[Antennas] Random Antenna Question

2003年1月25日 17:48:44

Previously Sean wrote:
>How many of you are using random wires? If so, how does it compare to
>your dipole? I'm not asking about theory, I am asking about your
>experience.

As a follow up he wrote:
>I was looking for some ideas. Sorry I brought up the subject. 

No need to be sorry. Your original question was incomplete. It is hard to
give guidance when the meanings of "dipole" and "random wire" mean so many
things to too many people.
While you decidedly eschew "theory" in favor of "practical experience" in
your query, I don't know how you can avoid it. An anecdotal comparison
made by one guy at his location, with antennas that will surely be
different from what you might use at your location will be essentially
worthless.
However, making some rash assumptions that by dipole you mean a near
half-wavelength antenna, fed in the middle, I can say that this is the
"gold standard" of predictability. Freely available software or tables and
charts in any number of publications will show you what to expect. Sorry,
I know that's theoretical.
Continuing with the assumptions that the antenna is erected at a reasonable
height (in wavelength terms) and reasonably clear of surrounding objects it
will deliver the expected performance. 
If (big if) by random wire you mean a dipole of arbitrary length fed in the
middle with a low loss line and tuner, and erected similarly to the dipole
above, then the performance will be different for different lengths. Again
freely available software or published tables and charts for a number of
lengths can help in your predictions/comparisons.
If by random wire you mean a single wire brought into the station or
end-fed with a (now unbalanced) "balanced" transmission line then I suggest
that this almost defies analysis and enters the realm of "cut-and-try".
Certainly there is nothing wrong with cut-and-try, however, to ask for
comparisons from someone else who has done just that brings us back to my
original caveat.
If I may ask one more slightly rhetorical question, why do you want to
avoid theory? When you start to construct a half-wavelength dipole for 40
meters do you start with 500' of wire and prune until it's resonant or do
you use the approximate formula, L (ft) = 468/F (Mhz)?
Wes N7WS