You can purchase CMC chokes ready-buillt, or better yet, make one yourself. It's not difficult.
You will get efficient and thoughtful service from mide.
HERE ARE 5 METHODS:
A) COILED COAX CHOKE: Coax wrapped to form a coil. These work, but they are not always good enough. It is the easiest method to try.
B) IMPROVED COAX CHOKE: Wrap coax onto a PVC pipe, using nylon rope to space the turns. This reduces the mutual coupling and helps keep symmetry in case you are using this as a balun.
C) FERRITE BEADS OVER COAX: Slide a number of beads over the outside of the coax, and cover with heat-shrink tubing. Then solder the PL connector onto the end of the coax. Tip: using thin Teflon insulated coax enables you to reduce size and weight but still run high power. I usually use one of these just before the coax enters the house. I also use it with commercial
D) COAX WOUND ON A LARGE TOROID: This is also easy to make. I usually use this one for my home-brew wire verticals because it can be built to provision for the radiator, the radials, and even for a series capacitor (if required). I will show this in detail below.
(A) Coiled Coax Choke
The coiled coax choke is the easiest to make but also the least effective. If it works, use it. Otherwise use something better.
For details see the ARRL HANDBOOK or ARRL ANTENNA HANDBOOK.
For impedance details, see the bottom of the chart on Steve Hunt's (G3TXQ) web page on Common Mode Current Chokes: http://www.karinya.net/g3txq/chokes/
Scrambled-Wound Choke? OK or not?
I honestly do not know for sure. I have heard that although it will work, it doesn't work as well as when keeping the turns in a straight row. I have also heard that on 160m it doesn't matter; scrambled is OK. It seems to me you could have unwanted, stray coupling if you scramble the turns.
Until I find out for sure, I will continue to wrap my turns in a straight line as shown in the picture (A) above.
(B) Improved Coaxial Choke
This is a very simple choke to build. The material costs are low, and it is very rugged. You won't burn this one up.
GOOD OR BAD?
You hear success stories with this choke, but you also hear unfavorable reports about them
What is the truth?
The reality is, this is a good method of building a choke but it does not have a broad bandwidth that chokes wrapped on ferrite have. You can't build one and expect it to work from 160 thru 10m. When you build one, you need to decide which bands you want it to cover.
How much impedance must a good choke have?
In most cases, 1K Ohms is enough. If the problem with CMC is not severe, you can usually get by with just 500 Ohms. It is difficult to measure Common Mode Current, but it is not difficult to understand that the more imbalance in your antenna, the more CMC you're going to have. Here are some basic indicators:*
*Any antenna can incur additional imbalances if there are conducting objects in near proximity of one of its sides. In that case you may need more impedance than shown above.
Especially above 7 MHz, if you use more turns than necessary, the chokes performance will degrade. You can get a graphical view of this by studying the chokes at the bottom of the chart on Steve Hunt's (G3TXQ) website. See: http://www.karinya.net/g3txq/chokes/
Here are some general guidlines for winding coax chokes on a 10cm (4 in.) PVC pipe,
For more MINI Choke ring antennainformation, please contact us. We will provide professional answers.
using RG-58 for low power or RG-213 for high power:
As stated above, it is best to space the individual windings by about 6mm (1/4 inch) using nylon rope between the windings. For more exact details see the link to G3TXQ's website shown just above the chart.
NOTE: When used with antennas with a lot of Common Mode Current, this "Ugly Balun" is not the best solution. In that case a 1:1 Guanella balun would be far more effective! See "D" (below).
I find it practical to build the CMC choke into a plastic box and include connections for the antenna I will use it for. For instance in the case of a Vertical, I use a box with 5 holes, one for the SO-239 coax jack, and 4 for screws.
Note: The shield (ground) side of the SO-239 should only connect to the shield of hte coax.
It must not be connected to the radial screws.
All components use for this choke were sourced from Spiderbeam, but this is not a standard Spiderbeam product.
(Modern Ugly Chokes)
Ferrite purchased in small quantities is very expensive. In severe cases of common mode current, the solutions presented above may be inadequate, unless you choose Method D and use several (expensive) toroids.
The concept shown here was first introduced by George Cutsogeorge, W2VJN in the issue of the ARRL Handbook.
Ian White, GM3SEK designed the 3 chokes pictured here, using common low cost ferrite cores. Left: Low-Band; Right: Mid-Range & Low-Band.
These chokes have high CMI (over 4 k-Ohms), [but only] across a limited frequency specturm. Therefore it takes 3 different chokes to cover the entire HF spectrum. You can use them independently of each other to cover specific ranges, or all three in series to cover 160 thru 10m.
I was unable to find these cores here in Germany so I ordered several dozen from the UK. In the meantime I have built and tested these chokes and found them to be an excellent compromise between effectiveness and cost. Their ability to impede CMC rivals that of the Guanella (type D above), but these are cheaper.
I will update this space after I have done more testing.
In the meantime you can click on the picture for more information.
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