[Antennas] Q loaded versus unloaded

Wed, 1 Oct 2003 09:10:50 -0400

Dear Hue,
> Has this been already discussed here?
> What kind of figures for Q would you typically get for
> a wire antenna without any loading? For example,
> a one-wavelength antenna would act like a high-z
> tuned parallel resonant circuit. Assuming wire size
> is large enuff so ohmic resistance isn't a substantial
> factor, what might be a typical ballpark figure Q?

It's a factor of so many variables that it is not possible to give a
quantitative answer. Here are some considerations that must be taken into
account:
 Since the reactive components of the antenna impedance (seen at the
feedpoint) are dependent upon the electrical length of the antenna, the
inductance and capacitance components will vary.
 Since the ohmic portion of the antenna impedance (seen at the feedpoint)
is a function of ground losses and the ohmic losses in the antenna and the
electrical length and the height above ground, the "loading" will vary.
Everything considered, it would be necessary to use antenna modeling
software in order to obtain a quantitative value for a given set of antenna
parameters. All of that having been said, the bottom line is that the higher
the antenna, the lower the ground losses, and the larger the wire size, the
lower the copper losses. Both of which will contribute to a higher Q. The
bottom line is that for a half wave, resonant system in free space there is
a nominal value of Q that will obtain, and there is very little that you can
do to improve or degrade that significantly with traditional antenna
construction practices.
> Would proximity to ground affect this "Q"? If so, which
> way? In other words, would a dipole at lower wavelength
> above ground have narrower or broader bandwidth?

I think the answers to those questions are indicated above.
> Also, forgive a dumb question, but where can i find
> the formula to calculate impedance along a wire, in
> one of the ARRL or other books?
> Okay to just point me to a good but not extremely
> mathematical book. Probably not an EE text.

The best answer that I can give is to suggest two things - one, there is no
easy formula available that I know of, but EZNEC software is the accepted
answer to all those questions. The other is that you are probably worried
about factors that are not easily controlled. The more important factors are
the radiation pattern of the antenna and the question of getting your
transmitter's available power to be transferred to the antenna efficiently
and to then be radiated efficiently. Using traditional antenna construction
practices is hard to beat - ie., get it up as high as is possible and in the
clear, as much as is possible and practical. All of the factors that work to
the contrary of those guidelines will usually reduce the efficiency of the
system.
Bob Lay (W9DMK) in Dahlgren, VA
http://www.qsl.net/w9dmk