Posted by
Stuart Rohre on
Jun 29, 2006; 11:07pm
URL: http://elecraft.85.s1.nabble.com/KX1-1403-Antenna-Project-tp391631p391632.html
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
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