RADIO HABANA CUBA
Coaxial line RF choke
A really useful and easy-to-make-yourself addition to your antenna systemBY ARNIE CORO
Host of "Dxers Unlimited"SEND YOUR COMMENTS, QUESTIONS AND IDEAS DIRECTLY TO ME AT: arnie@radiohc.org
Flexible coaxial cable transmission lines have existed now for more than fifty years… Previous to the large scale manufacturing of the polyethilene plastic dielectric and copper braid + copper center conductor flexible line, professional installations used rigid coaxial lines, made from copper tubing and internal spacers placed along the line to keep the inner conductor at an exact spacing.
Coaxial cables, as they are also known, exist in many different forms, from miniature ultra-thin designed for short interconnecting runs in electronic equipment, to very special low loss dielectric lines that are almost as good as their rigid counterparts.
Here you are going to learn how to build yourself a very useful antenna transmission line accesory, one that can't do any harm to a system and that can certainly help reducing one of the problems faced by users of coaxial cables… the so called ANTENNA CURRENTS.
Antenna currents are caused by inbalances, and they can certainly destroy the radiation pattern of an antenna system. For example, a three element tri-band amateur antenna for 20, 15 and 10 meters, a very popular Yagi array for the ham bands, when fed directly with coaxial cable, may develope those dreaded antenna currents… which will in turn cause a severe distortion of the antenna's vertical radiation pattern…
In other words, you may be puzzled by poor performance from an otherwhise very nice looking recently installed antenna system, only to learn that ANTENNA CURRENTS, circulating on the OUTSIDE of the shield or braid of the coaxial cable are killing you!
Here is how to avoid this problem with ANY antenna system directly fed with coaxial cables…
USE A COAXIAL CABLE RF CHOKE COIL…
Which is easily built by winding a certain number of turns of the coaxial cable downlead, as close as possible to the antenna feedpoint… The diameter of the coil for the frequency range 15 to 30 megahertz may be around 15 to 20 centimeters, and the number of turns should be no less than SIX TO EIGHT, while TEN will do no harm at all.
The COAXIAL CABLE RF CHOKE COIL can be "air wound" or you may use a length of plastic PVC drain pipe to serve as a convenient coil form.
You can also use another practical version of the coax choke, consisting of from 15 to 20 turns wound using a 50 to 75 millimeter ( two to three inches) PVC plastic drain pipe coil form.
Whichever of the two designs you decide to build and use, your system will benefit, as the dreadful antenna pattern distorting currents will be effectively suppresed by the coaxial cable choke.
I make a point at using the fifteen or twenty turn coax choke on all my HF dipole antennas… it is certainly less expensive than using a ferrite toroid one to one balun transformer, and it does exactly the same job, at practically no cost…
Yes my friends… keeping those currents from flowing on the outside of the coax cable braid or shield is essential for obtaining a clean vertical pattern, and a CLEAN VERTICAL PATTERN is good not only for transmitting!
When you are using your antenna for receiving and the vertical pattern is clean, due to the presence of the coaxial cable choke coil, the system will pick up less noise for one very good reason… MAN MADE NOISE… is vertically polarized, so it will be picked up very easily by the distorted antenna pattern… With the choke coil, the horizontal antenna pattern will be nice and clean, and no vertically polarized waves will be picked up…
YES… you are just an hour or maybe two away from installing your new COAXIAL CABLE CHOKE COIL "antenna currents" suppresor
Good luck and tell me about your results!
Arnie Coro CO2KK
Havana, Cuba
1 August, 1998Postal address:
Arnie Coro, "Dxers Unlimited"
Radio Havana Cuba
PO Box 6240, Havana, CUBA 10600
phone: 53-7-814243
phone res: 53-7-301794
e-mail: arnie@radiohc.org
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