RF Chokes with RG-213 cable.
I was planning an antenna project and was wondering how much coax would be needed to make a 10 turn 10 inch diameter coil using RG-213?
Also, what is the best way to go about making the form?
Last edited by WD9IDV; 03-05-2009 at 01:46 PM.
Licensed since 1977
From what I gather, it's not the diameter or number of turns so much as the legnth of the cable used in the choke. Here is some good form ideas:
Thanks all for responding to my question.
I am planning on upgrading my Mosley TA-63N yagi with the 40 meter add on kit. I already have a 5 turn RF choke on a 6 inch diameter. I will need to change this to 10 turns with a 10 inch diameter when I add the 40 meter kit according to Mosley's instructions. Just wanted to make sure I have enough RG-213 coax to do upgrade....Might have to buy more.
That sounds like an awfully big choke which could reduce the choking impedance on the higher bands where it will be well above self-resonance.
Take a look here for some measurements of choking impedance:
Of the chokes that K1TTT measured 8 turns on a 6-5/8 inch former looks a more sensible option.
"In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be." (Lord Kelvin 1824-1907)
Remember your 7th grade arithmetic class: Multiply the diameter (in this case 10 inches) by Pi (3.14159...) and then you get the length of cable for 1 turn (about 31.5 inches). Then multiply this by the number of turns (10) and that gives you about 315 inches. Divide that by 12 and you get 26.25 feet. You need to allow a little because of the fact that these figures are not exact. Therefore, you will need probably between 28 and 30 feet of cable to make a choke that is 10 inches in diameter and that has 10 turns.
There are concrete forms available from home improvement centers that are 10 inches in diameter. However, you can make a form by drawing a circle on a piece of plywood and then use those nails that are sold for installing gutter around your house. Drive them into the wood at 3 or 4 inch intervals and then wind the coaxial cable. Secure the cable with cheap black plastic tape wrapped between the nails (for spacing). The coil will be able to support itself if you wrap the tape around 3 or 4 times for each one.
For smaller diameter coils you can use 2 litre or 3 litre soft drink bottles. It is optional to drink the contents before or after winding the choke.
Choke on RG coax cable
I would not use that large bulky type of choke as it catches the wind and may fall apart.
Get ahold of one of the suppliers of the ferrite beads made for RG 8 or 213 cable. These beads are type 43 and slide over the coax just right. They are about 1 1/4 long or so.
For a beam project of 14 to 30 mHz, you will need about 8 or them.
For 7.0 to 10 mHz, you will need about 12.
For 1.8 to 3.5 mHz, you will need about 16.
Palomar Engineering in California has these and can advise you best on the use of these and probably supply a heat shrink tubing to slide over them.
See them at:
re: Ferrite Bead Chokes
Yes, if you put the 16 beads on the coax so that you can operate as low in frequency as 1.8 mHz, the system WILL work all the way up to 30 mHz. The reason I listed approximate numbers was in case you were only interested in the higher bands on H.F., you could save money by buying only what you need.