Re: [TowerTalk] SteppIR 40/30 and loading

[Jim Lux <jimlux@earthlink.net>]

At 12:50 PM 11/5/2003 -0500, K3BU@aol.com wrote:
> In a message dated 11/5/03 12:45:42 PM Eastern Standard Time,
> tongaloa@alltel.net writes:
> <<Interesting charts, but in the real world wouldn't one be comparing
> antennas to which equal
> power is delivered rather than antennas with equal currents at the
> current maximum?
>
> -Bob
> ah7i<<
>
> In examples and comparisons we would feed the same power, at the base of the
> quarter wave antenna, loaded or otherwise we will get the same current. What
> happens to it along its "travels" along the radiator, that's what defines
> efficiency. The longer portion of the radiator radiating maximum possible 
> current,
> the better efficiency, more signal out.
> ON4UN explains that nicely using cosine formula.
>
> Yuri

If your goal is simply to radiate the power, then a uniform current would 
be the best (since power dissipation goes as I^2, anything other than 
uniform I will suffer), and all the interesting analysis is valid. (I 
haven't given it much thought, but there are other arrangements with 
varying currents that might be as effective.. think of 3 phase power 
transmission viz DC)

However, this seriously neglects an important issue: that of the 
pattern.  An idealized dipole (or a monopole over ideal ground) with the 
classical cosine current distribution has 2.15 dBi of gain, presumably 
squirted in a desirable direction. Make the current uniform along that 
radiator and the gain starts to look different. Taken to an extreme with a 
physically small lossless radiator, and the pattern can be arbitrarily 
close to isotropic.
The monopole over ground has a pattern that has maximum gain at the horizon 
and radiates nothing directly vertically.  Start shortening the monopole, 
and you start radiating power at higher angles above the horizon.  Might be 
good, might be bad, depending on where you are and what sort of propagation 
you're looking for.

I think that the average ham might be willing to give up a few tenths of a 
dB in loss in exchange for a few dB of gain in a desired direction.


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