The whole concept of radials is to improve the conductivity path for return current to the base of the vertical antenna, when the antenna is not a complete half wave dipole, (and therefore, balanced and a complete radiator unto itself). You are replacing always lossy (to some degree), earth or rock, with copper conductors.
The vertical of whatever length produces return currents that flow to couple into the earth, from the high tip of the antenna. There is some curvature of these return current paths, but generally speaking the tip current will traverse a path and reach ground (earth) about the same distance out from the base of the antenna, as the antenna element length or less due to the curvature of current paths. Therefore, there is probably no reason to make the radials a quarter wave long exactly, if the radiator element is not a quarter wave long. In addition, the presence of radials close to earth couples with capacitance such as to detune the electrical length of the physical radial. That quarter wave length you cut will not be electrically a quarter wave lying upon the ground. When you are backpacking, you may be upon stony ground in one place and good earth somewhere else. (Good in terms of RF character). You will have differing amounts of earth coupling. To get the full benefit of quarter wave radials, then, they would need to be elevated some distance above earth. Fortunately, with elevated radials, you get some benefit as to length and it has been found you no longer need a physical quarter wave length. Thus, if you pick an antenna length for its being easy to backpack, and at least 60 per cent of the full resonant antenna length, you get a shorter load to carry, but also you benefit from the current return path being no taller height than your antenna, such that radials can be less than quarter wave and still provide a good current return to the base of the antenna. SWR can be "good" as being near one to one, and you can still have a lossy vertical or other antenna. Don't get too wrapped up with seeking 1:1. A good quarter wave vertical is not 50 ohms, ever. And thus, if the vertical is quarter wave resonant, you will never have 1:1 SWR. But, that is OK. The goal is maximum field strength. The character of the surrounding terrain 2 to 5 waves out from the antenna will have more effect on reflections that could give you reflection gain, and help your antenna work well in terms of skip. In every back packer's kit should be a simple field strength meter, which can consist of a miniature meter movement, a diode and a RF bypass capacitor. This detector and a short whip can be used at some standard distance from your antenna over flat terrain to establish a "normal" field reading. Then, when you set up in a camp, you can quickly check if the antenna is radiating as well as it did when you first tested it under controlled conditions. Note the meter reading or mark the scale where your antenna tuned up best on your standard "antenna range", and you will be able to ensure you are getting out if that mark is reached by the field strength during backpacking operations. -Stuart K5KVH _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
