RF Choke Coil (Made Easy)
Insert and tighten the PL-259 into the antenna’s SO-239. Wrap the connector with a few turns of electrical tape if you wish, to keep the connector waterproof. Tape or wire tie the coax to the mounting pole. Then use 2 or 3 more of the heavy duty plastic wire ties to mount the choke directly to the pole or mast as shown below. Then continue supporting the rest of the coax down the pole. When you are finished, your installation should look like this.
This photo is the choke I made, mounted just under my I-MAX 2000. As you can see, I stuck to the 8-8 rule.
In this photo, you can see the chokes I made on both the I-MAX 2000, and on my Dominator 6M antennas. I also have one on my CHA250B HF Vertical.
How Does It Do That? You may ask.
The way the choke works is very simple. A small amount of RF radiates from the shield wire in the coax. The RF emitting from the antenna surrounds the antenna with a very large pattern filled with RF. Some of it gets on to the coax and can travel down the entire length of the coax. Also, if there is a slight mismatch enough to cause even just a 1.4:1 SWR, some of that reflected RF comes back down the surface of the coax.
The RF choke creates an electromagnetic field on the chokes surface, and within the donut hole. This field attracts the stray RF and chokes it off before it travels down the coax, and it is dissipated within the EM field. Hence the term “Choke”. In some ways, it almost acts like a ground radial and reflects slightly the RF radiated from the antenna, to a more upwards angle sending more of the signal towards the horizon where it belongs, and not in your radio room. But unlike a ground radial, the choke does not tune the antenna to any specific frequency. So it is good for all frequencies from 160 through 6 meters.
Antennas like the I-MAX 2000 that cover more than one band, and do not require ground radials, benefit greatly from an RF Choke. As the antenna is used on different bands and across a wide range of frequencies within those bands, the SWR can vary quite a lot. As the SWR goes up, more stray RF likes to seek out the source of the RF (Your Radio) by the shortest and quickest route possible (The Coax). This RF choke prevents that from happening which helps your SWR a little, and keeps RF feedback out of your radio room.
Dipoles, Yagis, and single band antennas also benefit a lot from RF chokes for all the same reasons. Horizontals, verticals, slopers…etc. Every antenna of every design still emits RF, and it can travel along the coax.
NOTE: Before we go on, I need to make one thing clearly understood. An RF choke will not prevent the reflected RF caused by a high SWR from damaging your radio. Reflected RF caused by an impedence mismatch, and is reflected back to your radio through the center conductor of the coax, and can still hurt your equipment. So do not assume that a choke will cure your high SWR problems.
An RF choke only stops the stray RF on the surface of the coax from traveling down it’s length that causes RF feedback. Removing the STRAY RF can in many cases help the SWR, but just a little. Only correcting the impedence mismatch of the coax and antenna can fix your SWR problems.
Now you know how to build and install an RF choke, the easy simple way. You may read about how chokes should be cut to a specific exact length, and each loop should be all side by side and spaced properly, then adjusted perfectly to a specific frequency then hung in a specific way to…………OH PLEASE! Give me a break. You are not building an antenna for the International Space Station. It’s a simple RF choke to stop stray RF from traveling down the coax. The design I just walked you through will do exactly that. Can you count and make 8 loops, can you read the number 8 on a ruler, and do you understand how a wire tie works? If you answered yes to these 3 questions, you will do just fine. Just remember to add about 17 to 20 feet to your coax purchase, because each 8 inch loop uses just over 2 feet of coax (about 26 inches).
By David – K3DAV (4/18/2012)