 & 9913F7, LMR400 & LMR400UF.. And those type as well as how many needs/cores needed and were at?( just at PL259 connector?).


     
    #31 Heavy Metal, Mar 5, 2015
    Last edited: Mar 5, 2015

  2. fourstringburn

    fourstringburn W9WDX Amateur Radio Club Member K5KNM

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    The choke I made was terminated with RG8X coax.

    I made 7 turns making about a 4 inch center diameter coil and used one 3/4 inch mix 31 snap on ferrite bead. I believe it took about 9 ft. of coax to make those turns.

    I'm not aware of any snap on ferrite beads large enough for RG213 size coax.

    It doesn't really matter if you terminate your larger coax with a barrel connector and downsize to RG8X coax like I did as long as you have the choke right at the antenna feed point. I still do this but with My DX engineering choke. I run RG213 to the DXE choke, then use a small RG8X jumper to the antenna.
     
  3. Heavy Metal

    Heavy Metal Active Member

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  4. Heavy Metal

    Heavy Metal Active Member

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    Can I place something like that at the coax switch? And do turns of coax and ferrite beads/cores( snap on type ) at each antenna at the pl259/so259?
     
  5. fourstringburn

    fourstringburn W9WDX Amateur Radio Club Member K5KNM

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    These ferrites listed hear are no way large enough to wind several loops of RG213 size coax. These ones simply snap on the RG213 size coax which does hardly anything for creating a choke much less being much effective for common mode rejection unless you use 50 snap on ferrite beads on it.

    A choke should look like this using smaller RG8X size coax. DX engineering sells 3/4 inch ferrite beads like the one pictured here which can take about 7 turns of coax.


    [​IMG]

    The general idea of using a choke is to keep common mode currents from entering your shack in the first place. Therefore, as I posted earlier, you need to place a choke at the antenna feed-point. If you are using RG213 size coax, then as I posted previously you need a barrel connector and at least 9 ft of RG8X coax to coil up with a ferrite bead to connect between your antenna and larger coax.

    Here is a paste from K0BG webpage that gives some technical detail even though it is a Mobile HF information website, the theory still applies.


    After proper mounting, the next solution to control common mode current is to install a choke. In this case, the choke is nothing more than a ferrite split bead, with a few turns of coaxial cable threaded through it. In fact, you can use the same type of split bead that you use for the motor control leads. That is, mix 31, and preferably the 3/4 ID units. They can be purchased from DX Engineering (et. al.) for about $30 per bag of five including shipping. These beads will allow 6 to 7 turns of RG58 or RG8X (as shown in photo). Note that the coax is not tightly wound around the choke. In this case, the diameter is about 3 inches. Any tighter, and the core could migrate and cause a short.

    The resulting choke exhibits an impedance of about 2.2 kΩ at 10 MHz, which is typically enough for 80 through 10 operation. However, if you're really careful, you can wind 7 turns of RG8X through a 3/4 inch ID bead. The resulting choke impedance would be about 2.7kΩ at 10 MHz, and may even prove adequate for 160 meter operation (≈1 kΩ at 1.8 MHz). If you need more than that (trunk lip mounting?), simply snap on a second bead which will (almost) double the impedance. It should be noted that the impedance of the above chokes are primarily resistive at 10 MHz, an important design parameter.

    The choke should be installed as close to the base of the antenna as possible. The last place to install them is inside the vehicle. After all, we want to keep the RF on the outside, not the inside of the vehicle. Fact is, the inside of a vehicle is almost as RF noisy as the engine compartment. Knowing this should be prima materia about where to mount a common mode choke.

    How much choking impedance you'll need is not a cut and dried scenario, but there are factors which need to be considered. Those nifty appearing, clamp type mounts seemingly are the rage. However, the fact remains they add to the level of common mode current. Remember, the further away from the ground plane the base of the antenna is mounted, the greater the common mode currents.

    It should be noted that antennas mounted through sheet metal via ballmounts will exhibit a bit less common mode than a similar height antenna fed outside the vehicle. Nonetheless, a proper choke is still required to minimize both egress and ingress RFI.

    Here is an important fact to keep in mind: A multi turn choke will have much better common mode suppression than an equivalent one made up of a series of single turn ones. Further, in order to duplicate the total reactance of the choke shown above (7 turns, 3/4 inch ID, mix 31 split bead, ≈2.2 kΩ @ 10Mhz) on RG213, would require 49 similar split beads. That's about $275 worth, instead of just $6! However, there is another factor which needs to be considered. Impedance wise, it is imperative that the resistive value (in ohms) be larger than the inductive value (in ohms) if the choke in question is to be effective over a given bandwidth. Referring to the common mode impedance chart on Steve Hunt, G3TXQ, web site, the black line in the chart represents the resistive value of the represented chokes. In order to achieve this, the SRF (self-resonance frequency) needs to be within the frequency band of interest. To better understand this premise, read the verbiage accompanying the chart. Here's a hint: Compare the length and position of the black line in the chart with respect to the number of turns comprising the chart. Remember too, coax wrapped around a ferrite core is a tuned circuit.
     
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  6. Heavy Metal

    Heavy Metal Active Member

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    I see but is their an alternative way with larger coax rg9913f7 and lmr400uf to do same and if so how many winds and how wide to wind it like shown above without the core that will work as well. I don't have a lot of probs but I enjoy knowledge and how too vs buying way out.
     
  7. M0GVZ

    M0GVZ Sr. Member

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    Several turns will do enough. If you get the data sheet for the ferrite you're using it will tell you exactly how much impedance it has per turn at various frequencies (it is frequency dependent) and that is the same regardless of the coax you're using. You want to be aiming for 2k Ohms or more. So if the data sheet says for each turn you get 250 Ohms at 28 MHz you want at least 8 loops.

    Large coax is usually unsuitable as it doesn't have a sufficiently small minimum turn radius. You therefore make a choke out of a smaller coax with better bending characteristics and place it inline or even better, make a bifilar wound one with insulated wire which will handle a lot of power.

    [​IMG]
     
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  8. Oatmeal

    Oatmeal Active Member

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    LMR 400UF, I've been told to use a 6" forum for it, and use 4-5 wraps....

    I no the choke is suppose to be like a foot away, but whats it going to hurt to go a little more, maybe 2ft from the feed ?.....

    I'd like to put one of these chokes on a beam but instead of mounting the choke on the boom, mount it on the mast pipe...maybe less than 2ft...

    Because looks like using a 6" forum and 5 wraps of LMR 400UF coax is going to be a little heavy, mounted on the boom of your beam..
     
  9. Heavy Metal

    Heavy Metal Active Member

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    Better to do more wraps than needed or what? Sorry just trying to learn lol.
     
  10. The DB

    The DB Sr. Member

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    A lot of people think this way, but it actually isn't. In this case more is not actually better.

    What is actually happening with a properly designed choke is you are creating a something called a parallel resonant circuit. This type of circuit has a very high impedance, but only for currents traveling on the outside of the feedline.

    The fact that this type of circuit is almost purely reactive means it has functions over a rather narrow effective bandwidth.

    Adding another wrap will change the values that make up the parallel resonant circuit, which has the effect of changing the frequency that it functions at. More wraps lowers the effective choking frequency.

    This chart, which was already shown on page one of this thread is a very good reference chart.

    [​IMG]

    If you look at the line for five turns on a 4.25 inch form, the best frequencies are between 25 and 28 MHz, and adding just two wraps pushes that range to between 19 and 21 MHz, it also brings down the effectiveness at the CB frequencies we are talking about pretty drastically, essentially to 1/16 of using the correct number of wraps for said frequency.

    All that being said, I personally recommend and use ferrites. There are any number of advantages to doing so, not the least of which is the fact that such chokes are more resistive than reactive, which has the effect of it being use able over a much wider range of frequencies.


    The DB
     
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  11. Oatmeal

    Oatmeal Active Member

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    The LMR 400 coax looks a little stiff to try wrapping it around a 4" form....this is why I was going to use a 6" form...

    When I thought about using a choke I was looking into setting this beam up for the 11m band nothing else, but now I'm looking at using it on the 10 and 11m bands....according to what I have been told thats all I can use it on....

    Still debating on doing this or not, don't no if its worth doing this or not......also have that Imax 2k up here, and the swr is all good on it on the 10m band from 28.325.00 up to 28.500.00 meters, with just using the tuner in the radio..
     
  12. Heavy Metal

    Heavy Metal Active Member

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    What about using LMR400UF or RG9913F7 these are stranded centers.
     
  13. Beetle

    Beetle Sr. Member

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    I always use a bending radius of 10x the cable OD. For RG-213, 9913 and other cables of that general size, the OD is specified as 0.405 inches.

    So if the RADIUS is 10 times 0.405 inches, the RADIUS is 4.05 inches.
    And therefore, since the radius is HALF of the diameter, the diameter must be just over 8 inches. Call it 8.

    The smallest-diameter for a coil form for 0.405 inch coax would be 8 inches.

    Don't confuse "radius" and "diameter". :)

    The USN uses a factor of 13 times the cable OD for installing coax. I've shot down hundreds of cable installations on Navy ships because the bend radius was too small.
     
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  14. Heavy Metal

    Heavy Metal Active Member

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    So make a turn on something 8" wide( or what is an 8" circle? And how many turns on it?
     
  15. M0GVZ

    M0GVZ Sr. Member

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    Without getting the same coax, building one and using a grid dip oscillator to measure it, nobody can tell you. All the figures in the table from G3TXQ came from ones he built and tested himself.
     
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